New Tight House – Negative Pressure – Wood Stove Causing Issues – Make up Air issue?
Hello!
Just recently finished and moved into our new Construction house. First (and last) house I built and designed myself with zero prior experience.
Western MA, Zone 6. 3 indoor mini split units, and a brand new Vermont Castings 2040 CAT wood stove, with fresh air intake system installed. The stove pipe is a straight shot up, no bends. Is single wall where pipe is exposed on the 1st floor and insulated pipe through 2nd floor chase, attic and up through the roof.
House is 32′ x 24′ with two floors and a basement. 8′ ceilings on 1st floor, 2nd floor has 6′ walls at eaves side, sloping up to 8′ ceilings. the basement ceiling is approx 6′ 6″ in one half and about 4′ 6″ in other half – an “almost crawlspace”. Vented attic, 10/12 pitch
The basement is insulated on the walls with 3 inches of foam, and is therefore considered conditioned space.
*Important Information*
We thoroughly sealed off the wood stove flue, the fresh air intake system, and the stove pipe seems before our blower door test. (the insulated stove pipe seems that are buried in wall chase are permanently sealed). Having no experience, I was scared I would fail my blower door if I didn’t do this, which apparently is common practice…(?)
*Blower door Results as followed
*Total conditioned space = 2,336 sqft
*House Tightness = 263.2 cfm50 (0.86 ACH50)
*x2 Continuous Bath vents were to set by HERS rater to 35cfm and one at 40cfm. Both have boost options on a timer which are used for shower, etc.
*Kitchen hood vent above our induction range which maxes out at 400cfm – I chose this model under the impression that I would not need make up air.
**Issues**
I’ll start by noting that the fresh air intake for the wood stove, BY DESIGN, is not connected in an air tight manner to the firebox. Something I only now am noticing, as I didn’t install the wood stove system. The fresh air intake is screwed in place behind the firebox’s air control flap, with a box surround that has plenty of air space for air to leak in or out. The fresh air duct goes down into basement, where the duct insulated, turns 90degrees and heads out the south west side wall.
* When the woodstove is cold – With 30cfm of bath vent exhaust turned on, no air is pulled into the house through the stove pipe seems, but air does come in from fresh air intake. At 60cfm of bath vent exhaust, cold air is pulled through stove pipe seems into the house and noticeable more cold air is pulled in from fresh air intake leak. When no bathroom vents are running, there is still cold air leaking in from the fresh air intake, but at a lower rate, and the stove pipe seems no longer leak inwards. Once a fire is established in the stove, smoke/co2/air is able to resist being pulled into the house by the bathroom vents (though I haven’t experimented with running both of them at specific cfm to see where the threshold is) But I’m guessing its safe to assume that at certain point as the fire slows down, it will begin to slowly leak into the house if even a single bath vent is turned on. *Needless to say, when the kitchen hood vent is used in conjunction with or without the bathroom exhaust vents, it can pull plenty of smoke into the house, within seconds, even with a well established fire going. (cracking a window when using the kitchen hood is an easy enough of a habit to get into – but is not an elegant solution and doesn’t solve the other issues).
The potential for a slow leak of smoke/cO2 as the fire dies out is my main cause for concern. As well as the pulling in of cold air when the wood stove is cold and any exhaust is. being used. It’s my understanding that having continuous exhaust via bathroom vents is important for air quality/air exchanges, but the exhaust vents are pulling in cold air and likely smoke/cO2 at low firebox temps.
Does anyone have any reasonable solutions/insight. I can’t bare to rip open walls and run ductwork. This business of balancing variable air pressure scenarios is over my head and I’m not sure how to proceed in finding an inexpensive solution. We were originally quoted 10,000$ for an intelli-balance ERV which we could not and still cannot afford.
Ive attached photos of the spots where there are noticeable leaks. Thank you all very much in advance! – MD
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Replies
Those seams look shoddy. Those single wall pipes are difficult to seal.
The fresh air intake feeds the stove and the air then leaves the stove, travels up the pipe and leaks out of the seams when the stove is cold? Hopefully, the stove is tight enough so it doesn't leak out of the stove.
I would investigate a damper to close the fresh air intake when not in use. This should solve one problem. The second problem appears to me to be that your fresh air intake is not supplying enough air during stove use. This is a stove installer design problem.
Your entire stove system, stove, pipes and fresh air system seems leaky. Maybe replace that single wall with double wall pipe also.
the fresh air intake feeds into the stove yes, but also leaks air right at the place where the fresh air kit meets the firebox = the fresh air intake is by design not air tight sealed to the firebox - As shown in the 3rd photo, you can see the gap along the red line. It's difficult to say weather or not the air coming out from the stove pipe seems is coming from the fresh air intake or from the top of the stove pipe.
md1986,
You shouldn't have to worry about having air-intake ducts or chimneys being entirely airtight to counteract pressure imbalances. The leaks between pipes are a symptom, not a cause.
If I have understood your description correctly you have an exhaust-only ventilation system. Unfortunately thats a very bad match for a house with combustion appliances like wood stoves. Ideally you would have balanced ventilation, and additional make-up air for the exhaust fans in the bathrooms, dryer, and range hood. In the absence of that you should at least add passive air intakes to provide an easier path for the make-up air to enter the house, rather than doing what it is now, which is using the wood-stove vent.
If your bathroom fans are actual going to function and move any air out of the house that air must be replace with unconditioned air that will find the path of least resistance. It seems you fresh air intake is the current path of least resistance for air to enter. My point is there will always be a leaky spot somewhere they maybe multilabel smalls leaks that can’t identified.
My opinion is wood stoves and tight houses and are poor companions and best avoided.
My guess is what you have will work well as is but I would have 3 CO detectors around the house just to make me feel better.
When the fire is dying out, I think there will be more than enough heat in the stove to keep warm air rising up the fly pipe.
Sealing the pipe joints of the cold air intake with foil tape would not be a problem if you wanted too.
My opinion is that you are smart enough to decide if the air in your house is stuffy and crack open a window when necessary. If your wintertime indoor humidity exceeds 45% crack open a window until it drops. Yes, opening the window uses more energy than an HRV but it would be a very long time before an HRV could save $10K in fuel.
Walta
Thanks for the reply.
Yes, unconditioned air has to come from somewhere.
Do you think outside air is coming in through the woodstove at a much higher rate that it should be/is required for air exchanges? Being that with the continuous bathroom exhaust running at a rate that was set to pull air from a house that was much tighter = the blower test results, based on the house with the sealed off wood stove system. If they did the blower test and I hadn't sealed off the wood stove system, would I even need continuous bathroom exhaust or would the wood stove be acting as a passive exhaust system?
You know your home better than anyone else ever will.
The inspector required you to install the fan and made sure you did install it. If the fan happened to fail in somehow, it is unlikely the inspector is going to stop by and notice the failure.
To my way of thinking if the winter time indoor humidity exceeds 45% you have a moisture problem and need more ventilation. If you find your house is stuffy you need more ventilation.
Where air leaks into the home generally does not matter the volume of air does matter.
Walta
Exactly, the AMOUNT of air leaking in is what I'm worried about.
I also have a new self designed and self built house with a woodstove in your general area (southern VT) and also find what Walta points out, that wood stoves and air tightness are a difficult match. I've been heating with a minisplit so far and am curious how far into the season that will take me before needing the stove. My solution for the leaky stove while not in use is to seal off the cold air intake and remove the single wall pipe inside (my stove goes out the side wall so I only have about 6 feet of pipe inside) and seal off the double wall pipe as it goes out the wall. But I do have the intelli-balance erv that you mentioned so I don't need the air flow. In your situation it almost seems like the "leaky" cold air intake is necessary as without it seems unlikely your tight envelope would have the makeup air necessary for the exhaust. With air being available from the cold air intake I would doubt your exhaust would pull smoke from the chimney even if the fire was smoldering but it's certainly worth testing with CO monitors.
The HERS rater set the exhaust fans to a CFM to pull a sufficient amount of fresh air in from what little air leakage was present during the blower door test (the wood stove system sealed tightly for the blower test). By leaving the exhaust fans running at the set CFM, and with the wood stove now un-sealed, I'd be curious to know if the exhaust fans are now pulling in much more air than was designed by the HERS rater. Because now there are more holes in the house to pull from, and with little to no resistance. Making me think I should quit with the continuous exhaust for now. When the exhaust is off, and the stove is cold, fresh air still trickles in from the stove's intake.
Smoke does not pull in through the wood stove when the bathroom fans are on, even when on boost for showers. The kitchen hood vent however, even at its lowest setting = 120 cfm pulls smoke in through the pipe seems even when a ripping fire is going. Ive been cracking a window when I use the hood.
Without sealing up the fresh air intake when the stove isn't in use, for now I'll just be pulling in excessive air whenever any exhaust vents turn on and the stove is not running. You can feel the breeze from the fresh air system get stronger in relation to the CFM increase of the exhaust. When the stove is burning, the cold air from the intake is pulled right into the firebox as designed.
I bought an airflow meter on amazon to verify that my erv vents were hitting the cfm they were set to. That should give you some insight as to if your exhaust fans are pulling out more cfm than you expect. I also use A CO2 meter and adjust my erv settings to maintain a CO2 reading at 800 or below. So if you know your CO2 levels are in good shape you could reduce the exhaust. With the smoke being pulled from the chimney when the hood is on, looking on the bright side at least you know you built a damn tight house and seems like opening the window is a suitable workaround for the time being. From your experience living in the house so far do you feel like the hood is a necessity with the induction range? I have induction with no hood but an erv exhaust vent directly above the stove but that particular vent is only typically pulling about 20 cfm.
It's hard to say if the hood vent is a necessity at this point. We've only lived in the house for 3 months. I'd imagine in warmer/humid seasons, it's probably a good idea to remove any cooking moistures as to not make the house too humid. In the winter, maybe that steam from boiling water etc would be a good way to help keep the house from being too dry. Another reason why we got the hood vent Is because our 1st floor is one open room (and a very small 1/2 bath). We didn't want cooking smells to linger for long periods in the bulk of our living area.
And yes, the house is damn tight overall! And very easy to heat, even with the air coming in from the wood stove.
Deleted
You got your house half right: the saying is, "build tight and ventilate right." You have a house that's more airtight than probably 99% of existing homes, but you're continuously exhausting 75 cfm, with another ±400 cfm when the range hood is running, plus ±25 cfm when the woodstove is running, and if you have a conventional clothes dryer, that's another ±125 cfm. That's up to 625 cfm that HAS to be replaced by outside air.
Your house's interior volume is around 18,700 cf. That means that every 1/2 hour, ALL of the air in your house needs to be replaced if all of the venting appliances are running. Round up or down, but that's a lot of air to move. The easiest place for that up air to enter is through your woodstove makeup air vent. (It's common for the makeup air not to connect directly with the wood stove, to prevent embers and smoke from being sucked out in an air reversal.)Assuming a 2" diameter makeup air pipe, air is attempting to enter at up to 30,000 feet per minute. Ducts are normally designed for 500-1,000 feet per minute.
Some of that air is also likely coming in through your range hood vent, dryer vent and the few remaining gaps in your building envelope.
The IRC requires makeup air for range hoods over 400 cfm, but that's assuming their tightest building envelope of 3 ACH50. I imagine as tighter buildings become more normal, they will update this requirement.
The solution is to pressure-balance your house. Ideally you would have both a balanced ventilation system (ERV or HRV) instead of bath fans, and a dedicated makeup air system that turns on when your range hood is on.
Because the wood stove is only drawing 15-25 cfm when it's running, once it's running it should rarely "backpuff" when the outside air pressure drops suddenly, and a balanced, whole-house ventilation system would take care of the little bit of smoke that enters the house. It's very common in relatively airtight homes like yours to open a nearby window a few inches until the stove and chimney get up to temperature, then they usually draw ok by themselves. But having a house at a severe negative pressure AND running a wood stove at the same time is a dangerous combination. Running your house at negative pressure can also pull radon into your conditioned space.
Thanks!
That all makes sense. The wood stove intake duct is 3". We have a heat pump dryer as well, and would rarely, if ever, run the kitchen hood vent to full 400 cfm. Our county does not require radon venting/testing because apparently it isn't very common here.
The whole house ERV/HRV would be ideal but was and is still to0 expensive. Do you think shutting off the continuous bathroom exhaust is a good idea, during the winter time especially?
If you know of any ways to help remedy some of these issues/ideas without the whole house ERV, please do let me know!
md1986,
Here is what Martin Holladay said:
" If your house approaches Passivhaus levels of airtightness (aiming for 1 air change per hour at 50 Pascals or less), an exhaust-only ventilation system may be starved for makeup air. Most Passivhaus homes have a balanced ventilation system (an HRV or an ERV). Builders of very tight homes who prefer to install an exhaust-only ventilation system should consider the installation of one or two passive air inlets."
https://www.greenbuildingadvisor.com/article/designing-a-good-ventilation-system
I understand; few people want to pay the premium for balanced ventilation, when it still seems like "advanced" technology. (It's been around for 40+ years.) But if you want an airtight home AND a combustion appliance, I don't consider it optional--it's every bit as important as the air-sealing, or more so, since it relates directly to both indoor air quality and life safety.
I've tried passive makeup air systems on a few projects, either homemade or the Lunos passive system. But they aren't very effective unless they are huge. It's better to have a small hole with a fan that you control or that is controlled automatically. Just like exhaust-only systems, there are supply-only fans available.
Michael,
I agree - and that advice from Martin is 16 years old. For the best part of a decade balanced ventilation has been required here by our code. But In the OP’s case balanced mechanical ventilation may practically not be an option if the house is already complete.
Would you be especially concerned about any part(s) of my current setup in any way?
Yes my house is complete, and our debt hole is deeper than we expected. The only way getting an ERV installed would jump to the top of our current priorities list is if it posed a very serious health/safety concern.
The only ERV option at this point might be to have the unit in the basement with ducts that dump air through floor vents on the 1st floor only. I'd assume that wouldn't cost 10k, as the 10k design I was quoted had ducts ran to both floors. Maybe that would be simple enough to even do myself.
I've been in touch with the wood stove installer, and they plan to come check out the stove soon. If they suggest sealing up the fresh air intake as well as the stove pipe seems, would that be advisable? Something tells me that may not be a great idea.
md1986,
As I said in my post #5 above, the leaks in the vent and chimney should be seen as symptoms not the cause of the problems.
Negative pressure in houses with combustion appliances is definitely a cause for concern which needs remedying. I guess the question we are left with is whether passive vents, or even just cracking a window, will be sufficient to fix the problem, or as Michael says you need to introduce mechanic ventilation.
Deleted
Supply only fan sounds interesting and that it would could be helpful. Do they make supply only fans that would respond appropriatly to pressure changes in the house? Since the basement is part of the conditioned space (though it's not directly heated) maybe a supply fan could come into the basement through a portion of our basement that has some wood framing and not through the concrete.
Thanks Malcolm.
Do you think my wood stove's fresh air intake is essentially acting as a passive vent? Especially when the wood stove is not in use, and therefore the air from the fresh air intake is not being pulled into the firebox.
Currently, If I decided to heat only with mini splits, and I left the wood stove's fresh air intake un-sealed, it would pull more air, in relation to how much CFM is exhausting. I wonder if the amount of air being pulled in when the stove is not in use, is putting a burden on heating/cost of mini splits, or if it is supplying a proportionate amount of fresh air in accordance to what is being exhausted.
md1986,
Yes that's exactly what I think happening. It's another circuitous path for make-up air. Even with balanced ventilation many people put a damper on the wood stove intake to prevent that. And even if there is no negative pressure to bring it into the house the damper stops the introduction of cold air coming from outside and exhausting up the chimney.
If your house is being depressurized by the exhaust only system I don't think the vent is supplying too much make-up air or affecting the mini-split's efficiency. That could only happen if the supply air exceeded the amount exhausted, and your current experience seems to show you are a long way from that.
Thanks so much Malcolm.
I'm assuming that all applies to when the wood stove is not in use, and it makes sense to me.
When the wood stove IS in use, all windows closed, and the majority, if not all of the cold outdoor intake air is being pulled into the firebox, is there any reason there would be a concern about not getting air exchanged into the house for breathing. Or do you think at that point, the exhaust fans would be pulling air in from other points of entry. Meaning the exhaust fans would pull air from different locations, depending on if the wood stove is running or not..
md1986,
- No I don't think that in general there is any danger of having too little fresh air for the occupants. Like other combustion devices - furnaces, gas water heaters, etc - the danger is that the negative pressure draws combustion gasses into the house, in the case of the wood stove this is likely to happen when the fire is dying down.
Why I said "in general" is that with an exhaust only system you may well have rooms, like bedrooms where the doors are often closed, where the system is supplying very little fresh air, and others where it is supplying more than enough. That's the disadvantage of relying on leaks in your building enclosure to decide where the supply air comes from, and only exhausting from bathrooms.
What if you keep the exhaust fan running and close off the chimney with a damper when you are not using the stove and then shut off the exhaust fan and open the damper in the chimney when you are using the stove?
What about the smaller ERVs about the size of a restroom fan that can bring in fresh air at a neutral pressure? Could you use those for your kitchen exhaust and restroom exhaust (panasonic whisper comfort)?
Would be wise and inexpensive to invest in multiple CO monitors especially in each sleeping quarter, regardless of your path forward.
By the way, some minisplit units have a knockout in the back or side to allow fresh air to be ducted into the unit. Is that an option with your minisplit units?
To shut of the continuous exhaust in the bathrooms, requires a step ladder, removing the grill, so that is not something anyone would want to do often. Only the exhaust's boost function, which is on a timer, can be turned on by using a wall switch.
There is a smoke/c0 detector right above the wood stove, and 3 of them on the 2nd floor, one in each bedroom and one in the hallway at the top of the stairs.
Im not sure about our mini split units having that option.
Do you have any CO2 monitors in the house? That would be a real and practical way of making sure your getting enough or not too much fresh air at any given time.
Wondering if you heard anything back from the stove manufacturer about sealing around the cold air intake. I would be interested in what they said.
I didn't reach out to the manufacturer, but I did the wood stove installers. They havent come out to the house yet. But on the phone, they did mention that they have been sealing flue seems more often as houses have been getting tighter. He didn't say anything specific about the fresh air intake.
Just curious - what was the specific purpose of installing a woodstove?
Is it your primary heating source or for aesthetics (or both?)
We've got 16.5 acres of woods. I enjoy cutting/splitting/stacking wood. We live in a rural area, and lose power often during storms. The way I see it, it's free heat. Even if I buy firewood, it's cheaper than running the mini splits. We ran the mini splits for about half of November and our electric went up over a hundred bucks. Being that this is our first winter in the house, I am curious what it would cost to heat solely with mini splits for a month during the dead of winter. But I don't think it'll be worth the cost of finding out. Wood stove is our primary heat. With the house being so tight, it doesn't take much wood at all to heat both floors. Our mini splits are for keeping things from freezing up when we're out of town/heating during shoulder seasons when the wood stove would be too hot. As well as for cooling in the summer, which is very inexpensive to run.
Have you given any consideration to moving the wood burning outside of your house?
begreener,
What is the advantage of doing that?
Maybe consider moving to a wood boiler (combustion outside of house) & use a hot water tank "thermal bank" that then circulates to indoor fan coil emitters
https://store.tarmusa.com/
https://store.tarmusa.com/collections/indirect-hot-water-tanks-1
https://www.123zeroenergy.com/ductless-split-fan-coil.html
https://toyotomiusa.com/product/hc-190b-toyotomi-heat-convector-with-wall-bracket/
begreener,
I’ve still got the same question. You go from a simple, efficient stove and chimney that has all the components inside the enclosure - meaning you can feed the fire there, with minimal heat lost, and you can enjoy looking at the flames - to a wood fed, mechanically complex heating system.
Your situation is very similar to mine. I've been using my minisplit to keep the house at 72 down stairs and 68 upstairs so far this season and a 2000sqft house. With current temps in southern VT the minisplit is using around 12-17 kw per day which equates to about 90 per month. I have 5 1/2 forested acres which is why I installed the wood stove and will have plenty of firewood going forward but with this being the first winter in the house and spending all of my free time on the build I have a limited supply this year so will run the minisplit for a good portion of the heating season. Our house is very tight with 12 inches of dense pack cellulose in the walls so putting in the wood stove and introducing what is more or less a permanent air leak was a painful decision but as you said, when it's free heat it just made too much sense not to do it.
I'd imagine with 12" of cellulose, you won't have to burn much wood at all to heat your house for the season. We have just 5.5" cellulose and I keep overheating the house with small 3 log fires. Ive been letting my burns almost die out completely before re-loading. I wait till there's only a small amount of coals left, then reload, otherwise it's pretty easy to overheat.
Josh,
"putting in the wood stove and introducing what is more or less a permanent air leak"
If the unit has dedicated make-up air, how is it a permanent air leak?
Because the term "air tight" in regards to modern wood stoves is nowhere near the same meaning as when we are talking about building envelope. epa stoves have a damper to control the amount of air that can come in but even if it is in the "closed" position it isn't closed all the way, so air can leak into the stove that way and there are joints around the cold air intake that aren't completely sealed, apparently by design. I'm searching for something like a dryer vent cover in reverse that will allow air in but shut when not drawing air. But even then you have the issue of the stove pipe itself, which the seams could be sealed, but even if they are there is still a 6" or 8" hole at the top of the stove pipe which allows air into the stove box where it has the same avenues into the house as the cold air intake.
And you feel like an idiot when you talk about this stuff in any kind of wood stove forum because the air leakage issue seems so minimal under normal construction but when that small amount of leakage is far and away the single greatest air leak in your building envelope it will always be in the back of your head.
"And you feel like an idiot when you talk about this stuff in any kind of wood stove forum"
Also, in general wood burners have minimum outputs that are so high that air infiltration just isn't a concern.
There's a reason that Americans didn't really start insulating houses until they stopped burning solid fuel in the house.
Josh,
If those things worry you - and I’m not sure they are significant, you should install an airtight damper or valve on the dedicated outside air intake. The chimney isn’t much different that the plumbing vents that penetrate your roof, but that too can have a cap damper installed.
Thanks Malcolm, I am looking into those options. I am also not convinced this makes any significant difference, although the CO2 levels in the house do seem to be a bit lower sense I unsealed the chimney to connect it to the stove for the winter. My main issue is the psychological effect of feeling cold air around the unsealed parts of the wood stove after all the effort that has gone into making the house air tight. But in the grand scheme of things it likely makes no material difference to my energy consumption.
Josh,
I take your point. Wood burning appliances are really vestigial devices. In general they don't have much of a place in a new, efficient house - but there are still places and circumstances where a wood stove is a great solution. Because they are mainly used in these marginal situations I doubt there will be much effort put into making them integrate into a house like yours. I suspect we will just have to put up with a bit of inefficiency.
To confirm, you have no dedicated make up air in the house except the fresh air intake for the wood stove? With a tight house (0.86 ACH50), that is your fundamental issue.
We don't have the complication of a wood stove or 400 cfm range hood, but we have a tight house (less than 0.50 ACH50) in NH and Panasonic bath fans for ventilation. We started out with 2 Panasonic passive air inlets, but that wasn't enough, and we now have 4 (2 downstairs and 1 in each bedroom). Those provide enough make up air that we run one bath fan at 30-40 cfm continuous depending on the season.
First, I would recommend installing some Airthings monitors in your house so you can track various levels (CO2, radon, etc).
I would also consider replacing your range hood with a much less powerful version or going re-circulating if you're not heavy duty cooks. The 400 cfm hood is going to seriously de-pressurize your house except during times of the year when windows can be open.
In the end, despite the costs and complications, I think you're going to have to retrofit your house with an ERV or install passive air inlets in rooms that need fresh air (bedrooms, living areas). Panasonic stopped making them, but Lunos has some expensive ones (https://475.supply/products/lunos-ald-r160-passive-air-inlet-kit).
"We started out with 2 Panasonic passive air inlets, but that wasn't enough"
I'm curious how you determined it wasn't enough.
With only one bath fan running at 30cfm, the house was still getting de-pressurized (air wooshing in when opened). The Panasonic passive air inlets were advertised as letting in up to 18cfm, but in reality probably only let in 10cfm.
Correct, we do not have any make up in the house, except for the fresh air intake of the wood stove.
Thanks for recommending the Airthings monitors. I'll check that out.
We would rarely if ever use the range hood at full setting = 400cfm. the lowest 2 settings is more than adequate and are 120, and 240 cfm. We are pretty heavy duty cooks. I don't see an issue with cracking a window when the hood vent is on.
The passive air inlets you suggest do not condition any of the of incoming air, correct? And do they adjust the volume of incoming air based on the changing pressures of the house?
Maybe If my house wasn't so tight, I wouldn't have to worry about breathing air inside of it. Makes ya think..
md1986,
"Maybe If my house wasn't so tight, I wouldn't have to worry about breathing air inside of it. Makes ya think..”
But you would have a myriad of other problems. Potential moisture issues in the walls and roof, lack of comfort, and poor efficiency. Houses that are insulated and air-sealed need a source of make up air. The issue you have isn’t that your house is too tight, it’s that your ventilation strategy didn’t include it.
Malcolm --
"Houses that are insulated and air-sealed need a source of make up air."
I know you operate under a slightly different code, but under my code only houses with combustion appliances in them need make up air. All-electric houses don't.
I feel like there is a fetish around depressurization. Even houses that are considered very tight have sufficient leakage, you're not going to notice a small pressure difference with the outside.
"Even houses that are considered very tight have sufficient leakage,"
I would disagree with this based on our experience. Houses that test below 1.0 ACH50 (and certainly below 0.5 ACH50) have very little to no pathways for fresh air to make it in.
DC, as I've mentioned before, my experience is different, requirements aside. One house tested at 0.14 ACH50 and has an outswing entry door. We included a passive makeup air inlet, 6" diameter but with a long duct run. The owners run the exhausting dryer frequently and have a powerful range hood. Trying to open the entry door felt like someone was on the other side pulling on it.
Another project, that I think I mentioned above, tested between 0.5 and 1.0 ACH50, again without makeup air. The range hood, 400 cfm max, barely works unless a window is opened.
Both homes are around 2200 sf, plus one has a full basement, so it's not like there's a lack of interior air volume.
Oh for sure, I hear ya, I was just being pesky. Im glad my house is good and tight.
I will say, I do find it frustrating that our HERS rater only suggested ERV or continuous bath fans as our make up air options. I do think that if they had suggested those simple make up inlet options I would have done that to begin with. Air pressure differences and make up air was never talked about. But maybe that's just not their area of concern.. Of course, being that I designed and built the house myself, it's ultimatly my own fault for not learning about these things.
md1986,
"it's ultimately my own fault for not learning about these things."
Maybe, but it sounds like you did a pretty good job, and if adding a couple of passive vents turns out to be all you need to solve your ventilation problems, I'd say you've done really well.
DC,
I put that poorly. What I meant was tight houses need mechanical ventilation, and if that or other appliances unbalance the house excessively, or it includes combustion devices, then it needs make-up air.
Malcolm --
I agree with two thirds of that but quibble with "unbalance the house excessively." What does that mean?
I would say it means it reduced the effectiveness of ventilation fans, which is going to depend upon how tight the house is and the size of the fan and the shape of its fan curve. That's a legit concern in the absence of combustion, and once the blower door numbers are known the effect can be calculated.
My point is that people should be discouraged from routinely making holes in their nice tight houses out of a fear of "depressurization." Unless there are practical issues that arise there is nothing wrong with having the house at slightly below atmospheric pressure. And in the scheme of things the pressures involved are miniscule. Fifty pascals is the pressure change associated with a 13 foot change in altitude, it's what you experience going between floors in your house. It's not going to cause your ears to pop or the water to slosh out of your toilet traps.
I think the consensus is that you have lots of paths for air leaving and few paths for air in. And on top of that the problems of using a woodstove in a super tight house (comfort issues, back draft risk, poor combustion due to negative pressure, interactions with other appliances.)
So one solution is outside makeup air—poking a hole or holes in your nice tight house. But that seems absurd given the trouble you’ve gone to seal things up.
So you definitely need an ERV—basically a heat exchanger. That way fresh air comes in and stale air leaves and most of your heat stays in the house. And your woodstove is happier—as long as nobody burns anything in the kitchen. (And even without the woodstove, makeup air with a motorized damper on a pressure switch is a really good idea in a tight-house kitchen with a big hood.)
A Panasonic Intelli-balance would be a good solution, and you can probably DIY or find somebody willing to tackle a simple installation. If the unit is in the basement, you have limited ducting to do—a wye with one supply branch in the basement and a second for the first floor, and the same on the exhaust side, with some good separation between them so the fresh air circulates and mixes. The manual has all the details and Home Depot probably has all the stuff you need.
Definitely a learning curve with all this, but sounds like you got the big stuff right. This is just the right appliances.
"And on top of that the problems of using a woodstove in a super tight house (comfort issues, back draft risk, poor combustion due to negative pressure, interactions with other appliances.)" I don't think any of these issues are inherent problems with having a wood stove in a tight house. They are only problems in this particular house because of no make up air which is an unusual circumstance for a new house with this level of tightness.
I agree that the ERV is the real solution here. If you built the house yourself you certainly possess the skills to install the intelli-balance yourself. I did my install and it was about $1,500 all in. They also make single room units that don't require duct work, perhaps 2 or 3 of those would do.
Woodstoves tend to overheat tight houses—it’s easy to get 20,000 btus/hr; it’s really hard to throttle down to 5000 btus/hr. So you end up in hot-cold cycling.
I think the idea of wood stoves inevitably overheating tight houses is overblown. With modern day EPA approved wood stoves you have a lot of control over airflow and therefore burn rate. Also like any other heating/ cooling appliance if you choose the right size wood stove (mine is rated for 1200 sqft in a 2000sqft home) you can avoid those issues. Sure, I could continuously pack the stove full and heat the house up to 85 if I wanted, but I can also maintain a steady moderate burn and keep the house 72-75 as long as I want.
What I still don't understand is - will an ERV also supply the make up air, and adjust to pressure changes related to the hood vent etc, as well as exchange air continuously?
Do I need both an ERV and make up air in order for my wood stove/kitchen vent to work without cracking a window?
If I crack a window when I use the hood vent, the wood stove doesn't backdraft. How much energy/heat am I really losing overall with that system vs how much money would I spend running/maintaining mechanicals.
We have no problems with poor combustion or physical comfort here. we lived in a one room, 400 sq ft cabin with hardly any insulation for 5 years before we finished and moved into our house, it's plenty comfy in here.
I think the only way I'd be convinced to spend anymore money is if we aren't getting enough fresh oxygen, or too much co2, and to know that, I'll need to an air quality monitor.
I think you're thinking about it the right way. The main reason I installed the ERV was because without it I thought our CO2 would regularly be too high, and I have found that the CO2 levels regularly rise above 1200 when the ERV is off so I do think it is needed. But the only way to figure out if your situation is similar is to monitor your CO2 and VOCs which is very inexpensive and useful information.
So right now you have multiple exhaust appliances that fight one another. The woodstove will work better with an ERV because it is “balanced” in that it bring in its own makeup air, unlike the bathfans.
From a money standpoint I’d look at it this way: an ERV buys you the value of what you’ve already spent on the air sealing, insulation, and mini splits. All of those features are degraded by running bath fans 24-7.
Mini splits in the shoulder seasons in a tight low-load house will be cheap to run and comfortable whereas your woodstove won’t have good draw and will probably overheat the place constantly.
Anyway good luck, hope that’s helpful.
So right now you have multiple exhaust appliances that fight one another. The woodstove will work better with an ERV because it is “balanced” in that it bring in its own makeup air, unlike the bathfans.
This is true but the wood stove still needs makeup air for the air it exhausts. The ERV isn't going to provide that.
A wood stove is an exhaust appliance. It doesn't need balanced ventilation that an ERV would provide, it needs a dedicated supply of makeup air to match the air that it is exhausting. The best way to provide that is a direct feed to the firebox.