Increasing Relative Humidity in Tight House
New house build in zone 7. First full winter in the home. House is .2 ACH50. Fully electric with a Mitsubishi heat pump and air handler ducted throughout the home. Fresh air from the ERV is primed with an electric duct heater and is injected into the air return to distribute throughout the house and their are dedicated exhaust ducts.
The ERV (Renewaire EV L Premium) is set (and designed) to run 100% of the time on low with a boost mode for bathroom exhaust.
The house stays very comfortable in every way, even when very cold, except for that the relative humidity is around 20-24% during the winter and dries everyone out.
What can I do to improve the RH conditions? Should I play with reducing the runtime of the ERV, going against the intended design and just incorporate a 80, 90% timer or something?
Should I consider a whole house humidifier? Then, my concern is that I am chasing my tail given that I’d be injecting humidity and then immediately exhausting it with the exhaust and fresh air supply regularly changing over.
Help! We’re drying out!
Thanks in advance.
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How big is the house/how many occupants? Are you monitoring any IAQ metrics other than RH?
Those things will likely help you decide between moderating the ERV and adding supplemental humidification.
Working through a similar thing, but down here in climate zone 4A. First winter in the house post-"deep energy retrofit" and substantial tightening. RH stayed decent except during some extreme cold snaps (15F and below), during which I just put some water on a simmer which easily helped things.
Hi Patrick,
The house is 2,000 sf with 2 adults and 2 kids. We're not measuring anything else, no. Mostly just the "sniff test" and the overall sense is that the air quality is always very fresh and clean... but dry, of course.
Forgive the ignorance but should I be measuring other things specifically?
We tried simmering some water but you've got to boil a whole lotta water for a long time to get even a few points of RH difference.
Thanks!
You can get a controller that monitors humidity, CO2 and VOC and only vents when one of them is high.
As to the question of whether excessive venting is the problem, I'll try some analysis. The source of humidity in the house is occupant activity -- bathing, cooking, breathing. The rule of thumb is each occupant produces 0.2 lbs of water per hour, so 0.8 lbs per hour for four people (that might be overstating what the kids produce). If the house is staying at 20-25% RH that means enough air is being exchanged with the outside to remove 0.8lbs per hour of water. Air at 70F and 25% RH contains 28 grains of water per pound of air. You don't say how cold it is there, but air at 20F and 50% RH contains 7.5 grains of water per pound of air. So each pound of air you exchange with the outside sheds 20.5 grains of water. A pound is 7000 grains, so 0.8 pounds is 5600 grains, so you'd have to exchange 273 pounds of air per hour, or 4.5 pounds per minute.
The density of air at room temperature is about 0.075 pounds per cubic foot, so 4.5 pounds per minute is about 60 cubic feet per minute. So yeah, running the ventilation could account for the dryness.
If the indoor humidity was at a more comfortable 40% RH, the air would have 44 grains of water per pound, each pound exchanged would shed 36.5 grains. Venting 5600 grains of water per hour would require exchanging 153 pounds of air per hour, 2.5 pounds per minute or 34 CFM.
So reducing the ventilation is something to look into.
Gotta love traditional units.
Bravo for stepping through those calcs for this question :-) I didn't see an allowance for moisture transfer efficiency via the ERV core though?
Data is here for this ERV: https://hvi-1491.quickbase.com/db/bh6688vwb?a=dr&ifv=1&rid=17526&dfid=12
That's left as an exercise for the reader.
:-)
Wow DC_Contrarian, that's a lot! Thank you for doing that and giving a scientific backing to what I need to do. I'll pull on the thread a bit related to measuring and reducing the air flow given that between all of you, there's pretty good consensus that I am well over the desired air flow and could reduce RH by reducing. Thanks!
Thanks. I think the place to start is measuring RH, CO2 and VOC, and seeing how much ventilation you need to keep all three at acceptable levels. Then the question is whether you can maintain RH while also having necessary ventilation. If not, then I think you'll have to do supplemental humidification.
You are over ventilating the way it sounds. We had the dry air problem with airtight, superinsulated houses in the 1980's. Did not have timed controls available to us then and too much fresh (dry) air was brought in through the HRV. We then got VanEE (I think) controls that could be set for 20 minutes on and 40 minutes off on low speed. Remote timers in the exhaust areas and a humidity setpoint on the control would override and go to high speed for the HRV.
The first question is what is the air flow your ERV is set to. If the installers measured this you can start from there, outside of that the only way to check this is with a differential manometer, you can get pretty inexpensive mechanical unit such as Dwyer 2-5001 for this. The manual has a table and pressure port locations to connect the gauge to get the air flow across the ERV core.
If you are running well above ASHRE flow ventilation rates, it will definitely over dry the house. There are dials on the unit you can adjust to reduce the flow rate to bring it down if needed, no need to cycle the unit. Both fresh air and stale air needs to be adjusted to keep the unit in balance, you can use the same gauge for this.
An ERV does recover moisture but not 100% efficient, when it gets really cold (thus very dry outdoors) even low ventilation rate can dry the house too much. For example, during cold snaps here, I need to run a humidifier to keep indoor humidity up. If I remember correctly your place has exterior MW insulation, so higher indoor humidity should not be an issue even in zone 7, there should be no problems with running a humidifier set to a reasonable RH.
Is the Dwyer differential manometer a single-use item for this specific purpose or is there a tool that I can get that will do things I may need to do in the future as well? Never used one of these and just trying to anticipate future needs before buying a new tool.
I don't know what the flow is set to. I have a mechanical design that has exhaust CFMs but I don't see any supply CFMs since it is ducted into the return air supply. If I add up the exhaust CFMs on the design it totals 175 cfm.
The contractor did say they balanced it but I'd like to probably fix this myself since they've been a pain and I need to learn the ropes.
If I order the Dwyer is it going to come with what I need to hook it up to the Renewaire and figure this out? Thanks!
175 CFM running 24/7 for a family of four is likely about twice what you need. My system modulates on C02 levels in the house (the system targets 700-800 PPM co2) and is often ramped down to 50 CFM if a few family members are missing etc.
There are many ways to track/log but I'm using an Ecowitt WH45 which tracks C02, PM2.5, PM10, temp and RH. I'm using this one specifically as it integrates with our automation system, however it's not expensive and works very well. You can view live data as well as the historic graphs via their web app or at ecowitt.net
You'd only need 175 likely on "boost".
Hey DennisWood,
Thanks for the help on this. I've got the tools now but I still haven't completed this. I am coming back to it now.
Wondering if you could tell me about this Ecowitt unit and how it integrates with your automation system? Once I get this all tweaked, I am wondering if I should get something else to track and monitor and make sure that I always have the right amount of flow.
Dennis, what fan or ERV are you using that modulates remotely via software? On mine, and on others that I'm aware of, somebody actually has to go to the unit and manually spin a dial to increase or decrease the airflow.
Once you are done adjusting the ERV, you can connect the gauge to air filter on your heat pump. This lets you monitor the pressure drop across the filter and know when you actually need to replace it when it starts clogging up. Being able to stretch your filter will probably will pay for the cost of the gauge over time.
You'll need some latex tubing and reducer to hook the gauge to your ERV, should be able to find this in your local box store.
Akos, what amount of pressure increase is the trigger point at which you decide it's time to change the filter?
The rule of thumb is 2x initial resistance. Filters get more efficient as they load up, so you want to stretch the use as much as possible. The limiting factor is usually is usually your furnace blower, so if the filter initially was oversized, you can stretch it until it hits the blower pressure limit.
Check this video out : https://www.youtube.com/watch?v=A3UPImODwIY
The Dywer is an analog version of the digital one he's using in the vid. Same method though.
I agree with the others on the over ventilation. The air is extremely dry right now, and all of your vapor is being pumped out of the house. In a house that tight, with no ventilation you ought to be having high humidity problems, so I think you'll be able to figure out a solution that lies somewhere between just turning it off and running it 'full steam'.
Kyle- timely comment. Over ventilation, for same reasons , can cause substantial summer humidity increases in a tight house. Much of the experience and research and dialogue on ERVS is north climate centric ; it can be risky to extrapolate to southern climates or even mid Atlantic. In cold climates, obviously an ERV should be the start of your humidity Control system, but south of the mason and Dixon line, you should be buying a ventilating dehumidifiers before you even consider an ERV. And in the Midlantic, you will most likely have to purchase both from the outset…imo there is entirely too much emphasis on ERVS in journals like GBA and too little about ventilating dehumidifiers……in ANY climate zone, too much humidity is a much greater risk than too little.
Ventilating dehumidifiers can work but they are unbalanced. I think a standard in-line dehumidifier plus a separate ERV for fresh air exchange would be the most ideal setup in humid climates.
The ventilating dehumidifier would have a higher latent heat penalty than the ERV, because there is no humidity exchange occurring when bringing in the outside air. It would be cheaper to just have one piece of equipment but I think the optimal situation is both if you can afford them.
You could try running your design past Corbett Lunsford or Alex Meaney.
Corbett Lunsford
https://buildingperformanceworkshop.com/
Alex Meaney
https://www.meanhvac.com/about-us/
‘Balanced’ ventilation is over rated, even if achievable.it is very easy to create negative pressures in a house (and all the attendant bad consequences) while it’s very hard to create large positive balances, and thus in practice not a problem. See Corbett Lunsford take on this at https://youtu.be/7nhXP3v5Zy0. Indeed sl positive pressure has significant moisture entry , radon etc. benefits. And of you are not in a ray cold cookware, ervs are over rated , and should arguably be part of your fresh air/humidity solution after you have installed a ventilating dehumidifier. In the south, you are better off with. Ventilating dehumidifer and exhaust fans , timed or not. Ervs exacerbate any humidity problems you have in summer .