ERV Installation
jehjr68
| Posted in Mechanicals on
Folks … just built a new fully enveloped home. Being concerned about air quality, I had an ERV system installed. We have dedicated stale air exhaust but the fresh air intake is plumbed into the Air handler return. When the ERV runs, the fresh air seems to be coming out of the return filter and not the existing supply side ducting. I would prefer not to have to run the air handler fan all the time. Anyone else experience this issue ?
Thanks
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Replies
Johnny,
Once you introduce fresh air from your ERV into your forced-air duct system, it can go any way it wants (as long as the furnace blower isn't running). That's OK -- as long as your duct system is tight and entirely within the conditioned space of your home. After all, you house is getting fresh air. It doesn't matter too much where it ends up.
You really don't want your furnace blower to run every time your ventilation fan runs. Furnace blowers use a lot of electricity and move too many cfm for ventilation.
Of course, if you prefer a ventilation system with dedicated ductwork -- one that delivers your fresh air exactly where you want it -- you can always install ventilation ductwork.
Martin,
I've been reading your articles like the one in Fine Homebuilding on ducting and replies on ventilation systems and was wondering if you thought it would be beneficial with a simplified system to install a damper in the supply duct between the intake and supply to be closed when the air handler was off and open when the air handler is on so the air handler fan wasn't required to be on constantly.
I'm in the home building process right now and the builder has budgeted for continuous fans in the baths to meet code rather than a dedicated system and fear it would be too costly to add dedicated ducting at this point but also worry about efficiency of running a 2 stage system (non-ECM) continously in addition to the ventilator.
Bosman,
First, I would strongly advise you not to install a so-called "simplified" duct system. For more information, see "Ducting HRVs and ERVs."
You asked whether "it would be beneficial with a simplified system to install a damper in the supply duct between the intake and supply to be closed when the air handler was off and open when the air handler is on so the air handler fan wasn't required to be on constantly."
I'm not exactly sure where you want to install this damper (presumably, a motorized damper -- but you didn't specify that). What do you mean by "between the intake and the supply"?
The intake of what?
And the supply of what?
Yes, a motorized damper installed in between the intake and supply of the ventilator on the return air duct for the air handler.
The idea would be to close the damper so the supply air from the ventilator would be mechanically separated from the intake for exhaust.
I am under the impression that preventing contamination of the intake/supply for the ventilator was the reason for having the air handler fan running while the ventilator was on.
I realize that a simplified system is not ideal, but costs are a barrier in my case and I wish the builder had not used bath exhaust fans to meet code. The damper was just my idea to make it less of a less-than-ideal option.
Bos,
Do you intend to operate the ERV for 24 hours per day -- with the ERV fans (there are two) operating for 24 hours per day, plus the air handler fan operating whenever the air handler is operating?
Or do you have some other control mechanism in mind?
Yes I was planning on having it run continously. I live in Michigan and was planning on running at a slight negative pressure in the winter and slight positive pressure in the summer to attempt to control vapor drive through the wall.
Bosman,
I suppose you can install a motorized damper in that location if you want, assuming that you are working with an HVAC contractor who is sophisticated enough to wire the necessary controls. Note that you have introduced a new motorized component to your already-complicated system, raising the chance of future maintenance and diagnostic headaches. Note as well that the ventilation rate is likely to differ greatly during the two operational modes (the mode with two operating fans and the mode with three operating fans), making commissioning a matter of compromise.
Finally, you should know that vapor drive has nothing to do with pressurization or depressurization. Pressurization and depressurization affect air leakage rates through the envelope. Vapor diffusion is an entirely different mechanism.
I probably stated that incorrectly.
My thought is houses still leak air through the building envelope no matter how tight. The goal is to drive lower humidity air though the inevitable leaks in the wall assembly rather than pull higher humidity air into the wall cavity though those leaks where it could condense and cause vapor drive within the wall cavity. That is what I meant by attempt to control vapor drive.
I am worried that the proposed continuous bath exhaust fans will create a negative pressure environment in the summer pulling humid air into the wall cavities and inviting it to condense and create potential moisture problems.
I am probably way off base.
Bosman,
Your worries are baseless, as I explained in one of my articles ("Designing a Good Ventilation System"). In that article, I wrote, "Some builders worry that a supply-only ventilation system (for example, central-fan-integrated supply ventilation) won’t work in a cold climate, because the ventilation fan will drive interior air into building cavities where moisture can condense. This worry is needless. As energy expert Bruce Harley explains, 'The upper portions (walls and ceilings) of every home — typically most of the second floor in two-story homes — already operate under positive air pressure in cold weather, due to the stack effect. The relatively small airflow of most supply-only ventilation systems (75 cfm to 150 cfm) will have little effect on this situation other than to shift the neutral pressure plane down slightly, in all but the very tightest of homes.'"
Thank you very much for your responses! It has been very helpful and definitely has me considering other options.
Thanks for the scoop Martin. Our system has the return duct (filter plenum) pretty close to the inlet on the air handler. The installation manual recommends to tap in to the return at least 10 feet from the air handler. Our geometry won't allow that so it went where it would fit. They are coming back next week to "balance" the unit since they left the flow meters at the shop yesterday during the install. It will be interesting to see if the system will balance properly with and without the air handler running.
You don't need to run the air handler in order to distribute air from the ERV--the ERV will push the air into the ductwork and it will find its way into the house, as well as out of the house if there is any duct leakage to outdoors.
The issue I would be concerned about is pressures in the ERV when the air handler is running. There is a significant negative pressure in the air handler ductwork on the return side of the fan, so it may suck extra air through the ERV, reducing the efficiency of the ERV quite a bit. Is there no way you can simply pipe the supply air from the ERV directly into the house, preferably into the bedrooms?
I'll report back after we try and balance the system next week ...
Adding extra duct work is possible just allot of trouble. If the system can't be balanced we will add dedicated supply ducts.
Just curious, how do they propose to balance? Does your unit have ports to measure pressure drop using a manometer, or are they actually proposing to measure airflow somehow?
If they can measure actual airflow with the air handler on versus off, that would be interesting. Please keep us posted.
They have flow collars that attach to the ducts. The ERV has adjustable dampers.
I just installed the Panasonic ERV in my attic bath. I keep the door open when bathroom is not in use so that fresh air can be introduced throughout the attic. When I run the vent hood or dryer on the main floor it will create negative pressure bringing in air from the basement via stack effect. Will the ERV balance out the pressure by bringing in more fresh air? I just close cell sprayed my rafters and dense packed my main floor with cellulose so the house is a lot tighter.
Well .. not sure. I'm no expert. Maybe some of the experts on here know. Wish I could help.
Michael, hard to say exactly what will happen in your house without doing some simple testing, but I would expect some of the makeup air for the range hood and dryer to come in through the attic ERV, and I would expect some to come through other openings in the shell. Do you have any combustion appliances in the house, fireplaces, woodstoves, etc? If your house is "a lot tighter" then I think you probably need to evaluate the ventilation needs and combustion safety. It doesn't seem likely to me that a spot ERV in the attic can serve the whole house.
Thanks for you feedback David, I plan on adding another spot ERV in the basement and also insulate to air seal the whole house. I was just curious if the makeup air would pull through the fresh air intakes of the spot ERV at a higher clip then the exhaust when my vent hood and/or dryer are running. Air sealing the whole house would limit the amount of air coming in from the shell.
I have a fireplace insert. The other concern is the oil furnace in the basement. Last thing I want is the bad gases to be sucked up into the living space so eventually plan on separating out the mechanical room from the living space in the basement and giving it it's own access to air with a dedicated vent.
Am trying to balance energy efficiency with an equal or greater focus on a healthy home for the family.
Thanks.