HRV supply to bedroom closet floors
I recently changed my plans from ventilating with an AirCycler on my new build to installing an HRV. After some research, I was concerned that my wife would find the unheated air flow of the furnace fan uncomfortable. I turned the fan on without telling her one day and within minutes, she was asking why cold (or cold feeling) air was coming out of the furnace vents.
Unfortunately, I’m late in the game. My house is nearly finished, so I am trying to put together a simple dedicated duct plan that will go unnoticed by my wife’s temperature sensitivity. I’m comfortable with my exhaust duct plan, but has anyone tried putting supply grilles in the bedroom closet floor? I know the wall is better, but I don’t know if that would matter in a closet, and the wall would be more difficult at this point in the build. Doors will be solid, with an approximately 1″ x 48″ long gap at the bottom. House is located in zone 5.
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For starters, closets don't usually have a register to supply heated air, so if your dumping unheated air (heat recovery or no, still below room temperature) into the closet, you will be cooling it. A good quality HRV in my climate will deliver 12°C air to a room. Your closet would eventually come down to somewhere close to this temperature. Then you would have a cold draft coming out under your closet door, not to mention cold clothes! Also creates a great spot for condensation at the back of the closet to occur, voila! moldy clothes/boxes.
I ran recently ran ventilation ducts in my house, which has been around since 1970. It's harder for sure, and getting well sealed ducts is a fun time, but it's definitely possible, even in finished walls. Hopefully you still have access in the basement.
Another option would be to build a dedicated chase, outside of your finished walls. But that hardly seems better than opening up a wall.
While dedicated ducts are the A++ best system, interconnecting it with your forced air system for a simplified system might be a good idea in this situation. Set the furnace fan to run on low speed whenever the HRV is on, typically 400 cfm, mixing the ventilation air with house air. The velocity out of the registers should be barely detectable since the design airflow is "probably" two or three times that. And the air will be room temperature.
Help me. Why are dedicated ducts better than introducing vent air into the forced air system?
Furnace fans, even ECM blowers, still consume a LOT more energy while running than an HRV blower, so running them both increases cost. Noise increase is also a concern for some. The largest difference is you can't determine where the fresh air is going. Some rooms are going to get a lot of fresh air, some are going to get very little. The air handler causes pressure imbalances on the HRV, making true balanced ventilation very difficult to achieve under operating conditions. Some people are opposed to room temperature air coming from their heating ducts.
So dedicated ventilation is an A++ system. Interconnecting an HRV with an air handler is still an A system and usually a perfectly acceptable solution.
If you're going to run the furnace fan whenever the HRV is on, would it make sense to just run the furnace whenever the HRV is on?
The fraction of time that the furnace needs to run to satisfy the heating requirement may be less than the fraction of time that the HRV needs to run to satisfy the ventilation requirement. Running the furnace whenever the HRV is on would result in a too warm house. However, control logic that coordinates HRV runs with heating or cooling calls can significantly reduce the energy penalty for running the furnace blower for ventilation. AirCycler has logic to do that with a CFIS system. I don't know whether any HRVs or ERVs have logic to do that.
The furnace doesn't run with an interlocked HRV, just the blower.
Would it make sense to put ERV/HRV exhaust ports in closets?
Keep in mind the HRV air volume will be a small fraction of your furnace fan volume. It will be cool though.
That's a good point on the closet vents. So that plan is out. I would like to avoid the furnace connection as well since that's what my wife found uncomfortable even on low speed at room temperature. Is there a reasonably priced duct heater that could bring the supply air to room temperature. That would make a floor mounted dedicated vent a good solution. The only option I've been able to find is in the range of several hundred dollars and is quite over sized. I calculate that I would only need about 100 to 150 watts max to bring the air up a few degrees and make it unnoticeable, and the heater could be shut off at night when no one is awake to notice.
My gut feeling is that using resistive heating would eliminate any cost savings from using an HRV instead of just bringing in outside air.
There are lots of different inline electric duct heaters available, your best bet would be to call up an HVAC contractor or your local supply house and ask for one. (You are unlikely to find one in the 150 watt range, thermolec makes a 6" one for this purpose, the smallest size is 1 kW. It does modulate though.)
For a retrofit, you best bet is to go with ducting for a high velocity system such as this:
https://www.unicosystem.com/Shop/duct-system/aluminum-core/aluminum-supply-tubing-2.5x25--upc-225-1/
You can fish this through existing walls with minimal effort. A 2.5" duct should be good enough for a master bedroom feed and you can use 2" ones elsewhere.
You can also take the insulation off them to make the install easier, since there is no heat/cooling in there, the insulation is not doing anything.
Akos suggestion is a good one. If the outlets are positioned with some thought and high on the wall you won't be able to notice the air is cooler.
There is also 3" x 8" oval duct that can fit into a lot of places.