Shared Registers for Heating/Cooling and Ventilation Systems
Situation: Following what seems to be the consensus for the “best” way to duct balanced ventilation systems, I’m installing an ERV with independent supply and extract ducts (extract from bathrooms, laundry, kitchen, basement, and a large closet; supply to bedrooms and living room). On the 2nd floor, I’m also installing a slim ducted heat pump, with a central return grille in the hallway and supply to bedrooms / bathroom / study.
The bedroom ceilings, however, are getting to be a bit crowded — I’ve got lighting, smoke detectors, sprinklers (still planning), and both heat pump and HRV supply diffusers — which are going to basically end up next to each other.
Question: Has anyone seen a way to run independent supply ducts to a single register / diffuser? I was thinking that it wouldn’t hurt to mix incoming fresh air with conditioned air anyway, for comfort since ERVs aren’t 100% efficient, and having one register looks less silly than two registers right next to each other. It doesn’t sound right to tie the ducts together before the register, feels like that would cause balance / pressure issues as the two systems both modulate airflow. Are there register boots with an internal baffle and two duct connections to make it look like a single register but function like two next to each other? Am I overthinking this / is there another way to implement?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Yes, duct the fresh air supply to the return of the air handler. Make sure to use an autobalance ERV.
It's a good thought, and would definitely simplify ducting, but I've got a few concerns that maybe you have an idea on:
1: Presumably if the ERV is ducted to the heat pump return, then the heat pump fan needs to be always on. The fan in the heat pump (Fujitsu ADUH12LUAS1) modulates between approx. 250~400CFM. It doesn't clearly specify how much power it consumes, other than one line in the specs that specifies 81W as the "fan motor output." This is about equal to the power of the ERV (Brink Flair 325) at max speed, effectively halving the power efficiency of the ERV during times that the heat pump could otherwise be off. I suppose that only amounts to a few dollars per year, <$20?
2: The design cooling load for the area served by this heat pump is ~5400BTU/h. The heat pump modulates down to 3100BTU/h. We only rarely hit design conditions, and have kind of long, humid shoulder seasons (N Ohio / zone 5). I can imagine that the heat pump would need to cycle on and off during a fair bit of the cooling season, and leaving the fan constantly on seems like during otherwise "off" cycles it might re-evaporate moisture condensed on the cooling coils, reducing the already limited dehumidification capacity of the unit.
3: About 55% of the heat pump output is sent to the two bedrooms, the rest goes to the study, bathroom, and circulation spaces. This means half the fresh air from the HRV is being sent places other than the bedrooms, so it'd be less effective than bedroom-only supply at providing fresh air to sleeping people at night
On the plus side, ducting is simplified and the fresh air gets well-mixed and conditioned before being introduced to any rooms, so there's that.
Again, maybe I'm overthinking these potential negatives -- just the things that popped into my mind. Thoughts?
1: " Presumably if the ERV is ducted to the heat pump return, then the heat pump fan needs to be always on."
The fan on most (all?) modulating heat pumps is on 24/7, so no matter what you are paying for that power use. That 87W is at max flow at rated pressure, at min flow these typically use 20W or so.
2." heat pump would need to cycle on and off during a fair bit of the cooling season, "
The heat pump will modulate down to its min capacity and then it will start to cycle but the fan will keep running (this is not the best for humidity control). If your unit can be configured to fan off that solves this issue.
In this case you have two options, you can have it go fan off and the fresh air will be delivered to the hallway through the return or interlock it. I would vote for the non-interlocked version, the fresh air will eventually be delivered to the rooms when the unit turns on again.
3. The extra fresh air that is delivered to those other rooms will eventually make it to the return of the unit which will send it to the bedrooms. Overall, CO2 levels in the bedrooms will be a bit higher this way, if that matters or not is up for debate. I would rather have some extra fresh air in the study anyways when working from home, CO2 levels there would be more important than in a bedroom at night when sleeping.
Akos, thanks for the thorough and thoughtful reply! I think I'm about convinced to go with your suggested approach -- it'll certainly save me some time on sheet metal work, which is one of the less enjoyable construction tasks.
If using an air handler that by default is always running, then it does make a lot more sense to share ducts :-) If you think about it, an HRV/ERV would have to be running at considerably higher wattage to overcome static pressure in a standalone system. By connecting to your air handler supply (which is running any way) then the fresh air intake fan on the ERV will be using much less power if ducted correctly. The exhaust fan will also run at a much lower speed if you decide to interconnect to your air handler return. The cost savings with respect to ducting and labour would be considerable, and power use differential may in fact be a wash. In fact, with your air handler calling for heat/cool, auto balance EC fans on the ERV may in fact idle back considerably as the air handler fan ramps up.