HRV and MUA for new build in zone 6A

5 years ago

We are in the process of designing an HRV/MUA system for a new build in zone 6a (Update NY) and are confused about HRV capabilities. Can an appropriately sized HRV also provide MUA for a kitchen range hood?

This is the HRV system we have been recommend:

Fan-Tech SHR series HRV. This unit has auto fan defrost control and 6” side ports (5,000 square feet of coverage @ 200 CFM).
6” runs to the supply and return lines throughout
One supply line in the toe kick of a cabinet next to the range (not in front of the range) to provide MUA.
Digital humidistat

Details on our house, heating system, range hood, etc....

House size: 2,100 sqft with relatively open floor plan. 3 bedroom and 2.5 bathrooms.
Heating: Mini splits with baseboard electric back up. No wood stove or fireplace.
Insulation/Envelope: Pretty tight with spray foam and Zip System R-Sheathing.
Range hood: 1,000 CFM (two 500 CFM motors)
Range: 36" Bertazonni all gas range. No other gas appliances.
ACH50 - Unknown at this time but blower door tests are required in our area.
Other ventilation: Four bathroom fans at 110 CFM each.

Will this system adequately address MUA, pressure and ventilation requirements?

Featured Answer

PRO

Charles Ross Homes

5 years ago

If your mechanical contractor spec'd the HRV as you've noted it above you do need to have a conversation about the system-- not with them--but rather, with someone who knows what a HRV is, how it works, and why it doesn't take the place of a range hood make-up air (MUA) system.

The function of a HRV is to maintain indoor air quality. If your home is "tightly constructed" as indicated based on the results of a blower door test or inferred from the use of spray foam insulation and/or ZIP-R sheathing, you'll need one. A HRV exhausts air from your home at a particular flowrate and brings in fresh, outside air at the same flowrate (after all, the solution to pollution is dilution.) It also transfers energy from the higher temperature air stream to the lower temperature air stream. That function is the "HR" in HRV. It stands for heat recovery. Warm air being exhausted in winter pre-heats the incoming outdoor air.

The need for a make up air (MUA) system is to replace the quantity of air that is exhausted by your range hood. Accordingly, the make up air (MUA) system operates only when the range hood is operating. The International Residential Code (IRC) requires a make up air system if the capacity of a range hood exceeds 400 cfm. Your range hood is rated at 1000 cfm so you'd need a system to make up at least 600 cfm. For a tight home, I'd recommend you install a system capable of matching the exhaust rate of your hood.

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Comments (7)

User
5 years ago
This is a joint discussion with your HVAC person and GC.
dan1888
5 years ago
No. 200 CFM has nothing to do with MUA for an exhaust of 1000 CFM. It is meant for ventilation. Fantech has a MUA product.
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opaone
5 years ago
last modified: 5 years ago
HRV and MUA serve two different purposes and generally must be two separate systems (it is possible to make a combined unit but it's not economical as you're basically making a huge HRV and incorporating bi-directional control).

I agree w/ @Charles Ross Homes, if your HVAC contractor spec'd that then you need to question everything that they do. That is irresponsible and will endanger your families health.

In a house of that size with that many bedrooms the HRV's should feed the bedrooms directly if you are going to maintain healthy air at night (the biggest IAQ problem is high CO2 at night in closed bedrooms). Daytime is generally not much of a problem as people are scattered about the house and exterior doors are opened and closed routinely.

MUA s/b 1,000 CFM (or whatever your range hood actually is) and control locked to your range hood (ideally variable speed to match).

Ideally the MUA should feed air directly to your kitchen (opposite side from your hood, NOT underneath the range as with consumer products that risks the air short-cutting behind the range to the hood and significantly reducing hood effectiveness). It can feed in to the return duct though if necessary for conditioning.

More: https://www.gardenweb.com/discussions/5161173/hood-faq
jackson28 thanked opaone
jackson28
Original Author
5 years ago
@Charles Ross Homes and @opaone, thank you very much for your detailed replies. It is frustrating that we can't trust our HVAC contractor but here we are...

Based on your comments, numerous conversations with our HVAC contractor and extensive research, I think we have devised a decent HRV and MUA plan. I'll be eliciting input from another HVAC contractor, code officer and engineer before I pull the trigger and purchase this equipment.

The current plan is :
1. Fantech SHR 200 HRV with supply runs to each bedroom and return lines from the kitchen and laundry room to the outside. I believe this solves the bedroom air health issue @opaone mentioned.
2. Broan 8" Make-Up Air Damper with Pressure Sensor Kit. We can have the MUA feed fresh air from across the kitchen through several toe kicks or maybe a floor register.
opaone
5 years ago
That Broan is a passive MUA and appropriate only for about 200 CFM. You need an active MUA similar to http://www.electromn.com/gen/makeup_air_ii.htm (EM-MF series).
PRO
Charles Ross Homes
5 years ago
The MUA system you have in mind will pull unconditioned outside air directly into your kitchen whenever it's in use. I grew up in upstate NY. The winters can be long and brutal. I would not want that type of system operating in my home in the winter.

Fantech makes some active MUA systems that speed match the supply blower to your exhaust (you might say they customize the flow of make up air so you only pay for what you need.) Plan on adding a make up air heater if you want to introduce the make up air in the cabinet toe kicks or stock up on wool socks.