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Community and Q&A

Make up air strategy

Toyoland66 | Posted in Mechanicals on

This is for a home being constructed in zone 7/8 Colorado mountains. The home will be built with attention to air tightness and will be blower door tested, an ERV will be installed. Heating system will be a direct vented propane boiler with in floor radiant and an indirect water heater.

The house will have a 600 CFM range hood and two electric clothes dryers, these appliances will need make up air provided in order to operate correctly without depressurizing the house.

My idea is to install an electrically controlled make up air damper that supplies make up air into the mechanical room, from there I would run a duct that exhausts from the mechanical room into the laundry rooms or kitchen with a supply grill behind each dryer or the range. The reason for running the air through the mechanical room is to temper it.

-Is this a good strategy?
-Are there any code or safety issues with doing this?
-How is an electrically controlled make up damper actuated by 3 separate appliances?
-Would I be better off ducted directly to the supply grills without running through the mech room to temper?

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  1. STEPHEN SHEEHY | | #1

    I suggest you consider heat pump dryers. They are more efficient and don't need venting. That's two fewer holes in the building envelope.

    Propane is very expensive where I live (Maine) and I opted for minisplits. Maybe it's cheaper in CO.

    At 600 cfm, I doubt the make-up air will warm up any going through the mechanical room.

  2. SwitchgrassFarmer | | #2

    Chris, first, I think the standard disclaimer you will hear, is that you better ask your local code person these questions.

    I will whet your appetite though by telling you that I have a make up air damper in my mechanical room. It opens when the clothes dryers and range hood fan run, when the HRV goes into defrost, and even when our masonry heater (MH) is looking for combustion air.

    MH pulls air directly out of the mechanical room through a CAPE backflow damper and redundant input dampers. Probably didn't need the backflow damper (or the second input damper either), as we have incredibly strong stack effect on the MH; at times you would almost think we have a combustion air blower. There is also a clothes dryer in the mechanical room. 1st floor clothes dryer and our range hood just pull air through the house, or through an open window.

    Our make up air damper is a simple flue balancing damper turned backwards. Works for me because of the tightness and geometry of the house, mechanical room/wall penetration location, and how the prevailing winds work here. YMMV.

    Someone will chime in with this exact data point, but I recall at a certain range hood CFM it's mandatory to have a make up air source. I know that some range hood makers offer systems where that fresh air supply vent damper talks to the range hood. What you are proposing is one better, not only will your range hood open the fresh air source but so will your clothes dryers. I think it would be possible to build a logic system to do that using current transformers like these: The electric feed for each dryer, fan, etc would be routed through its own current transformer (CT); once one or more CTs sensed fan operation they would drive a circuit to open your input damper. What kind of engineering drawing or cert your code person might want for such a custom system, if any, is TBD.

    Now to another code question; I have a gut feeling that since you have a combustion appliance right in your mechanical room, even one that is directly vented, that may raise eyebrows with the mechanical room as fresh air plenum approach. I don't have that situation, so can't give you any feedback there. Again this is something to talk to your code person about.

  3. charlie_sullivan | | #3

    By the time you are done with all the controls and ductwork, the Whirlpool Hybridcare heat-pump dryer is probably about the same cost and it will use a lot less energy, and provide you some useful heat in the house in the winter as well.

  4. Toyoland66 | | #4

    The heat pump dryers sound good on paper, the reviews are somewhat mixed but It seems that's the norm for pretty much any appliance. Thanks for the suggestion. Both laundry rooms have a door, and will have exhaust to the erv which should address most of the negative comments. The thing I am most concerned with is that they dry effectively and timely. My employer also has a buying program with whirlpool so I can get a pretty good price.

    Concerning the range hood, I will still need to provide make up air for that, there are several kits on the market that have their own control switch that seem pretty straight forward. Would it be best to supply the make up air to a supply grill behind the range, or supply it to a remote room? I am thinking that supplying remotely would be best to prevent short circuiting. I didn't think about it earlier but we have a storage room that is much larger than the mech room that could be supplied to allowing the air to temper before it enters occupied space.

  5. GBA Editor
    Martin Holladay | | #5

    Concerning your clothes dryers: I urge you to either (a) specify heat-pump clothes dryers, or (b) consider installing ordinary clothes dryers without worrying about makeup air. Approach (b) works for almost all houses, including fairly tight houses, because all homes leak.

    Concerning your range hood fan: I urge you to either (a) choose a range hood fan rated at between 200 and 300 cfm, or (b) include a standard solution for providing range hood makeup air, such as the motorized damper kits sold by Broan. For more information on this topic, see Makeup Air for Range Hoods.

  6. SwitchgrassFarmer | | #6

    Darn it Martin et al, here I had already started the ladder diagram for the make up air controls for his house. Kidding of course, but I think Chris still has a valid question out there about tempering.

    We might want to learn more about Chris's household. Does he have ten rugby playing kids and a catering business that he runs out of the house? If so, maybe his original query makes sense, the clothes dryers and range hood would be running constantly. Based on his initial response I am still not sure on the level he intends on using these appliances, because he remains interested in tempering the make up air for the range. At a minimum he may be thinking that the range hood will be running quite a bit.

  7. charlie_sullivan | | #7

    My opinion about range hood makeup air is to put it near the range. Kitchens can get hot during intensive cooking, and so the influx of cool air might be better there than elsewhere. And if the cold draft is a little annoying, that might be a useful reminder not to vent more than needed.

  8. Chaubenee | | #8

    If you have a central heating system that is high efficiency and has the ecm motors in the fan, perhaps you can tie your motorized damper into that in the return line. You would have your mechanic hook up a switch that sensed the pressure change in the house. Then this would work for range, dryer and perhaps central vac. Make sure you put a large enough duct, as they have 6,8 and 10" versions.

  9. Toyoland66 | | #9

    I do have 4 kids.

    This is a second home for now, it will be a part time rental when we are not there. It has 6 bedrooms, two of which are bunk rooms, it will sleep 20 people. The reason for two laundry rooms is the amount of bedding and towels that will have to be washed between occupants.

    Maybe this is a silly question, but, we do have an ERV. Will make up air not enter through the supply duct if there is a pressure differential between interior and exterior?

  10. GBA Editor
    Martin Holladay | | #10

    Neither an ERV nor an HRV can be depended upon to provide makeup air for a range hood fan.

    For one thing, these are balanced ventilation appliances (cfm in = cfm out).

    For another, all ERV and HRV manufacturers specify that these appliances cannot be used to provide makeup air.

    Finally, a range hood needs very large volumes of air -- in this case, 600 cfm -- requiring a large duct. ERVs and HRVs operate in the 60 cfm to 120 cfm range.

  11. SwitchgrassFarmer | | #11

    Mmm, I think this puts a slightly different light on things Chris. Sleeping twenty people with laundry for same sounds like some of the homes here near Penn State that are rented out for football weekends.

    I thought the idea of going with heat-pump dryers was great, but I am not so sure now. My understanding is that some of those dryers require discharge line filter cleaning etc, and that they have somewhat longer run times. That is no problem for one family, I would definitely buy one if I had it to do over again. For a situation where you have a boatload of laundry, that you have to turn around quickly, I wonder.

    I have an HRV, it's pretty dumb, it's "set it and forget it". Wish it was smarter, but as Martin has pointed out a few times, there is a steep jump in price for one that can anticipate demand, adjust based on CO2 or moisture levels, etc. Can't justify it for my home.

  12. STEPHEN SHEEHY | | #12

    Re: Heat pump dryers-Our whirlpool dryer give the option of using regular electric drying w/o the heat pump setting if you need a quicker cycle. The longest cycle is about 1 hour 20 minutes at the most economical setting. Cleaning the lint filters (2) takes a few seconds. The moisture gets dumped into the same drain-pipe as the washer uses.

  13. Toyoland66 | | #13

    So, it sounds like I should provide make up air for the range hood, using a common motorized damper kit. Install conventional vented dryers but don't worry about make up air for them.

    I ran some numbers as a sanity check, the house is roughly 49,000 cubic feet. Even at 0.6 ACH50 the allowable leakage is about 490 CFM which is greater than the CFM of two clothes dryers. 50 pascals is 0.00725188690035 PSI, so not an uncomfortable I thinking correctly?

    Thanks all for your input.

  14. Chaubenee | | #14

    Chris, read the brochure here by Broan. Although you may get a Honeywell or Aprilaire model, this explains the use of a make up air damper that operates on a PRESSURE SWITCH.

  15. user-5888278 | | #15

    Yes Chris, the total cubic ft and allowable leak is definitely on track with the real world. One issue to consider is lingering cooking odors in such a nice home as suggested. Sounds like a commercial cooking appliance and hood. No problem! The automatic pressure switch could be reliable for a while sensing the high pressure of the exhaust hood duct, but grease and steam would foul the sensitive control unless the duct is tapped near the outlet more than 10 ft from the hood (you probably have speed controls on vent hood so the lowest pressure should actuate the switch). Now 24v may pass through the normally open contacts ran down to the air handler.

    I will jump ahead to tell you the result. When fresh air is introduced to the return side of the furnace blower (ahead of the filter) the negative pressure draws in the fresh air when the 24v damper is open and the fan is running. Fresh air will be drawn into the whole of the ducted system replacing air at a level equal to the air loss by the kitchen Hood if sized for full makeup. Draw back is air temperatures. In extreme cold outside events the thermostat will not sense the call for heat fast enough to warm the air from the air ducts which will be exaggerated by high velocity air duct design. In your large house only the occupied seats near the supply air vents might pick up a sweater..? Lol

    OK!! Back to the wiring. A transformer relay kit called "Fan center" placed into the furnace will deliver your 24v. Ideally, the vent hood switch will power the relay and damper. The relay should be 2pole 2throw design using 1 N.O. set to parallel the furnace low voltage power and fan thermostat [ (R) power to (G) Fan] to start the blower....the second set of contacts could be used to warm the air as an option..... this is a standard " WHOLE HOUSE FRESH AIR" hook up for a furnace guy..... I did this hookup with a remote Fantech centrifugal "XL" duct booster in place of the WOLF SYSTEM FAN MOTOR, for quieter operation, plumbed in the attic, in a 9000 Sq ft Residence in Washington State.. as a reference.

  16. Chaubenee | | #16

    Bernard, the pressure switch I am suggesting would not get fouled by grease bc it is somewhere other than the kitchen and is part of the return ducting, leading to outside somewhere away from the kitchen area and into the furnace/AC air handler. I am sure you are talking about a switch which is on the outside of the range hood fan. I certainly agree that if you are using 700 CFM or something the amount of air that comes in in January will not be heated up fast enough to come out of the resisters warm! But conversely a 200 CFM draw might not be a heavy lift for the system. Unfortunately when we are talking make up air, there is very little we can do to temper it is we need to quickly make up 700-1000 CFM. I HAVE A FRIEND WHO RAN HIS MAKE UP AIR UNDER THE SLAB to try to bring the air closer to fifty degrees F by time it hits the air in the house. I really don't know how successful it is in zero degree weather after a few minutes but he actually only has a 100 CFM fan. So his use of 6" PVC was doable, but I wouldn't advise it. Also the system I described would also work with bath fans and dryers. But if the house is leaky enough to pull air in without it in small bites here and there that no adverse negative pressure is experiences then maybe that will do the trick. My 1957 ranch needs no make up air for the mere 100 CFM bath fan, that is for sure. (Even after upgrades)

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