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Community and Q&A

Combustion venting and HPWH cold

Brady W | Posted in Energy Efficiency and Durability on

I have a 1960’s Denver home and I had it air sealed and insulated in the attic. I have a (finished) basement mechanical room with an 80% gas furnace with the 2 high-low outside vent openings into that room. Given we also have a 6.3 kW PV solar system, I also just removed the gas water heater that was right next to the furnace and installed a 50 gal Bradford AeroTherm HPWH (given the rebate available, it was kind of a toy over buying a standard electric but working OK so far in heat pump only mode). It does get a little colder in the basement though (not surprisingly).

I was exploring options for limiting that cold effect (which would happen irrespective of the HPWH but of course it aggravates the problem). One contractor suggested I might not need the combustion vents. Some back of the napkin math though w/ this 75k BTU/h furnace tells me that the 730 cubic ft room it’s in is not big enough (need 3700-ish cubic feet) to supply combustion air without either the existing vents or a louvered door to the rest of the basement. Any good reason the contractor would say that? Even if it would be safe to do that, the code to me clearly says ‘no’ on this and it seems like a red flag for future inspections/home sales/etc.


It seems like to me the other/better ways to mitigate the HPWH coldness a bit are:

1.     Get a sealed combustion furnace and then no need for the existing high-low (furnace was brand new before moving in May, so don’t want to toss it yet).

2.     When the furnace gets enough ROI under its belt, replace it (and the exterior A/C condenser) with a heat pump and close off the high-low vents that way, maybe using a hydronic system for the heat pump backup, although the GPM usage I’ve seen on hydronic handlers might be tough for a HPWH to keep up with w/o falling into electric heater mode.

3.     Insulate the mechanical room. Although the rest of the basement might be better off, this to me seems not ideal because the mechanical room could get even colder than it is now.

4.     In the summer, I could do something with the cold air by putting an actuated register on the return duct (which is right next to the HPWH) and use some zwave/nest home automation to detect when the HPWH is running, then open that register, turn on the furnace fan/blower, and get ‘free’ air conditioning. I’ve read Martin’s take on how stuff that requires lots of hoops for the home owner to jump through often doesn’t work out. Hopefully automating it would.


To me it seems like #3 is a no go and the other options come down to money, timing, and electrical consumption. Any ideas? Thanks.

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  1. Jon R | | #1

    How about just providing lots of air flow between the utility room and the rest of the basement?

  2. Expert Member
    Zephyr7 | | #2

    #1 is a great option, and eliminates the need for combustion air to enter your house since the furnace will have its own source of air piped in from outside. This is really the best and safest option for a very tight house though. Of course, the trade off is as you mention in #2, your existing furnace isn’t worn out yet. I have no idea if anyone would buy your old furnace as a used unit, but you might ask your contractor.

    Related to #2, consider an evaporative (swamp) cooler. They work awesome in Denver due to the crazy low humidity there, and Xcel energy has (or at least did recently) offer an incentive program for the installation of these units. I have family in Parker so I’m out there several times a year.

    #3 is a problem. Heat pumps work by moving heat from one area to another. In the case of your HPWH, the water heater essentially sucks heat out of the area around it and uses that heat to heat up the water. If you insulate and seal the room the HPWH is in, that room will get cold, and then the heat pump doesn’t have a good heat source to use. I’d try to ventilate the room more so that the HPWH has the whole basement as a heat source.

    #4 is my favorite way to do these things. I love maximizing efficiency of systems. You are correct, your HPWH can help to cool your house, a little. You need some venting, perhaps a way for some air to circulate in your mechanical room with the blower on the furnace. You’d need to be careful this system was interlocked such that it would not operate while the furnace burner was in operation since you don’t want any possibility of backdrafting with your non-sealed combustion furnace. Check out BACnet too — it’s usually used for commercial building automation, but it has a lot of HVAC applications and might give you more options than zwave.


  3. Expert Member
    Dana Dorsett | | #3

    It's a too bad the furnace is non-condensing and oversized for the heat load, and too bad the HPWH isn't the type set up to be ducted.

    The minimum room size for the HPWH is 700 cubic feet (or 240 square inches of free air communication to a larger room), but the presumption is that it's inside of conditioned space. The make up air requirements with the high/low vents for the furnace make the mechanical room a somewhat less- conditioned space. If you subtract out the volume of the furnace & ducts you're probably less than the 700 cubic feet minimum(?). But couple of decent sized grilles to the conditioned basement adding up to 4-5 square feet of free-air might allow the furnace to draw it's combustion air from indoors, AND run the heat pump water heater, allowing you to seal off the exterior vents.

    Some furnaces can be reasonably set up with ducted combustion air directly from the outdoors, which may or may not be the case here.

    Assuming it hasn't been already, insulating the exterior walls of the mechanical room (and the rest of the basement, if you can) to the current IRC code minimum R15 continuous insulation would help some, and would have a somewhat less chilling affect on the house should you ventilate the mechanical room with conditioned space air to keep the heat pump water heater running in heat pump mode. Do NOT insulate between the partition walls between the mechanical room & finished basement, which would increase the risk of freezing the cold water feed to the water heater.

  4. Brady W | | #4

    Thanks for the replies. @dana - I could buy the ducting kit for the HPWH. With regard to option 4 above though, I think that would make it more complex because I'd have to make sure the HPWH exhaust flow wasn't putting too much air into the furnace return and then have a damper/tee to exhaust it elsewhere in the winter when I'm not cooling. It would seem easier/simpler to louver the door/put in interior vents as you describe, seal up the high/low. Then when I get around to it, do #4 without any 'forced' ducting. Then the furnace isn't having potentially too much air going into it (although it would have a bit of a breeze since the HPWH exhaust is around 8 inches from where I'd put the motorized register).

    Is there anything wrong from strictly an HVAC standpoint with having a register in a return duct that you open/close?

    1. Brady W | | #5

      Another thing I'm not quite smart enough to understand is that by capping high/low vents and using a louver door, do I really improve the situation or just move the problem around? Instead of large volumes of outside (cool) makeup air coming into that 1 room, instead I'm drawing that cold air through 10/20/whatever smaller spots around the house via windows, etc., right?

      Perhaps distributing the problem makes it less noticeable?

      1. Expert Member
        Dana Dorsett | | #6

        With a ducting kit you'll have to install a return grille in the partition wall to keep the HPWH from depressurizing or pressurizing the mechanical room relative to where you're directing the chilled output air. There are no issues with putting a tee and hand-operable duct balancing dampers to change where that output is directed.

        If the vents to the outdoors are open even when the burner isn't firing it becomes a substantial parasitic load for the house (both heating & cooling.) The actual makeup air volumes needed to run the burners is pretty small- the minimum vent sizing is intentionally oversized to ensure pressure differences stay low and don't interact with the burner or potentially backdraft no matter how small or tight the mechanical room. With the air intake distributed across the house the pressure differences are very low, quite different from what happens in a small doored-off mechanical room, and the actual amount of air drawn into the is surprisingly small.

        Since the furnace is probably 2x oversized (maybe more) for the heat load it's actual burner-on time is a fairly low duty cycle. You can either measure that duty cycle directly, or run a fuel-use heat load calculation to estimate what the seasonal average is. iThe methodology for the latter is spelled out here:

      2. Jon R | | #7

        It's too bad that you didn't retain the gas water heater for Winter use. Using a gas furnace to generate heat and then paying more to move it into water isn't as efficient as heating water with gas directly.

        A HPWH is not designed to extract heat from Denver Winter air. So yes, spread the heat load around enough that it isn't much of an issue. Follow G2407.5.

        1. Brady W | | #9

          Hi Jon. I get that a garage in south would be more ideal but what do you mean by "not designed to extract heat from the Denver Winter air"? Is (lack of) humidity that relevant as an input (not output) to the refrigerant process? It seems to be performing OK thus far here as long as we can tolerate a little cooler basement (basement is conditioned).

          1. Jon R | | #10

            Humidity isn't the issue - cold is. If the mechanical room stays above approximately 45F, you are OK (with some efficiency loss).

  5. Brady W | | #8

    Not sure how techie you guys are but I've found that the Geospring control board is pretty much intact in the Bradford unit (same part number). Using a GE ConnectPlus dongle, I've been able to control it using the GE app just fine.

  6. Brady W | | #11

    Got it. The mechanical room is conditioned space so it stays fairly warm. (usually at least 65 degrees).

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