How many BTU required to heat air

Good Day folks,  this is my first posting to HEPF although I used to regularly participated in the LinkedIN group.  I am approaching year 6 on my build of a single family home.  Been quite the journey.

Anyway, I need some advise regarding the best approach to offset the cooling created by a Heat Pump Water heater.  I requested and received some data from RHEEM re their 80 Gal Rheem Hybrid DHW tanks.  The unit pumps out 4000 BTUH of cooling into a room.  If the exhaust was ducted to the outdoors, the flow would be 150 CFM.  And they do not recommend ducting the intake to the outdoors where the outdoor ambient temp can drop below 50F (so that option is not on the table for me as we get down below 14F in winter)

I will be placing this unit in a 3312 cuft (414 ft2) below grade room that is well insulated on all 6 sides and separated from the main dwellings building envelope.

I need to see what makes the most sense, not ducting the exhaust and just heating the room an additional 4000 BTUH   OR   Ducting the exhaust and heating the required makeup air from outside.  I just need to understand how to do the calculation. 

I used this site and entered a room size of 1x1x150 to simulate the makeup air volume.  It uses ºC so I used -5 and 20.  This gave me a needed BTU of 210 which would be per Minute as that is the air flow scale.  So this would be 12,600 BTUH.  Right??

In this case it is obviously cheaper to not duct the unit and just heat the room the extra 4000 BTUH.

If this calculation is correct, there would not really be a time when it made sense to duct the unit. The only time that the makeup air heating requirement was equal to the 66 BTUH (4000 BTUH / 60 minutes), was when the outside air was within 1 degree of the inside set point. And when the outside air is warmer, then obviously the cooling of the heat-pump is a beneficial reduction on the cooling load.

Are these calculations correct?? Any insight would be appreciated.

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  • I added a heat pump water heater to my basement, which had around 2,000 SF (all one big space) and I was able to gain enough heat from the concrete basement floor to supply the necessary heat for the water heater heat pump.  I would suggest you try installing new heat pump water heater and see if basement stays above 50F throughout the winter without needing to heat it.  It did for me.  

    • Thanks for feedback Kevin.

      I will be offsetting the heat extraction caused by heat pump with a hydronic ceiling panel.  The room that this will be in is about 480 ft2 and due to room uses, will be kept at between 63F and 68F.

      What Hybrid heater did you put in?

  • Hey Team,

    This in another instance where lots of researchers have tried various ducting schemes in many climate zones, such as FL, GA, and New England (check Building America Solution Center). The end result is that one can measure the impact of on/off of the HPWH on the temperature and humidity of the space to which it is exhausting, but only for a short time. It will not dehumidify an encapsulated attic or crawlspace. It will not really cool a laundry room.

    I think that complicated demand-controlled dampers with T/RH logic are total overkill. You will spend too much time and money getting that to work. KISS principle here.

    I suggest going with an unducted version. You can always add the duct kit if you install it with that in mind.

    Good luck!


  • Thanks David,

    Ya I was surprised when I recently went into tier 2 at the site only because I had added a 30 watt construction heater to the mix that ran periodically.  I currently average 36-40 kWh per day at the site.  This is running the computer network servicing the website cams and stuff, the duplex pumping station that pumps up over 1000 gallons of ground water a day from around the perimeter of the foundation (currently this also includes some storm water making its way down there until hardscaping is added on the north elevation), a 1400 W oil filled electric heater, any lights/tools I use when working at site, the led soffit lighting strips I have installed, and the periodic on time of the 30A construction heater.  After 666 kWh usage per month, my rate jumps to around $0.147 per kWh (inc all adder costs like connection fees and various levies).

    The Rheem HPWH has a heat pump only mode, so yes I will absolutely be trying to keep it within this mode of operation and will definitely be writing about my experiences on the project blog once we move in.  I am also working with Leviton on a partial sponsorship deal for there new smart distribution panel. With smart breakers installed on select circuits, I will be able to determine the exact electrical usage of that circuit for any given time period.  I will also work on adding a flow sensor to the domestic HW output to determine the volume of HW used and the timing of use. I will also see if there is a way to monitor the energy usage of the resistance coils on the HPWH separately from the overall appliance use, to capture that info as well.

    Thanks for letting me know the reheat costs will be a fraction of what I estimated.  I knew it would be but wanted, as you say, to set an absolute worst case number in my payback estimate.  At 20% the pack back will be 2.9 years.

    Yes LNG would definitely be the route many people would take (and I suspect even with monthly costs it would be cheaper from a monthly cash flow standpoint), but I have a conviction that we need to abandon fossil fuels, so it was never something I was going to consider.  I will have it on site to service the back-up generator, because the diesel version would only run for 40 hours before needed to be refilled, and it would be more polluting than LNG.  And because LNG will be on site, I will also utilize it for the BBQ, because reasonably performing electric BBQ's still do not yet exist.

    As we discussed today, I am doing a lot of things from an academic standpoint that are affordable because I am able to roll them out cheaply by doing them myself.  They would never make sense financially if I had to pay someone else to do.  But what fun we are going to have watching the results!

    Thanks for the invite, and yes, you are welcome this way any time.  Maybe at some point, I will even have a finished place for you to stay :-)

  • I want to Thank David for a marathon call we had today.


    We went through a lot of assumptions and data points. The end result was that I found an additional error in my spreadsheet that makes payback even less than what I reported (I was dividing by the cost to operate the HPWH instead of the savings between the two methods of heating).


    With the annual hot water gallons tweaked down to 30,700, and resistance heat cost increased to $.0147 to represent the fact that if I am doing resistance heat, I will for sure be in Tier 2 billing, the payback (not counting the cooling load imparted on dwelling) would be 2.75 years to offset the $1850 premium this equipment represents.

    I was then able to get the ThermAtlantic spreadsheet, later in the day, used to create the graphic I posted and modify it as needed. Assuming that the HPWH ran 24/7 for the 246 days of my heating season (which is obviously way more that it will actually run), and assuming that the HPWH would never be exhausted to the exterior during the heating season, the cost to supplement the 4KBTUH would be $191 annually using my planned DX2W and a 3 Ton Goodman Heat Pump. This would only drop payback to 3.84 years.


    SO – it is quite obvious that the Hybrid HPWH makes total sense for this dwelling and our use. Our hot water use would have to drop almost 50% to below 17,150, before the payback exceeded even a most pessimistic 10 year life span expectance of the appliance. Even if I gave up 100% of the baths, this would never be achievable.


    I am thankful for the education this post has brought me, and the need to ‘defend’ my plan to utilize this technology. Now I can confidently proceed knowing it is the right decision.

    • Yes, quite the phone session that was! Having spent time with you at the site early on, it's amazing and exciting to see it all finally come together!

      After slashing those sink estimates by 2/3rds, your new total seems realistic. But as 30k+ gal/yr is still well beyond what the 2-person household would consume (for good reason), this is really the ideal project for a HPWH, especially given your steeply tiered electric rate.

      One concern remains... By my math, assuming 6,000 BTUH capacity (which we don't know), your heater would need to operate 8 to 9 hours a day in winter to fully satisfy your average load. That's probably at or near the limit of this heater in terms of daily load without using the COP-robbing electric element. In fact, I think most folks would consider your load impossible to support without supplemental heat. But I KNOW you'll take that on as a challenge, enforced by disabling the electric element! It will be very interesting to see how it works out. If you end up needing to use the backup element, I hope you'll set up a circuit-level energy monitor to track supplemental kWhs.

      Regarding your original question... You'll be pleased to know your $191 estimate for reheating the exhaust air is probably high by about a factor of 5 (with the caveat that I'm relying on your spreadsheet calcs). You already knew it was worst case. Here's the rest of the story:

      The 246 heating "days" from the table (presumably derived from TMY3 '5F BIN hours' converted to '1C BIN DAYS') cut off at 15C (59F). We estimated the HPWH's 'leaving air temperature' at 45F, above which you'd want to exhaust to the outside until there's a cooling load. As it turns out, only about half the hours (days) on the table are below 45F, thus cutting your estimate by roughly 40% (not 50% since space heat COP goes down with ODT).

      Next, you assumed 24/7 operation for simplicity to put an upper bound on reheat cost. IF the HPWH is able to satisfy your load without supplemental heat, the compressor will run about a third of the time as noted above, so that drops you to about 20% of the $191 estimate, or $35 to $40 a year to offset exhaust air cooling.

      BTW, I estimate the interlocked temperature-controlled exhaust and make-up air system will save what would be an additional $20 to $25/yr in reheat costs if you don't do outside exhaust. Something you want to keep in mind as you journey down that path :-)

      As an aside, I think most folks with access to natural gas would go for a gas water heater without a second thought. Depending on your local rates, it might save as much as 40% vs the HPWH, or about $10/mo, but the pesky monthly (fixed) service charge would more than wipe out those savings!

      I very much look forward to visiting your new digs once you're past the crunch. Likewise, when you're ready to take a few badly needed days off, you guys are always welcome here where it's (usually) warm and sunny. Sorry, no sandy beaches.

  • If your room is air sealed and insulated on all 6 sides, where is the thermal energy going to come from? If you duct air from outside, the air is to cold in the winter for the heater to operate in heat pump mode (and COP falls off below around 65 degF). If you duct air to/from the house, where is the heat in the house coming from? A HPWH only makes sense if you are taking wasted heat or heat from the earth or from warm outside air or from a room heated with wood heat. This is only a heat pump, and the colder the air, the more electrical energy needed to pump the thermal energy.

    The 4,000 BTU/Hr quoted is for running time. It does not relate to gallons of water used, temperature rise of water to be heated, and temperature (and RH) of ambient air at HPWH.

    • Thanks for reply Brad - I am aware of the constraints you have stated.  The unit would be drawing heat from within the room during the 'cold' season as discussed above. The room will be heated with a Air Source to Water Heat Pump providing the hydronic space conditioning for the dwelling (ThermAtlantic DX2W).

      What I don't agree with, unless it can be proven, is "A HPWH only makes sense if you are taking wasted heat or heat from the earth or from warm outside air or from a room heated with wood heat."  A hybrid heat pump can make sense anywhere that the room heat being extracted by the hybrid, is created in a method that is more efficient than the option of a 100% resistance HWT.  

      I believe that the two heat pumps in unison still end up being way more efficient and cheaper than going with a 100% electric resistance heater when taking into account operating costs. Do you not agree?

      And as you say, this extra penalty of 4000 BTUH is only while DHWHP is operating and only during the heating season. During the 'hot' season, there will be no penalty and in fact it will reduce the cooling load of the room (and potentially the house if I create a ducted spill gate between that room and the rest of the house - as this would help greatly in reducing the latent cooling load). 

      The only question is what will be the cost for the extra 4000K of heating needed if I go with a Hybrid, and how does that effect the payback period.   Not sure that data for this exists easily. 

      Would depend on outdoor temps and operation time of the Hybrid which I do not know how you could estimate.

      One final comment - this has been done (combining two heat pumps) and the results measured by Alex at GreenBuildingAdvisor (  Yes the performance drops during the heating season, but it is STILL saving money.

      Heat Pump Water Heaters in Cold Climates: Pros and Cons
      While a heat-pump water heater will save significant energy on a year-round basis, be aware that in a cold climate the net performance (water heating…
      • I don't dive into numbers as well as David does because if you do analyze a situation only by numbers, you need to be very careful to include ALL parameters that might have an effect on the solution. When using assumptions, such as delta-Ts or gallons-per-day, you have to factor in some sort of error range when looking at the data, and the more parameters that are assumed, the more room for error from synergism. With the wide temperature swings we have been seeing lately, HDDs is becoming a less useful (but still very important) parameter. I love math and science, but I don't do that sort of analysis on a regular basis so I leave that to others. I am fairly good at looking at the whole picture and thinking about all of the parameters that could have an effect. Alex's blog post that you referenced made some reasonable assumptions but did not take all parameters into account, starting with the floor. He estimated the R-value of the floor and a delta-T between the house and basement but did not include air movement between them. Having done many blower door tests in houses, I find that rarely is the basement not reasonably well connected to the house for air movement. It is also interesting that he uses a Geospring HPWH, which has had a poor track record. GE stopped making them and Bradford-White has taken over warranty issues.

        Unless there savings in your situation are huge, a numerical analysis must be done with great care. Unless the savings calculated from measuring an existing situation are huge, you cannot compare the results unless you carefully compare all of the parameters, which would be many. I am looking forward to more studies and careful analyses of these solutions for heating our homes and hot water.

        • You can rest assured on one point - if I go ahead with hybrid, I will definitely be reporting on its performance and electrical consumption.  I may even put a flow meter on the hot exit to have accurate water consumption data.

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