# 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 https://www.simplex.ca/en-CA/btu-calculator 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.

Tags: BTU, Calcs, Hot, Hybrid, Tank, Water, air, heating, make, up

Views: 498

### Replies to This Discussion

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 :-)

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!

-Sydney

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?

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