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.