Anyone ever built a heat pump before?
I'm thinking of taking our old dehumidifier(which still works, just kept freezing up when it was in our basement) and using it as a HP for the hot tub. I already used it as a portable A/C and it does do that, it's just not big enough for what I'd use if for(pop up camper) at only around 6000 BTU. I noticed though, while taking temps. on the coils, that the condenser coil ran around 140. Seems like a great way to keep the hottub hot on those cloudy days. If the thing ran at 100% efficiency I'd gain around 3*F an hour. Even at only 66% that would be 2*F an hour which would heat the tub(albeit slowly) or maintain it easily.
Why don't I just use the resistance heat in the spa pack you ask? Because it's inneficient and SOO SLOW. I only have 120v out back so I only have 1.375Kw of heat. This means that for 1500w(1.5Kw) I get about 3000 BTU/Hr vs. the Dehu. HP which would give me(hopefully) about 5000 BTU/Hr but drawing only about 700w(.7Kw).
Doing a bit of figuring I end up with the numbers below...
Amount of Electricity used to produce a certain number of BTU.
.5w = 1 BTU ..... Resistance
.14w = 1 BTU ... Heat Pump
.5w = 3.6 BTU .. Heat Pump
1000w = 2000 BTU ... Res.
1000w = 7200 BTU ... HP
*** Our Electric rates are about $.10 KwH***
Theoretically to heat the tub 10*F it should take about 21,000 BTU.
20,750 BTU x .0005 Kw x $.10 KwH = $1.04 ...... Resistance
20,750 BTU x .00014 Kw x $.10 KwH = $0.29 .... Heat Pump
I believe that gives my Dehu. HP a COP of 3.6. (not sure if that's how it's figured) And that's figuring 83% efficiency on the HP. Seems like a good deal to me!
Obviously the Dehu. would be plugged into a GFCI outlet and I wouldn't use it when people are in the tub(Just in case) so nobody will get electrocuted if something leaks/shorts.
Does my math sound right? Anybody see any obvious holes in my plan?