Heat Pump v Natural Gas

[Michael in SJ]

It is time to replace my forced air heating/cooling system.
Is there a formula for calculating the savings, if any, for converting from Nat Gas to a Heat Pump. I have a bid for a new system and the price is almost identical for either system (the HP slightly less).
Extra information: I installed 8kW of solar on my roof in December/11 :) and it has more than halved :):) my winter electrical use which is very high as a run my business from my home.
Extra information: I live in San Jose, CA and the coldest it gets is the mid 30s (2 or 3 days/year) and the highest it gets is in the mid 90s (5 or 6 days/year)

[Dave Turpin]

The calculations can be quite complicated, but in general, we would need to know to specs on the NG and HP systems you are looking at, and the price for a kWh and the price for a Therm in your location.

From there we can, roughly, calculate how much it will cost to heat your home per 100kBTU. Your actual savings will then depend on the number of kBTU you need per year to heat your home.

Now, there may be other factors. A heat pump can cool as well as heat. In california, that might be very important to you. You can't cool a house with gas. (unless you are using an Ammonia refrigeration system, of course)

Also, if your furnace is going kaput at about the same time as your water heater, you might consider a hydronic air handler and a high-efficiency condensing tankless water heater. About $4000 in components for a 95% efficient system. Doesn't cool, though.

And congrats on the solar system! Using a rough estimate, you must be using about 16 MWh of power per year? Seems pretty high, especially considering you are using gas heat. Are you running a server bank or a pool or something?

[Michael in SJ]

Thank you for your reply:
You were pretty close on my electricity usage. Last year it was 16,931 Kwh and I run I server and 5 laptops.

My gas rates are $1.30/therm
My electric rates are $0.40 kWh

Here is one of the proposals I have:

Equipment is Carrier
Heat Pump system (heat and cooling):
# FV4CNF005000. 4 Ton, variable speed air handler with coil.
# KFCEH0501N05. 5kw back-up heat strip for ( +-) 14,000 Btuhs additional.
#25HCC542A003. Heat pump, 15 SEER condensor with precast pad

Gas Fired Furnace w/AC
#59TN6A080V17-14. 96% AFUE Infinity 2-stage, Variable speed gas furnace
#GAPAAXCC1625 Infinity air cleaner with 15 MERV, easy clean and KILL capacity #SYSTXCCU1201-B Infinity controller thermostat
#24ACC642A003 16SEER A/C condensor with scroll compressor and copper tubing, 3.5 ton
#C48A175C156 coil

There is a difference in installed cost of approximately #1,100 in equipment cost with the HP system being the smaller number.

I did replace my tank water heater with a tankless three years ago, but tried to save some money on the size and it barely supports two showers at one time. But it did save a significant amount of gas usage.

[Dave Turpin]

Okay.

ASSUMING:
1) You never have reason to use the back-up heat strip
2) The heat pump in question produces the 8.5 HSPF (low number)

Then, the HP unit will consume 24,790 kWh per kBTU/hr, annually, in the heat or cool direction. This is assumed based on a 70 degree inside temperature and an assumed average outside temperature in Summer and winter months in your area.

That would equate to $99.16 per thousand BTU/hr annually.

For gas, the calculation is simple. 1 Therm is 100 kBTU. If you consumed the same 1 kBTU/hr as the heat pump, at 96% AFUE, you would consume 8766 kBTU/year, or 87.66 Therms, or:

$91.31 per thousand BTU/hr annually in the heat direction only.
Using the same calculation as the heat pump, your A/C coil would consume about $96 per kBTU/hr, since this condensor is slightly more efficient.

Overall, the heat pump unit costs you 8 bucks per year per kBTU/hr of heat load. Even if your house is poorly insulated, you might actually save $40-50 per year. You will not save enough money over the lifetime of the furnace to make up for the additional installation cost.

So, in my opinion, the HP system is the way to go. Your power probably comes from San Onofre anyway, so it is not like you are polluting the skies more with this method.

[Michael in SJ]

Thanks Dave.

BTW, I get my electricity from PG&E and they are probably buying it from the Northwest grid - so it could be coal, hydro or nuclear.

[Shane mari Besh]

Thanks for sharing. it would really help me a lot.