Converting oil & AC to heat pump w/ electric coils
Loyal residential customer of HVAC pros here!
When I had my 100,000 BTU 80% oil furnace and 10 SEER, 3-ton scroll split system AC installed in my 1927 3-BR two-story w/ finsihed basement farmhouse 9 years ago, life was good. Now it's not! If I keep this system as-is, I'll spend $18,250 on fuel oil over the next 9 years buying an average of 450 gals of oil per year at $4.50/gal. With electricity at $0.077/kWh where I live, I figure oil would have to be about $2.56/gal to be competitive with pure electric resistance heating. I know I can do even better with the BTU multiplying power of a heat pump to handle most of the work on warmer winter days. Assuming a 50% rise in electric rates over the next 9 years, I figure about $8000 total cost of electric power for heating over the same period, even assuming 100 annual hours of 20 kW resistance heating (7300 kWh heat pump + 2000 kWh resistance per season). I don't know if those figures are reasonable but it's the best I could manage as an estimate. Even if I'm low by 50% it's still $6000 less than the cost of oil heat.
What I want to do is get the oil tank and furnace out of my basement and have an all-electric 15 SEER packaged heat pump installed on the ground outside. I can convert the newly available interior floor space into a small home office as a bonus.
My HVAC contractor told me that heat pumps are sized off the summer heat gain, not the winter heat loss. Is that correct? I was wondering if I could go with a higher tonnage 2-speed scroll unit to get more heating capacity in winter, and rely on the low speed compressor setting to match the lower tonnage cooling requirements in summer. My contractor said the low speed isn't intended to work like that. So, do I take his advice and go with the same tonnage as my current 3-ton AC (which seems to be just right)?
My other concern is how to ensure that the backup electric coils are sized big enough to satisfy the thermostat on subzero days when the heat pump COP is 1.0, but not so big that they cycle constantly when the heat pump output starts to fall (unless that would be normal?). In my area (Cincinnati) there are about 5000 degree days of heating per season. A cold month like Feb 2007 had about 1200 heating degree days. My oil furnace has a 0.85 GPH nozzle so the 100,000 BTU @ 0.90 GPH is actually more like 94,000 BTU @ 0.85 GPH. I wish I had thought to measure its duty cycle on a very cold day but I never have. I think it ran pretty steadily when the outside temp hit -20F.
I'm also prepared to live with the lower heat rise of the heat pump and electric coils compared to oil. My fan thermostat is already set back from the original 135F to 90F to get BTUs into the house faster with fewer going up the flue pipe, so we're accustomed to "cold' air blowing out of the registers.
A heat pump can be sized 25% larger then your cooling requires.
So if 3 tons is the right size for cooling, a 3.5 ton HP is ok.
By using the right thermostat, you can lock out the aux heat until its really needed. And have it come on in stages.
make sure you check the actual BTU output of the units your get estimates on. Some give less heat then others.
First off, if you are considering adding electric aux heat, double check your electric panel. Many people can't upgrade to electric heat simply because their electric panel is full. You can still do it of course, but replacing the panel or adding a secondary panel is not cheap.
Thanks for the comments. That's a good point about panel space. My 200A panel doesn't have enough pole spaces left to accomodate dual two-pole breakers, so I'm going to have to have some of the single poles converted to twins to make room.
Do you have any tips regarding recommended sizing of the resistance coils? I'm thinking 15 kW might be adequate.
I don't think anyone can tell you what size electrical strips to get without running the load calc. However electrical strips (or any resistive heat) don't have the high start-up costs (inductive load?) of motors, so oversizing the strips I don't think will hurt you like oversizing other systems - they don't run as often. Obviously the bigger strips will cost more. What say you pros?
I'm not entirely sure but I think you can be setup to run the HP and strips together, so even at low temps/COP you will still be getting limited heat from the HP. If you have a particular HP model and coil in mind try contacting the manufacturer to get the COP curve.
With a 1927 house, you need to be sure your service can handle those strips. If you end up having to do a bunch of wiring maybe consider electric baseboard heaters. Baseboard heaters have the advantage of zoning perfectly to each room.
I verified your cutover price on oil at $2.56. Sure wish my electric rates were $.077 KWH. Just got my latest bill - generation alone was almost $.12 KWH.
Most contractors can check your panel box to see if it can handle the load of the strip heaters.
The heat loss of the house determines the size of the strip heaters.
Guessing at them can be expensive.
Over sizing cost you more to install, due to larger wire sizes, could require a service upgrade to 320 amp, and incresing duct size to handle the excess air.
Under sizing could leave you with a cold house, and high electric bills.
10KW is 34,130BTUs at approximtly 42 amps 240 volt
15KW is 51,195BTUs at approximtly 63 amps 240 volt
20KW is 64,260BTUs at approximtly 84 amps 240 volt
25KW is 85,325BTUs at approximtly 104 amps 240 volt
Add the amp requiements of the OD unit to that yet, and you could force youself to do an unrequired elecgtrical upgrade.
The electric baseboard heat is a very good suggestion. I have considered that but wasn't completely sure how to maximize efficiency by coordinating all those room 'stats with the heat pump COP curve (speaking of which, I need to get my hands on that as suggested). I'm afraid I'd inadvertently use too much baseboard and not take full advantage of the available HP COP if the systems are fully independent. Plus there's the issue of automatic night setback (We like to drop back to 55F at night). I guess I'd have to have programmable room 'stats in each room.
My 200A electric service is pretty new. It was installed when the foundation was replaced in 1999. The other major electric loads connected include hot water heater, clothes dryer, and electric oven/range. I have heard that current transformers can be installed in the panel on those other circuits such that the heat strips shed load in 5 kW increments when those major appliances turn on.
Some of the more advanced packaged heat pump systems, for example the Bryant Infinity 15 SEER, seem to have sophisticated controls that stage the resistance heating to compensate for falling COP on the HP. I think some systems actually real-time modulate the current flow with solid state electronics instead of just staged contactors. In such cases, there doesn't seem to be too much harm in oversizing the strips since the last 5 kW stage may never come on in all but the coldest weather. Plus the initial cost of larger heat strips seems to be pretty low as a % of the overall system cost.
This is Kentucky coal country!
Originally Posted by dac122
You don't really want electric baseboard with a HP.
Your heat pump will still need a strip heater to temper the air during defrost.
With a heat pump, its cheaper not to set back the temp real far.
I know this is off topic, but how far of a setback is good for HP during heating season?
Originally Posted by beenthere
When I think about it, that makes sense. Otherwise the heat strips will come on with a vengeance in the morning. Thank you for this insight.
Originally Posted by beenthere
Originally Posted by Shran
That depends on the thermostat. My old thermostat wouldn't kick on the auxiliary heat unless the set temp changed by more than 4 degrees. So I could set back the heat by 4 degrees and not have AUX kick on uneccessarily.
Don't forget as beenthere mentioned there are stats with aux heat lockout. On mine you set the temp, and no amount of mashing the tstat buttons will bring on the aux heat above that setpoint outdoor temp. That's a good thing in my house where everyone seems to want it warm right now!
Originally Posted by badtlc