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I am getting ready to have a Goodman 18 SEER 2 stage heatpump installed in a home in Southern Delaware model DSCZ180361A. Cooling load is 26,000 BTUH and heating is 34,000 BTUH. Zip code is 19975. Balance point should be around 30*
The question I have is about the backup/aux heat. There are 2 choices: Metered Propane ( this is in a planned community ) or electric. The cost/btu between propane with a 95% furnace and electric is very close, propane is $2.70/gal and electric is $0.11/KWH. with propane having a slight edge
Looking at the Bryant Operating cost estimator for a similar system, they calculate that the propane system will cost about $200/year more to operate than the total electric system. Based on propane costs and electric costs, I would have expected the reverse.
What am I missing?
Global Moderator
My calculation shows it a tossup. Electric might have a slight advantage from the fact that the heat pump will continue to run with the backup helping out where the dual fuel will have the compressor shut off and switch to gas. I wouldn't expect a $200 premium from gas though.
With the heating load so close to the cooling load, I'd expect a lower balance point too.
God Speed, Jim. We'll miss you - 2-26-23
What indoor and outdoor design temps did they use for heating.
Professional Member*
Propane is EXPENSIVE ! ___ 29.3 Kw - THERM 100,000 BTUOriginally Posted by pendetim
PROPANE HEAT CONTENT =~ 92,000 BTU per HR / GAL
http://www.exothink.com/Pages/btu.html
The real question is how many hours < ~ 25'F will the heat strips need to be operating in S. Delaware.
+ Easy for small strips to come on/control during defrost cycle.
Looks like two 5kW would be appropriate.
2nd strip might not come on till < 14'F.
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With having both electric and LP, you always have a choice. Just think if a couple of nuclear power plants are sabotaged by Islamic terrorists, wouldn't you feel better with having LP?
Training is important!
Practical Training is a must!
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just make sure your tstat can control 2 stage heat strips and dual fuel. That way the 10kw (two 5kw elements) can call just one one as only 5kw could be needed most of time elect aux is called for.
Regular Member
Robin, this is the A"O"P not the ARP
Anyhows....
One advantage could be the use of a generator, less KW to be considered with the LP?
Folks like the "warmer" air coming from the vents, although both would heat the house to what you want without issue.
Could "Flip" from one to the other as the outdoor temperatures drop/rise as well to provide that "warm" feeling that many 1st Heatpump timers have an issue with?
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Propane will not work in conjunction with the heat pump. The pump will shut and switch. Can the Goodman stage grid heat on top of/with the heat pump?
If it can, go electric. (If it can't, switch manufacturers and go electric. In fact, if that unit doesn't defrost on pressure rather than timer, switch manufacturers.)
In other words, you won't ever be carrying your whole house with electric because the pump will always be doing most of the work. Electric will come in to give you the extra push to get over the high peaks.
Hybrid will shut the pump and completely switch to expensive propane. Propane will be carrying 100% of the load when you get to the high mountains.
Unless you think natural gas might make it to your area, go straight heat pump.
God Speed, Jim. We'll miss you - 2-26-23
Heat pumps don't do defrost by pressure. I think you mean on demand defrost. Goodman doesn't have it, and last I knew, Carrier doesn't either.
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One other thing is that propane usally goes up in the winter
Global Moderator
Few stats can stage the backup and most furnaces it isn't easy to stage 10kw without a complete rewire and a 2nd sequencer.
Professional Member*
If you set up the system for the furnace to operate on second stage of heat, the heat pump will always run primarily, only turning off to allow the furnace to fire when the second stage of heat is called for.
You can also set up the heat pump to shut off at a certain temperature to allow the furnace to take over. Or, you can switch the t-stat to aux heat and let the furnace do the entire heating job.
The point is that with a dual fuel system, you get to use the energy source you choose to use when it best benefits you.
Training is important!
Practical Training is a must!
Professional Member*
I was led to believe these things understood defrost based upon temperature. Given the source of the contradiction, I had to dig. I guess pressure increase does not trigger defrost. What does is really not very clear to me but probably will be to you:
Ideal Defrost / Intelligent Defrost - (I have the Carrier VNA service training manual)
What is not clear is it somehow knows how long defrost takes, and based upon that sets the next defrost run interval.The communicating control system monitors (counts) the compressor run time minutes and as the accumulated run time approaches the defrost interval time the communicating control system monitors the outdoor coil tempaerature sensor for a defrost demand. If the outdoor coil temperature sensor is 32f or lower for a period of 4 minutes, and the outdoor air temperature is below 50f, a defrost demand has been established. If a defrost demand exists, the defrost cycle will initiate at the end of the selected time interval. Failure to sense 32f or 32f for 4 minutes defeats the defrost demand.
This thing is pretty amazing. The controller can manage refrigerant charging check, pump down, tells suction pressure, compressor rpm, suction temp, superheat, exv position, airflow, lbs of refrigerant to add or subtract based on lineset length...
And in a mild climate where the btu cost of propane has a snowballs chance in hell of approaching the cost of high cop heat pump, having a few btu of electric resistance combined with the heat pump for a few hours a winter is going to be WAY WAY cheaper than paying the incremental on the furnace. Paying the additional cost of burning propane particularly when you factor the heat pump CAN run with the resistance (like an additional stage) and the propane CAN NOT.
I simply have not heard of 50c kwh electric, if it exists it's very rare, and that's still cheaper than propane at most temps.
Forward energy view is electric will become more and more closely tied to natural gas. The only source that can hold a candle to the btu cost of these high COP heat pumps is Natural Gas, and that requires some pretty cold temperatures. This is very likely to remain true for a long time.
God Speed, Jim. We'll miss you - 2-26-23
On demand defrost uses the temp difference between the air and the liquid line to determine if there is ice/frost on the coil to a point that it needs to be defrosted. After the outdoor temp drops below X degrees(varies with manufacturer) they won't go into defrost except for once every 6 hours of compressor run time to make sure the outdoor coil doesn't become oil logged.
Carrier tries to mimic the efficiencies of on demand defrost. But misses the boat a bit. Its better then just time and temp defrost, but still activated by only coil temp and compressor run time(with respect to outdoor temp being under 50). But when its 10 degrees outside, Carriers Ideal will still do needless defrost cycles. On Demand won't.
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Who has demand defrost on their units?
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I'd do standard electric backup. I don't see the additional costs of the LP furnace having a reasonable payback time.
God Speed, Jim. We'll miss you - 2-26-23
York, Lennox, Rheem(although they send time and temp replacement boards), ICP(at least they did in the early 2000s). Not sure who else.
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Trane
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Thanks BT. Not likely ever going to reach the point I don't have oceans to learn from you.
Awesome, that's great to know! Thanks Chris!
(I've moved over to AS as preferred brand. Anxiously awaiting inverter split. I guess mini's are coming in spring.)
Regular Guest
Thanks for all the insight, folks.
I will be having a re-quote done on a 2 stage 10KW electric backup, removing the propane furnace. I suspect the capital cost will be pretty close as I will need to run a 60A service about 80' from the garage to the crawl space for the strips.
As pointed out, this will make the heat staging a little more complex as I will want to be able to stage Y, Y2, W and W2. Would the EcoBee EB-STAT-02 be a good candidate for this?
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