Therms, KWH, Heat Pumps, Furnaces-where to set crossover

[cdh]

I recently purchased 3 new Carrier Infinity Scroll 3 ton heat pumps (25HNA9), 3.5 ton coils, and 96% (93.7%afue) furnaces (58MVB080) for back up heat. The dealer set the furnaces to take over at 45 deg. I used to have 17 year old, 100K BTU Bryant furnaces, likely less than 80% efficient

I'm trying to determine a "formula" for my area, Charlotte, NC to determine the most fuel efficient point, one that could be adapted to other areas.

Here, our electricity is 8.3 cents/KH, a therm of nat gas, about $1.25.

Last year I used therms ng, (avg temp): Nov-110 (51deg), Dec-180 (48deg), Jan-230 (46deg), Feb-300 (37deg), after which it fell off rapidly. I use about 19 therms baseline in the summer.

How does one determine this, or is it just 45 deg and forget about it.

Wilkpedia says a therm is equivalent to 29.3 KH (resistive).

I've looked through the manuals but there are no charts in them indicating the performance of the heat pumps at various temperatures for the pump/coil combination. These must be in some Carrier manual.

Anyone have any thoughts on where I should set my Infinity controller?

[amd]

It's kind of difficult to predict heatpump performance at lower temperatures because of defrost cycles, cycling, etc.

The heatpump will always be more efficient than the gas furnace but electricity is much more expensive than gas in most areas. I think that a COP of 2 (probably reached at a little bit below 32F) would probably break even with natural gas in most of the country; chances are you'll need to rely on backup heat a little bit at that point anyway.

Edit: $2.43 per therm for electricity. 8 cents per kwh is cheap for electricity - makes sure that includes everything.


Drop the setting consistently (few degrees per day) until the HP can't keep up any more or has to defrost regularly. (defrost kills efficiency; it's like running the a/c and furnace at the same time in the winter )

Note that this is pretty much theoretical to me so wait for others to post; heatpumps aren't common where I live.

Some manufacturers like Rheem and York have technical data available to the general public - I don't think carrier does.

[jkish]

Your cost of heating via gas per therm is 1.25/.96 or $1.30

Your cost of heating via heat pump per therm is .084*29.3/COP or $2.46/COP

The COP varies with temperature - lower as outside temperature decreases.

For all temperatures where COP > 1.89 (2.46/1.25), the heat pump should be more efficient.

You should be able to look up at least the 47 and 17 degree COPs for your heat pump.

Below some temperature, however, the gas comfort may outweigh efficiency.

[mchild]

You can drop the change over much lower than 45*, probably at least 35* if not lower. The one issue that amd brought up is the defrost cycle. If it isn't running very often then you can afford to set it lower.

COP stands for Co-efficiency of Performance. It is simply the calculation of how much energy the system is using as compared to how much it is consuming. Your units, if installed properly, have a rated COP in excess of 2.0 all the way down to 7* (COP of about 3.5 at 37*). A COP of 2.0 means for every KW you are using (each KW = 3,413 BTU) to run the system it is providing 6,826 BTU of heat. Can't beat a heat pump for efficiency.

Hope that helps.

[tigerdunes]

CDH

Your dealer is doing you a disservice at setting the changeover point from heat pump to gas and viceversa at 45 deg. This defeats the purpose of having a heat pump, much less a dual fuel system. The main idea is to let the heat pump perform the large majority of your heating needs because of your relatively inexpensive electric rate. I would ask the dealer to calculate the temperature balance point. I would then add 10% to that temp and have the changeover reset to that temperature. For example, if the balance point was 30 degrees, then I would set the changeover temp at 33 degrees and use this as a starting point that can be adjusted up or down depending upon your comfort and how the heat pump performs and warms your home.

I personally think a 95% AFUE gas furnace is a waste for your area/climate. Why? Look at your gas bill and you will find you will rarely use the gas furnace for heating. Still you have a great system-just get that changeover temp lowered before the heating season.

IMO


balance point

An outdoor temperature, typically -7° to 7°C (20° to 45°F), at which the output of a heat pump equals the heating demand. Below the balance point, supplementary heat is needed.

[cdh]

Very succinct explanation. I'll try to get the chart of COP vs. Temp for my units. It should be easy to make a spreadsheet that people can input the data from their own circumstances. I'll post it.

[BaldLoonie]

With those rates, use the pump as cold as it will go and still keep you comfy! Ours are similar, electric maybe even lower in the winter. I wait til the very last moment to turn on the gas. Even to the point of being a bit cool in the morning with the heat pump. Sure lots cheaper than burning gas.

[junkhound]

Also, if you don't mind a little cold air every few hours during defrost, diconnect the electric strip heaters in your heat pump, you will notice it on you elec bill.

Have mine disconnected (external switch if ever needed).

4T Rheem keeps 5300 sq ft warm down to 15F (has never gotten colder than that here -Seattle area). Son has dual heat, but the nat gas cock is turned off at the furnace.

[cdh]

That's impressive! I guess I didn't really need those gas furnaces after all.

[OOC]

I've got a 3-ton 16i and 80k BTU XV90 in Upstate SC (installed in late May so no winter info yet). The dealer set the changeover at 40° F. Of the 2 sales engineers from there that visited me, one said he likes it at 40° and the other said 35° worked for his home. A competitor told me he believes 35° would be fine as well, but his wife insists it be 45°. He did admit his mistake was showing his wife how to check what the changeover temp is on the t-stat because he thought she'd never notice if it got moved down to 35-40°.

[gary_g]

The Technical Information Manual for my new Goodman 3 ton, 14 SEER, 9 HSPF, heat pump lists the following info:

Outside
Temp (F)----COP

35 --------> 3.25
25 --------> 2.82
15 --------> 2.46
5 --------> 2.06

[tecman]

I agree that you should lower the changeover point. I would go to 25 or 30 and see how it feels. The only thing missing in the discussion is that while the COP stays high, even at very cold temps, the capacity of the heat pump starts to drop below about 40 degrees. At 17 deg it may have only half of the rated BTU capacity as compared to warmer temps. There will be a temperature where the diminishing BTU capacity of the heat pump equals the heat loss of the house - this is called the balance point. This can vary with wind, humidity, etc but there is a balance point that will be a good number for your house. This is the point where the heat pump needs help (such as electric backup) or changeover to gas. You can find that point by lowering the changeover temp to where the heat pump can no longer maintain house temps and the house will start to drop in temperature. This temp, or slightly above would be the ideal point to set the changeover.

paul

[cdh]

Thanks for your advice, it makes perfect sense to me now, as the limiting factor is not necessarily what is costing less (heat from the heat pump or gas), but at which temperature is your specific balance point for your home and systems, and that temperature again may vary due to weather conditions.

I'll play around with this as the season approaches. I now also understand why another poster suggested setting the changeover point a few degrees above what ever that point is so that the home heating stays predictable.