Questions on Heat Pumps

[Snagglefoot]

My air conditioner also functions as an air-to-air heat pump. I also have a propane powered boiler that provides heat to my in-floor system in my basement floor and through a heat exchanger provides heat for my forced air system for the entire house. During the shoulder season I have a choice to use the electric heat pump or the propane heated boiler system. We live in a rural area and don't have natural gas.

Converted to US dollars my power is 8 cents per kw hr ( due to BC's extensive hydro electric system and socialist subsides

) and my propane is $ 2.00 per gallon and the price contains an ill conceived 9% carbon tax. Using the efficiencies etc, the heat pump is cheaper but can anyone give me an idea about how much the life of the compressor would be reduced by using it as a heat pump during the shoulder season vs just using it as a AC unit for three months of the year. For instance, will it cause me to have to buy a new heat pump sooner? My existing one is 7 years old and has only run for 21 months as an AC unit. I'll probably sell the house in 10 years. Do you think it would make it that long if I add 40 months as a heat pump and 30 months as a air conditioner in that time?

The other question is what should the ambient temperature be before I start running the heat pump. Thanks. I'll gladly provide any extra info you think you need to answer the questions. Edit: My unit is a Trane .Thanks.

 

[ls1mike]

I have a heat pump/A/C unit. I am fairly close to you being in the Puget Sound area.

I had mine installed when I moved into my house in 2010. I leave the temperature at 64 to 70 depending on my wife's mood...

. I just let it run when it has to.
It has an electric furnace associated with it for when the temperature gets under 32ish degrees which is rare here.

I get it serviced yearly for 150 bucks. No issues. The neighbor does the same thing. Their unit is 21 years old

I have a Trane 16XLi it came with a 12 year warranty.

 

[dnewton3]

Most HP units are designed for 20 year life-cycle (based on average use cycles).

I'd say you're fine if you want to run 10 more years, on top of the estimated 3 you have accumulated so far.

 

[SubieRubyRoo]

Our heat pump also has a backup electric unit in the house, and it is going on 20 years old. I had the service guy out 2 years ago just to do a checkup and he said it's actually probably in better shape because it is used as a heat pump; the freezing and defrost cycles in the winter help to wash off accumulated dirt and debris. He said if it was just an A/C unit only, it would probably be in the scrap pile by now.

The unit is good down to single digits, but accordingly longer run times when it is really cold. When it's that cold though, I usually flip it over to run the electric element only. I know it's not as efficient, but the run times on the HP at those times are really, really long, and the air coming out of the registers feels very cold. Plus, on a unit that old, i'm trying to get as much life out of it. It's got a 10 SEER rating, so I know when I finally do replace it I'm going HP and natural gas furnace. Should be some nice savings to pocket when that day comes, after the initial outlay.

 

[Donald]

I would not worry about the life of the compressor.

Keep in mind that when the heat pump is in heat mode, sometimes the outside unit can get iced. So the system reverses itself for a bit of time and runs as an AC to melt the ice on the outside unit. When this happens typically electric booster coils come on so its not blasting cold air into the house during the defrost time.

The electric booster coils may also come on if the system cannot keep up.

 

[ecotourist]

Originally Posted by Snagglefoot
The other question is what should the ambient temperature be before I start running the heat pump. Thanks. I'll gladly provide any extra info you think you need to answer the questions. Edit: My unit is a Trane .Thanks.

I live on Vancouver Island and have a heat pump (with cooling, which we hardly ever use). It works well at all the temperatures we see here, which means almost never below -10C.

I'm told that these newer designs work well down to about -15C. Assuming yours is an older design, anything at or above -10C should still be relatively efficient.

 

[ecotourist]

Originally Posted by ls1mike
I get it serviced yearly for 150 bucks. No issues.

What do they do for the service (which costs $300 Canadian here)? I haven't been able to get an answer to that question from my installer so I haven't had it done.

I suspect they make sure the system works (check), no funny noises (check), filters clean (check), drainage hose for the AC is not blocked (check), and no build up of leaves, fluff or other debris on the outside unit screen (check). What else do they do? They might check system pressures (but I'll bet they don't).

 

[Snagglefoot]

I'm actually OK with leaving it until the night time temp is not freezing but sounds to me it's OK if it dips down a bit in the freezing range occasionally. I guess I might as well use it, because some home inspector for the future purchaser will write: "17 year old heat pump is near the end of it's useful life" and the buyer will discount the offer price anyway.

 

[dlundblad]

I wouldn't worry about compressor use.

We are seeing about a $15 difference per month ($.12 per kWh) after turning the heat pump off and running only gas heat. Our system is set to switch at 32*F, so as far as peak efficiency, I have no idea.

I've always been told our climate is too cold for one so I am surprised to see you have one that far north.

 

[MCompact]

My installer comes out once a year and he does check system pressure. He also cleans the indoor and outdoor coils.

 

[Fitter30]

Propane has 91k btu content boiler is close to 80% efficient= 72,800 btu for 1 gallon of propane
Electric 1000 watts @.08 x120% efficiency of HP 1200 watts
HP @0* are still the same efficiency of straight electric 100% = 3.3 btu's per watt
Have to figger tax in on the electric for total cost and any other charges for the propane.
Best system by far for efficiency is ground source heat pump seer factor 30+
Bosch has a HP 19 seer varible speed compressor and air handler.

 

[Snagglefoot]

The background on my heat pump was that AC was a must. We get three hot months, mostly from hot southern winds coming up from the USA. I live about 100 miles north of the border. We have fairly inexpensive power, equivalent to 8 cents US per kWh. We aren't serviced by natural gas and propane is the other choice but with the 9% carbon tax, it makes electricity look that much better. Our water comes from hydro dams so there is no carbon tax attached to it. Anyway, the electrician mentioned for a small incremental cost we could install a heat pump, so I did. Now I want to get that thing fired up. We are just around the freezing point at night and it sounds like it might be time to get it going. Obviously the rest of the winter is just too darn cold for it.

Thanks for the posts.

 

[Snagglefoot]

Originally Posted by Fitter30
Propane has 91k btu content boiler is close to 80% efficient= 72,800 btu for 1 gallon of propane
Electric 1000 watts @.08 x120% efficiency of HP 1200 watts
HP @0* are still the same efficiency of straight electric 100% = 3.3 btu's per watt
Have to figger tax in on the electric for total cost and any other charges for the propane.
Best system by far for efficiency is ground source heat pump seer factor 30+
Bosch has a HP 19 seer varible speed compressor and air handler.


I'm glad you pointed out that at the low ambient temperature the efficiency of the HP takes a hit. So, to finish off the math is it 3.3 BTU per watt hr? So dividing 72,800 btu by three would be 24,267 watt hrs or 24.267 kwhrs. At 8 cents per kwhr, that would be $1.94 for electricity and $2.00 for propane. Dang, that's no difference at all.

 

[mk378]

@filter30 assumes a COP of only 1.2 (COP = Coefficient of Performance = kW heat delivered inside house / kW electricity consumed). In mild weather a COP of 2.0 or higher is almost certain.

 

[Snagglefoot]

Originally Posted by mk378
@filter30 assumes a COP of only 1.2 (COP = Coefficient of Performance = kW heat delivered inside house / kW electricity consumed). In mild weather a COP of 2.0 or higher is almost certain.


Ok, that will help. We usually coast all night when the thermostat is lowered to 64f then kicks in at 5:00 AM. It might be freezing outside at that point and then it should gain efficiency as the day proceeds I guess. Thanks for the info.

 

[Fitter30]

Being a 10 seer its just a guess unless you have the factory engineering info. If a person has a system over 12 years old forget the seer factor just look at total name plate amps ( compressor, fan motor) on old vrs new and see the difference. Then if you plan to stay more than 5 years it might be worth it to replace system but before that insulation, windows ,doors and a blower door test should be addressed. By upgrading might be able to use smaller equipment. Blower door test see how tight the house is and alot of electric companies offer this service part of a energy audit. My coop charges $50 and came with light bulbs, caulking and some foam tape. Most efficient system by far is geothermal and through 2019 in the usa 30% tax credit.

 

[Cujet]

As with many things, some of today's heat pumps have an excellent CoP in colder temperatures. I have family that uses some of the best mini split heat pumps in Vermont. As they are engineer-types, they calculate the results. I was impressed.

Il ask for raw numbers.

 

[Snagglefoot]

Thanks everyone. I've been using the heat pump mode for a couple of weeks now and plan to continue to do so. The night temps are above freezing and that's what I'll use for my cutoff.