[drees]

I've got a 3-ton XL20i heat pump with dual-fuel (XV-95% furnace) that was installed about 4 months ago. Since it was installed, our electricity bills have gone up significantly - nearly double thanks to the tiered billing rates we have.

Right now we're using about 150w more on average 24/7 (~100 kWh/month more) compared to last year. I attributed some of it to the whole house air filter which we're running on circulate mode (on about 20% of the time, I think). I was able to put a Kill-A-Watt on the furnace and it measured about 15w when idle and 40w when air is circulating which is fairly reasonable.

I noticed that the compressor hums a bit when power is supplied to it, but I don't have a way to measure it's power draw. I assume I'd need some sort of clamp-on ammeter to do this effectively, but those cost at least $50 for a cheap one.

I found this post which suggests that the compressor uses about 100w to keep the compressor warmed and lubed up.

Frankly, I find that kind of usage outrageous considering our average power draw on the house before the HVAC system is about 500w.

Our electricity rates are about 13c / kWh for the first 303 kWh, 15c / kWh for the next 91 kWh and then jumps up to 26c / kWh above that. Rates will be slightly higher in the summer.

Assuming that 100w is correct, that's about $10/month at baseline rates and $20/month at the high tiers which the heat pump sitting idle has pushed us in to.

Is this normal with all AC/heat-pump units?

 

[drees]

On my Trane XL19i there is a thermostatic switch to cut the power to the crankcase heaters. The switch is mounted on the pipe coming from the compressor, which gets very hot when the compressor is running.

I have no idea at what temperature the switch opens,and have not yet found an answer to the question.

Does anyone know?

I don't, but let me know if you find out. I would gladly trade a 5-10 minute warmup-time before the AC/heat-pump kicks in to avoid drawing power 24/7. Given that most units sit idle the vast majority of the time, I wonder how much energy is being wasted across the country to keep idle compressors warm? That'd be like idling your car 24/7 - just in case you wanted it to be fully ready to go in case you need to go on a trip.

At 25 cents a KWH, Natural gas would have to have a delivered price of $2.32 a therm, for the heat pump at a COP of 3, to cost the same as a 95% efficient gas furnace.
At any COP lower, the heat pump will cost more to use.

Gas ranged $1.11-$1.21 this winter. No doubt it's cheaper to run the gas furnace, so that's what we've been doing this winter with the temperature cutoff set to 60*F. Anyone have a COP/temperature chart for a "typical" heat pump? I have a 3-ton XL20i.

We will be having solar panels installed which will offset most of our electricity usage, in which case running the heat pump should be about the same cost as gas as long as we're using 13c / kWh electricity. It should be cheaper with the relatively mild temps we see, though.

26 cents per kw-hr after the first 400 kw-hrs?

Higher summer rates?

Rates are "only" a couple cents higher / kWh in the summer.

 

[drees]

Wow, such a lively topic. Thanks for all the info, it has been very informative. A few comments/questions:

1. Installing a 3.24 kW PV system on the roof. Unfortunately, it's not turned on yet pending utility approval. But that should get me back into the "cheap" 13c/kWh electricity. Not sure how much extra electricity I'll be using in the summer to keep the house cool.
2. Does the thermostat measure crankcase/oil temps, or ambient temps? Or somewhere in between? If the thermostat opens at 105*F and closes at 73*F, does that mean that the heater typically cycles on, heats the crankcase up to 105*F, turns off, then turns back on when it falls below 73*F again?

 

[drees]

Yes. The key here is the compressor temp must fall below 73°f, not necessarily the ambient temp.

Awesome, thanks.

Like I said; 12 bucks a year. I'd be looking elsewhere for your increased electrical consumption.

Not quite. That's off by a factor of 10-20 depending on what my rates end up being and how often the heater actually ends up running (assuming 100w 24/7 and rates between 13-26c / kWh).

Not sure where you get $12/year - it's more like $10-20/month.

 

[drees]

Four bucks a month for 3 months. Please read all my replies to you.

OK - never mind that there are 2 heaters in my Trane XL20i heat pump so it's really $8/mo, or that because that extra usage puts me into the expensive tier of electricity that cost is more like $15/mo, or that the temperature information you've given is somewhat contradictory to what capecool has posted...

The only way to know for sure is to measure it.

A clamp-on ammeter would let me do so for about $40, but would be difficult to leave attached for any significant period of time to measure power draw.

Anyone have any good ideas on how to track this?

The best I've seen is to use a whole-house monitor like the TED, but at $200+, it's fairly pricey.

 

[drees]

Data logger on CCH with an outside air input for reference.

Have any sources for one that doesn't cost too much?

Post your full model number please.

4TWZ0036A1000A

 

[drees]

Isn't it true that not all heat pump condensers have sump heaters?

Anyone know what compressors don't? As an end-user, this type of information is very difficult to find.

 

[drees]

This is a difficult thing and not many people know how to do it. Weather effects for example, make it much more difficult to know whether one's hardware or nature is responsible within a range of +/-10 or 20%.

I am simply looking at my monthly utility bills and comparing last year's usage to this year. Yes, I know it's not perfect (which is why I'm looking for an easy way to track the electricity usage of the compressor directly), but when looking at multiple months, it's pretty easy to find a pattern as our bills are normally very consistent.

Last year, our winter electricity bills took a jump in Nov/Dec because we tended to use plug-in electric heaters to warm up the bathroom during bath time or a kids room instead of heating the whole house. Then in January we stopped using those as it wasn't too cold. This year, we didn't use the portable heaters at all after the new HVAC system was installed in early Dec. But electricity usage was the same or higher as last year.

We attribute the Jan bill to be higher because we were used the heat-pump for a week or two before setting the lockout temp to 60*F (it was 45*F). Our setback temp at night is 62* and normal temp is 66*. There may have been a few odd occasions when the heat-pump kicked in with the ambient temps were above 60*, but it wouldn't have run for long.

Here's our monthly electricity / gas data (kWh/therms):

     2009    2010
Dec 459/20  430/10
Jan 458/72  533/18
Feb 356/31  494/21
Mar 378/44  479/12

The billing dates were in the middle of the month, so Dec this year didn't include more than a week's time of the HVAC system installed.

In non-heating months last year, our electricity usage averaged 350 kWh/month. March this year was pretty mild so the furnace rarely ran. Our gas usage this March was lower than it was last year with the old furnace not running but with the pilot lit! Summer time gas usage (without the old furnace pilot lit) is well below 6 therms/month.

My hunch is that the data logger, decent analysis software, and accessories might cost more than leaving the crankcase heaters run for a year, maybe two.

I think you're right. But if it helps us save energy in other areas, it may be worth it. IMO, they are worth it for the geek factor alone (love having that kind of data), but convincing my wife is a different story!

 

[drees]

With net metering, I don't think I get one of those smart meters. Actually had a smart meter installed for a couple weeks, then when I signed up for net metering to install my PV system, they swapped it back out for a regular meter. I'll have to call up and check with SDG&E to confirm.

Looking at Wunderground, if anything it was slightly warmer last year than this year from Dec through Mar.