Heat Pump and power consumption

[MacLawn]

Installed a new Trane heat pump in Dec. 2016. Thermostat is set to 72 and the heat pump can maintain this temp without aux. heat strips kicking in down to around 15. 2 1/2 ton system and 1400 sq. foot house. Used to lockout old heat pump compressor at 10 degrees. Has been down to 0 recently and still left compressor energized as was still producing usable heat. Read recently that heat pumps produced in last few years have made lots of advancement in extracting warm air from frigid air that was not possible several years ago.

That's good to know! I've changed heat ac company, and they use Trane system. My current Carrier was installed in 2003, wha' 14 years ago?

 

[kcflhrc]

A 1400 sq ft home is very small and probably 1 floor. Try heating a 2500 sq ft 2 story and maintaining 72 degrees with a heat pump when it's 0 degrees out.

 

[KTurner]

Yes, that was a good reply by techman.

Partial followup... Heat pump service call scheduled for tomorrow morning. High usage (>6kw/hr) overnight again. I cranked up the wood stove this morning. Looking at the meter readings since the wood stove was started, usage is about 1.2kw/hr. So 6kw/hr with everyone alseep, lights/tv/etc off and heat pump running to 1.2kw/hr with tv/lights/coffeepot/woodstoveblower etc running but no heat pump. No doubt in my mind this is related to the heat pump, and I highly suspect it's defrost related.

doing some rough estimating... Running normally (non-defrost), the heat pump seems to use ~2kw. so assuming 20kw for heat strips, the defrost/strips would seem to be running 20% of the time to give that extra 4kw/hr.

EDIT: By the way, I didn't turn the heat pump off when starting the wood stove. It just isn't coming on as the wood stove is keeping the temp above the thermostat setting.

Tech inspected unit, only issue that could be found was pressure too high, so he removed 3lbs. Doesn't sound quite like it'd cause the spike in power usage to me, but if I were the expert, I'd have fixed it myself

Can't really gauge if it fixed the issue for a while as it's supposed to be highs near 60 for practically the next week.

 

[LD1]

A 1400 sq ft home is very small and probably 1 floor. Try heating a 2500 sq ft 2 story and maintaining 72 degrees with a heat pump when it's 0 degrees out.

A 1400 sq ft single story home is probably running a 2-3 ton system. A 2500sq ft 2-story is probably running a 4 or 5 ton system.

Its not about the size of the house. Its the size of the house in relation to the system. Size the heating system properly and there is no difference in trying to heat a 600sq ft shack when its ZERO outside vs a 5000sq ft house when its ZERO outside.

To make tractor related.......

Does a 200hp 20000# tractor labor harder pulling a 20' disc as a 100hp 10000# machine pulling a 10' disc of similar build? NO. Its about sizing

 

[Ford850]

LD1's nailed this. My heat pump runs down to 21*, with propane backup. No issues at all for 15 years now. I co-worker just converted from all electric baseboard to a similar heat pump/propane forced air system. Their HVAC contactor set it at 32*. I tried to tell them they are wasting money on propane especially on these newer systems, but they stick with the contractor's suggestion. I'm betting some contractors are setting higher lockouts to reduce call backs for first time heat pump users. If you aren't used to a HP running all the time, and producing lower duct temps than propane, the easy "fix" is to raise the lockout. Sell them on high efficiency, set them up for comfort.

 

[kenmac]

Tech inspected unit, only issue that could be found was pressure too high, so he removed 3lbs. Doesn't sound quite like it'd cause the spike in power usage to me, but if I were the expert, I'd have fixed it myself

Can't really gauge if it fixed the issue for a while as it's supposed to be highs near 60 for practically the next week.

3# too much refrigerant ? Well. I wonder how he checked the charge ?
If he's worried about charge, maybe he should have taken it all out and weighted the charge into the system . And if needed, adding refrigerant for longer run refrigerant lines

 

[ultrarunner]

LD1's nailed this. My heat pump runs down to 21*, with propane backup. No issues at all for 15 years now. I co-worker just converted from all electric baseboard to a similar heat pump/propane forced air system. Their HVAC contactor set it at 32*. I tried to tell them they are wasting money on propane especially on these newer systems, but they stick with the contractor's suggestion. I'm betting some contractors are setting higher lockouts to reduce call backs for first time heat pump users. If you aren't used to a HP running all the time, and producing lower duct temps than propane, the easy "fix" is to raise the lockout. Sell them on high efficiency, set them up for comfort.

Had a new Heat Pump installed 3 years ago and the contracted said never to run without aux below 32 or the unit would be damaged... it does sound loud when the temps get to freezing... this is my only experience as everything else has been natural gas.

 

[mjonesnh]

Had a new Heat Pump installed 3 years ago and the contracted said never to run without aux below 32 or the unit would be damaged... it does sound loud when the temps get to freezing... this is my only experience as everything else has been natural gas.

Wow! Never heard that unit could be damaged below freezing without aux heat before. We just finished a week of zero and single digits of night time temps. Happily the unit never missed a beat with aux. strips energizing as need to maintain 72 inside temp. Was very noisy at times due to frost on outside coil which restricts air flow over coil. current system has demand defrost instead of timed defrost and can go several hours without defrosting till unit sensors tells it to do so.

 

[ultrarunner]

Yep... it all new to me as previously the home was set up with electric only heat and it worked well but expensive in winter. The Electric only furnace is still set up for upstairs... with the new unit for the downstairs

The AC system went out and this is why the Heat Pump option chosen.

model number of RPQL060JEZ Rheem Pump and RHKLHM060 Rheem Air Handler

Mixed results but I agree keeping the thermostat set for a constant seems to be the way to go with a Heat Pump... tenants had complained they only get warm air from registers and no HOT...

It was a 12k install as I was not able to be there.

 

[kcflhrc]

A 1400 sq ft single story home is probably running a 2-3 ton system. A 2500sq ft 2-story is probably running a 4 or 5 ton system.

Its not about the size of the house. Its the size of the house in relation to the system. Size the heating system properly and there is no difference in trying to heat a 600sq ft shack when its ZERO outside vs a 5000sq ft house when its ZERO outside.

To make tractor related.......

Does a 200hp 20000# tractor labor harder pulling a 20' disc as a 100hp 10000# machine pulling a 10' disc of similar build? NO. Its about sizing

No difference but the electric bill, mine is a 4 ton unit in our 2500 sq ft 2 story. Lots of difference heating a 1400 sq ft ranch vs a 2500 sq ft 2 story. Air has to be pushed a lot farther and a lot more variations in temp between floors. Plus our house was built in 1970. Newer construction is much more efficient. Although ours is all brick