Heat Pump and power consumption

[kenmac]

When it's "full on" winter up here, I heat with a wood boiler & radiant heat. BUT we just put a small addition on and put a "2 head" mini split system in, primarily for A/C. Just for kicks the other day, (it was about 8*) I switched one unit on, it was throwing out hot air!
I'm not sure I would rely as a h/p in our climate without a backup, but that's just a guess on my part. Anywhere south of the Mason Dixon line I think that they are the way to go!

They also make a ducted / split VRF system that will heat down to -x without aux heat.

 

[Sigarms]

They also make a ducted / split VRF system that will heat down to -x without aux heat.

I had asked you a question per VRF in another thread without a response.

Are you talking a single phase VRF system wtith the ability to heat and cool multiple zones at the same time? If so, what brand, what phase and two or 3 pipe design?

 

[Turbys_1700]

We replaced our heat pump 3-1/2 years ago with a Goodman. I think it is 2-1/2 tons with a 13 seer. My wife checked last month and out total cost was $190. This was an extremely cold month for us with a cold week in the lower teens and single digits. My house was built in 53 and had insulation installed 25 years ago when we put the first unit in. All in all I don't think that's too bad for a 1420 sq. foot brick home with 18 windows. I need to replace all of them but have only replaced 8 so far. Would help out a bunch.

 

[techman]

I want to correct some comments posted here. A modern heat pump (made within the last 10 years) will produce more heat than the electric power it uses into the low teens or single digits. The issue with heat pumps is that the colder it gets, the less heat it will produce (colder outside air contains less heat), but the heat it is making is still more efficient than electric strips. So at 20 deg it may only produce 50 or 60 percent of the nameplate rating. At the same time, the colder it is, the home's heating requirements go up. The temperature where the heat produced and the heat required are equal is called the balance point. Below that temp, some electric aux is needed, and above none is needed. The balance point of a unit depends on efficiency, sizing, heat loss, etc but is generally in the upper teens to low 20's for a properly sized unit.

Defrosting can bring on the aux heat when above the balance point. About 90% of all units sold use what is known as Time-Temp Defrost. This can control can result in unneeded defrosts, especially at lower temps. A unit with Demand Defrost will only defrost when needed, based on temperature sensors. Demand defrost can save 10-15% on your bill, depending on where you live, and other factors. There is a defrost controller on the market, the Emerson 47D01U-843 which is an upgrade/replacement controller for most units on the market. You could get one installed for $300-$500 by your HVAC guy. That can eliminate unnecessary defrost cycles.

The other factor is the user. DO NOT set back the temp unless you will be gone for days. Constant temp setting is the best option. After a setback, the requirement for a rapid temp rise will cause most units to kick-in aux heat. Some modern thermostats that are programmed for a setback will slowly rise temperature and not trip the aux heat, but in general the savings from a setback are exaggerated.

Last item is your unit's condition. If you have a refrigerant leak, or an undercharge, efficiency will be reduced and the outside unit will tend to ice up more frequently and therefore defrost more often. Unfortunately properly checking the charge level in cold weather is difficult, and not done properly by many service techs.

paul

 

[Gem99ultra]

KTurner, with our Bryant heat pump, our electric bill was 3X higher than the same month last year, i.e. $324 vs $106. Only slightly less than the a/c cost for last August! I've been told by the HVAC tech that checks out my system twice a year that our heat pump is very inefficient at temps below 40*. For sure I can see the meter disc spin like crazy any time the heat pump goes on. And yes, I definitely had 'sticker shock' when I got my electric bill.

We've began supplementing the heating with a propane insert in the fireplace. That warms the main living area instantly, but does little for the adjacent rooms. With all of this extremely cold weather, I have no suggestion other than just hang in there. Spring is not far away.

 

[Bird]

The other factor is the user. DO NOT set back the temp unless you will be gone for days. Constant temp setting is the best option.

It seems that I've heard from most of my life that you should turn the heat down at night when you go to bed; sleep better in a cooler room. And of course I grew up in the days before air-conditioning and all the heat was turned off at night.:laughing: However, we never mess with our thermostat at all. It stays the same 24/7 and has for several years. The only time it's changed at all is when the service man comes twice a year to check everything.

 

[kenmac]

I had asked you a question per VRF in another thread without a response.

Are you talking a single phase VRF system wtith the ability to heat and cool multiple zones at the same time? If so, what brand, what phase and two or 3 pipe design?

sorry, I guess I missed the other thread you are talking about

condenser is similar to this Gree MULTI48HP1AO VIR9HP 3-VIR12HP 45, BTU

matched up to AH similar to below

G5CD-AU54-3_Submittal.pdf - Google Drive

 

[LD1]

Excellent post by techman.

Curious to see what the OP finds from a service call.

Going from 60kwh a day to 170kwh per day......and even on a day ranging from 24F-37F (mild in my book) just don't sound right.

Either electric strips aren't shutting off or something wrong elsewhere in the house.

Thinking outside the box......only two things in a typical house I can think of that will effect a bill that much....1. left stove on. Unlikely as most would recognize next time in the kitchen.

2. Hot water leak. This will drive a bill crazy high with just a slight drip/leak. So inspect hot water heater, T&P valve, etc

 

[KTurner]

Excellent post by techman.

Curious to see what the OP finds from a service call.

Going from 60kwh a day to 170kwh per day......and even on a day ranging from 24F-37F (mild in my book) just don't sound right.

Either electric strips aren't shutting off or something wrong elsewhere in the house.

Thinking outside the box......only two things in a typical house I can think of that will effect a bill that much....1. left stove on. Unlikely as most would recognize next time in the kitchen.

2. Hot water leak. This will drive a bill crazy high with just a slight drip/leak. So inspect hot water heater, T&P valve, etc

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.

 

[LD1]

Do you have a meter with an amp clamp? Or even the FLA of the compressor unit should get close to being able to figure out a kw per hour usage.

I know my Geo....fan, pumps, and compressor......pull 16.2a on first stage, and 27.5 amps on second.

That's right at 4 kwh per hour on first stage.....and 6.7kwh per hr on second.

I know this for two reasons. First.....if I have any issues such as what you are experiencing.....I know my baseline.

Second......because I can convert the 4 kwh and 6.7 kwh into BTU's.
Meaning......I can figure how many BTU's that electric would have netted me .....in the form of resistance heat. Like strips or baseboards.

I also know that my blower is 1000cfm on low and 1600 on high.

So I can measure temperature differential....combined with cfm.....I can figure my BTUs of actual output. And compare them to if I had straight resistance heat.

I have learned my Geo is 4.5-5x more efficient.

At my old house, I had a 15 year old....Rheem 10 seer 3-ton air to air unit.

Did the same calculations.....at various outdoor temps. I was still 2x more efficient than resistance down to 17 degrees. (But I had propane backup). And based on propane prices...below 17 was when I set the lockout and started to burn propane.

IMO....HVAC contractors get WAY too conservative with the lockout point on the compressor. Everyone too many contractors seem to think that below 35-38 degrees F you should run backup to be more efficient.

It all depends on your backup though. Electric resistance? Propane? Or natural gas?....and what the costs of those are.

I encourage everyone to learn this stuff themselves. Find out what is most efficient for your home based on costs of energy....and your system efficiency. The numbers aren't hard to attain, and aren't hard to punch into a simple formula.

With my 1995 Rheem being more efficient than both electric resistance and propane down to 17-20F.....I couldn't imagine a modern unit locking out at 35+. That's just crazy....yet it happens and is preached as gospel by many even to this day.

 

[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