Heat pumps and cold weather

[strength_and_power]

In North Texas this week, we’ve had a run of cold days, lows around 10-12* and some days not getting above freezing.
Our heat pump is an American Standard 5ton unit, 18seer I believe.
We recently had the inside unit replaced as a whole as there was a leak in the evaporator coil. When the tech came out to troubleshoot/diagnose, he noticed our Nest thermostat was configured as a dual fuel system which our system is not. He moved a wire and now we can see emergency heat in the app.
With the latest cold spell, the air coming out of the registers was definitely not warm and the unit seemed to run an excessive amount of time to get to 67*.
We don’t get a lot of super cold days and I’m new to heat pumps. What are some reasonable expectations of a heat pump?
Talking to the tech, he said it’s most likely the low temps. He is willing to come back out but I’d like to be respectful of his time and not waste it if the cold temps are the culprit.
Thoughts?

 

[lolaetype]

A very, very non-technical explanation, as I understand it.

Simply put heat pumps move heat from one location to the other. When it's cold they extract whatever heat is outside the house and move it inside. In the summer they do the opposite. The colder it is outside the less heat there is outside for the heat pump to move inside. This is why they often install electric het strips in the system. Once a certain outdoor temperature is reached the lowered effeciency is such that the heat strips are necessary to supplement heating the house.

This site explains it much better than I could.

Heat Pump Heating Efficiency: How Cold Is Too Cold?

Today's heat pumps offer efficient home heating ev

www.onehourheatandair.com

Our last house had a dual heat pump system, one for the upstairs and one for the downstairs. During a sub 30 degree old snap the downstairs unit quit and the only heat we were getting was from the heat strips. The electric bill that month was staggering compared to what it usually was.

 

[gmoss]

What sq ft is the space being heated? Do you see the emergency heat, or aux heat symbol come on, on the Tstat? It may not be coming on. When it gets that cold, it is needed to maintain warm air.

I am NC, and we have had those temps this winter. My HP is a 3 ton heating 2k sq ft. I have a 15k heat strip.

 

[strength_and_power]

What sq ft is the space being heated? Do you see the emergency heat, or aux heat symbol come on, on the Tstat? It may not be coming on. When it gets that cold, it is needed to maintain warm air.

I am NC, and we have had those temps this winter. My HP is a 3 ton heating 2k sq ft. I have a 15k heat strip.

2200 sq ft. I can manually trigger emergency heat through the app. On the display, it will say heat stage 2. . When I manually turn on the heat strips, I can feel warm air for sure.

 

[danski0224]

You need a load calculation, manufacturer heating performance data and utility costs in order to figure out (1) the balance point and (2) the economic balance point, which are not necessarily the same.

The economic balance point also changes with utility costs.

As the heat pump reaches the balance point, it is essentially "on" all of the time, which is why utility costs are important.

If you are doing thermostat setbacks to "save money", don't. Pick a temperature and leave it.

Of course, no one in this forum wants to do the load calculation stuff, so good luck.

 

[PopcornSutton]

As a rule, when the temp goes below 20, I just turn on emergency heat and let the electric coil heat. I heat with a wood stove as well. so a lot of run time is just moving heat around the house. I just don't see adding the hours on the compressor for non-efficient run.

 

[danski0224]

When I manually turn on the heat strips, I can feel warm air for sure.

This is normal.

Excluding "cold weather" heat pumps and inverter compressors, the typical heat pump with a normal single or two stage hermetic compressor will output ~80º air from the registers. This temperature will go down as the temperature drops outside given the same exclusions- to a point.

Fossil fuel condensing furnace might be ~105º F at the furnace. Electric heat coils will be similar.

Hot gas injection and/or inverter compressor is the only way to get "hot" air from a heat pump. I measured discharge air temp from a Unico System high velocity AC connected to an inverter heat pump, and got 102º F air at the register after a couple of minutes.

The Bosch IDS Ultra is hot gas injection, and it is double the wholesale cost of their "standard" inverter condenser. The inverter units are again around double the cost of a "standard" (not lowest model line) single stage scroll compressor unit.

 

[Yankeefarmer]

In North Texas this week, we’ve had a run of cold days, lows around 10-12* and some days not getting above freezing….

We don’t get a lot of super cold days and I’m new to heat pumps. What are some reasonable expectations of a heat pump?
Talking to the tech, he said it’s most likely the low temps. He is willing to come back out but I’d like to be respectful of his time and not waste it if the cold temps are the culprit.
Thoughts?

“Reasonable” expectations of heat pumps depend entirely on the specific heat pump in question. As others here have noted, traditional heat pumps‘ heating performance drops off rapidly at temperatures around and below freezing. Inverter and other cold weather heat pumps can produce useable heating at outside temps below zero. My shop is heated with such a unit, because natural gas is not available and I no longer wanted to deal with the cost and inconvenience of propane.

 

[danski0224]

“Reasonable” expectations of heat pumps depend entirely on the specific heat pump in question.

Yes.

It comes down to the load calculation and manufacturer specifications. Specifications being heating output btu's at a given outdoor temperature. This also varies with fan speed (cfm) and indoor coil size... and connected ductwork.

One can have the right blower and coil, but if the ductwork is ****, the results are ****. The ductless mini split eliminates the ductwork incompetence.

Math and design that apparently many/most HVAC contractors do not want to do, much less people posting in this forum.

 

[fitter30]

Standard efficient hp usually published at 17° and 47°. Can call the manufacturer and they can put up specs for different temps. Need complete model and serial numbers of outside unit, air handler and the evap coil if possible. Ask for tech. All heat pumps lose efficiency the colder the outside temp is except for some hyper heat minis.

 

[mm08822]

As a rule, when the temp goes below 20, I just turn on emergency heat and let the electric coil heat. I heat with a wood stove as well. so a lot of run time is just moving heat around the house. I just don't see adding the hours on the compressor for non-efficient run.

If your wood stove is large enough and stoked for 24/7 operation, wouldn't it make sense to just run the furnace blower?

 

[PopcornSutton]

If your wood stove is large enough and stoked for 24/7 operation, wouldn't it make sense to just run the furnace blower?

For the most part yes, except by 2 or 3 in the morning the fire is getting low, so the heat keeps it up before I get up and re-stoke the fire.

 

[PopcornSutton]

I have a temperature sensor outside near the condenser unit, one in the return duct close to the air handler and one in the supply duct a couple feet down stream of the AHU. The readouts are mounted on a small panel I made, so I can see exactly what the unit is doing in relation to the outside temperature. With a direct exchange coil, on a conventional r410a heat pump, a 20-22 degree difference across the coil is about all you'll get at 70 degrees outside. In cooling mode, I don't see a fall off of that 20 degrees until it gets about 90 outside. In heat mode, I notice the fall start at about 40. When it gets to 20 outside, I'm only getting about 16 degrees of exchange. I just don't see the reason to let the compressor grind away for that little increase, it would run almost constantly to keep the house at 68.

 

[mm08822]

I have a temperature sensor outside near the condenser unit, one in the return duct close to the air handler and one in the supply duct a couple feet down stream of the AHU. The readouts are mounted on a small panel I made, so I can see exactly what the unit is doing in relation to the outside temperature. With a direct exchange coil, on a conventional r410a heat pump, a 20-22 degree difference across the coil is about all you'll get at 70 degrees outside. In cooling mode, I don't see a fall off of that 20 degrees until it gets about 90 outside. In heat mode, I notice the fall start at about 40. When it gets to 20 outside, I'm only getting about 16 degrees of exchange. I just don't see the reason to let the compressor grind away for that little increase, it would run almost constantly to keep the house at 68.

That was my point exactly. Metered energy input ( compressor + blower, or comp + strips + blower vs. blower only while on wood stove) is what needs to be managed. Yes, I'm fully aware cutting/splitting fire wood, stoking/cleaning stove, cleaning chimney is indirect cost but for some is abundent and therapeutic (no need for gym memberships).

 

[theoldwizard1]

The air handler and the compressor need to be closely matched to get the best performance. I would be nervous replacing one, but not the other.

Also, try to find system performance numbers for cold weather operations. All systems drop off below freezing. Some dop off a lot and require a heat strip (expensive to operate) in order to get reasonable indoor temperatures.

 

[TRITOON]

I'm 1 state north of the OP.

I have a carrier heat pump at the lake and use a non-smart heat pump with emergency heat backup thermostat. Some of the digital ones have setting sthat after 30-45-1hour time amounts if heat set points arent reached it goes to emergency. God, i hate explaining it because they are all different with tons of settings and it gets complicated quick with heat droop, timers, is there outdoor thermostat, an all the other nonsense. As others have said, whats the savings when the heat pump runs for an hour when emergency heat would only need 15 mintues?

Just get a plain non-smart thermostat and manually switch it over when it gets below 10 - 20 degrees or so. I got tired of waking up during snow/ice storms during the night and hearing what sounded like a freight train as the heat pump did its thing. Is it working, is it defrosting, is there ice?....Nothing ever broke. But pay the price premium for heat strip heat during the rare couple of nights of really cold weather.

The rest of the time i love heat pump heat, but it does have some sort comings during really cold weather.

 

[danski0224]

I have a temperature sensor outside near the condenser unit, one in the return duct close to the air handler and one in the supply duct a couple feet down stream of the AHU. The readouts are mounted on a small panel I made, so I can see exactly what the unit is doing in relation to the outside temperature. With a direct exchange coil, on a conventional r410a heat pump, a 20-22 degree difference across the coil is about all you'll get at 70 degrees outside. In cooling mode, I don't see a fall off of that 20 degrees until it gets about 90 outside. In heat mode, I notice the fall start at about 40. When it gets to 20 outside, I'm only getting about 16 degrees of exchange. I just don't see the reason to let the compressor grind away for that little increase, it would run almost constantly to keep the house at 68.

If you had a load calculation and manufacturer performance data, you can get the same info as the.......






Balance point.

 

[dcg9381]

We don’t get a lot of super cold days and I’m new to heat pumps. What are some reasonable expectations of a heat pump?
Talking to the tech, he said it’s most likely the low temps. He is willing to come back out but I’d like to be respectful of his time and not waste it if the cold temps are the culprit.
Thoughts?

The standard "builder grade" stuff they install in Texas is generally not designed for cold weather performance. What you are seeing is normal, as it gets colder and colder (say below 20) the heat pump simply doesn't work very well and your temp rise against interior ambient will reduce.

These are great options for places where it's not cold often, but once it's cold, you may have to trigger EM heat (resistive). My biggest power bills were in the winter... I eventually started supplementing with a 30k ventless propane heater downstairs.

My 17 SEER ductless does the same thing.. It's just not a cold weather or performance (hyper-heat) model.

 

[firebirdparts]

. What are some reasonable expectations of a heat pump?

The colder it gets, the harder it is for the heat pump to keep up. They have improved dramatically, but obviously there is a basic problem. The house is losing more heat and the heat pump is pumping less heat. Fundamental. if it's sized just right for 30F, then that's not 10F and that's fundamental. With a variable speed compressor, you could tilt the economics so that you might choose to fight back more, and that is a feature nowadays.

In the thermostat, you will have the resistance heat (emergency heat) requested when the setpoint/actual offset is higher than some number (let's say 1 degree). This means that moving the thermostat around will call for emergency heat.

I have no idea how the thermostat is programmed to turn the emergency heat off. The basic expectation is that it would turn off when the offset gets to less than 1 degree, but your thermostat may do something else. Also, there has always been a potential feature where low outside temperature, measured directly, could cause your emergency heat to stay on until the setpoint is reached. This was always a fancier heat pump option in the electro-mechanical days. With computer controls, I don't really know what your thermostat is doing.

Finally, heat pump outdoor units will freeze water from the air and they have to defrost occasionally. That gets much worse when it's cold. Defrosting is commanded by the outdoor unit. While defrosting, emergency heat will be commanded by the outdoor unit.

This should be obvious but the emergency heat "mode" on a thermostat would not call for the compressor and then would reach the setpoint with resistance only.

 

[WildBill]

It depends on the heat pump, we mostly install the hyper-heat ones around here and they work fine at below zero temps. My shop unit is not a hyper-heat and heating capability drops off rapidly under about 10F. I recently checked and at 10F it was putting out around 75F air. It was running at -18F a couple weeks ago but only putting out around 50F air. Mine does not have any type of heat strips. They tend to spend a lot of time defrosting right around freezing, usually when it gets colder its less humid so they don't defrost as much under about 30F.

 

[dcg9381]

They tend to spend a lot of time defrosting right around freezing, usually when it gets colder its less humid so they don't defrost as much under about 30F.

In a cold snap in 2021, I eventually just shut off the ductless splits. They would defrost the coils OK, but the fans had so much ice on them that they sounded absolutely terrible when running...

 

[Model A Fan]

Is a heat pump worth installing if the temps in my area reach the teens? I have a propane furnace for half the house and a wood stove for the other half of the downstairs. Upstairs I have Cadet brand wall heaters. My house is 120 years old, so no "real" central heating.

Would it be worthwhile to install a heat pump for the upstairs rooms and have the ducting for them in the attic? How about the downstairs area that is primarily heated by propane? I would like to get away from propane unless its really cold out. My hope would be to have the downstairs be heat pump + wood for most of the fall and winter and use propane seldomly. The propane furnace doesn't do a great job unless on full blast and at $600+ per fill up, it adds up pretty fast.

 

[PopcornSutton]

If you had a load calculation and manufacturer performance data, you can get the same info as the.......






Balance point.

I had the unit (this is the second in this house but both 2-1/2 ton) sized by a mechanical engineer whom I used to work with.

It still remains, for the style unit I have which is a single stage 13SEER, when charged properly, coils clean and filter new, there is still only going to be a 20-22 difference across the evaporator coil within the optimum outside temperature. Whether it is over sized or under sized, the unit is relatively dumb, it runs the same. The outside temp and the return air temp effects the supply temp.

 

[ericm]

Is a heat pump worth installing if the temps in my area reach the teens?

Yes. Especially in the PNW where electricity is cheap.

People in Maine use mini splits. It reaches the teens in the part of Oregon where my new house is getting built and most new houses there use mini splits.

If it works for your install, mini splits are more efficient than ducted heat pumps.

 

[u2slow]

Many older heat pumps can't actually heat worth a darn. It's AC-primary, and lean hard the aux/em heat elements.

I had to beef up my element stack from 4.8kW to 9.6kW to be able to leave the house for 1-2 weeks in the winter and not have pipes freeze. When I am home, I burn wood for heat.

 

[danski0224]

Many older heat pumps can't actually heat worth a darn. It's AC-primary, and lean hard the aux/em heat elements.

Not necessarily true.

I have a now ~16 year old two stage (single scroll compressor) R-410A unit.

It heats just fine... as long as the design parameters are honored.

Heating btu output is determined by the outdoor temperature, and whether or not that's enough for my house is determined by the balance point... which is dictated by the load calculation and manufacturer equipment performance data (specifically heating output at 47º F).

Just based upon continual issues posted in this forum, I am willing to bet that most equipment is not installed properly, nor is it set up properly, and no one told the homeowner what they bought and how to use it.

 

[pcmeiners]

Just based upon continual issues posted in this forum, I am willing to bet that most equipment is not installed properly, nor is it set up properly, and no one told the homeowner what they bought and how to use it.

When performance issues occur I believe most equipment is the wrong equipment, either due to the homeowner unwilling to pay for the correct units or the seller is taking them for a ride. Sadly the latter happens to often.

 

[danski0224]

the seller is taking them for a ride.

This.

I just quoted a job, homeowner told me that my price was a little higher.

However, the other bids included ZERO WORK to the ductwork, just a box swap.

My price included part of the additional work required, and the rest was recently discussed.

Homeowner said that the "other guys" spent maybe 30 minutes discussing a $20k job.

Criminal.

 

[u2slow]

Not necessarily true.

I have a now ~16 year old two stage (single scroll compressor) R-410A unit.

It heats just fine... as long as the design parameters are honored.

Heating btu output is determined by the outdoor temperature, and whether or not that's enough for my house is determined by the balance point... which is dictated by the load calculation and manufacturer equipment performance data (specifically heating output at 47º F).

Mine is similar. It is charged and working properly.

It came with the house, and was squeezed onto a 100A service. It might be undersized, house likely under-insulated, and duct work could be tighter . Whatever the case, I stop trying to use it for everyday heating when the outdoor temps swing below 14°C. For the AC side of it though, I can't complain.

 

[mm08822]

It all makes sense if you can follow this example:

The "boiler" energy input could be from any other source such as heater elements.

 

[ericm]

A hyper heat unit heats well at low temps. For example the Mutsubishi MSZ-FS18N, an 18k unit, can put out 23k btus at 5 degrees F using 3100 watts. A heat coil would need 7500 watts. It will make 17k btus at -15 degrees F though they do not give the power consumption.

Unless it gets really cold where you are, if you get the right heat pump you no longer need a second heat source to take over at low temps.

 

[theoldwizard1]

But pay the price premium for heat strip heat during the rare couple of nights of really cold weather.

If they are RARE, you are correct ! Probably cheaper than paying a big time premium for a heat pump capable of producing warm air down to 0F !

 

[Hobby_Man22]

In North Texas this week, we’ve had a run of cold days, lows around 10-12* and some days not getting above freezing.
Our heat pump is an American Standard 5ton unit, 18seer I believe.
We recently had the inside unit replaced as a whole as there was a leak in the evaporator coil. When the tech came out to troubleshoot/diagnose, he noticed our Nest thermostat was configured as a dual fuel system which our system is not. He moved a wire and now we can see emergency heat in the app.
With the latest cold spell, the air coming out of the registers was definitely not warm and the unit seemed to run an excessive amount of time to get to 67*.
We don’t get a lot of super cold days and I’m new to heat pumps. What are some reasonable expectations of a heat pump?
Talking to the tech, he said it’s most likely the low temps. He is willing to come back out but I’d like to be respectful of his time and not waste it if the cold temps are the culprit.
Thoughts?

Mine pretty much runs nonstop below freezing. It's my understanding most are rated down to zero degrees now. Mine is only good to 40 for some reason. It's a 5 ton goodman installed in 2020

 

[strength_and_power]

Mine pretty much runs nonstop below freezing. It's my understanding most are rated down to zero degrees now. Mine is only good to 40 for some reason. It's a 5 ton goodman installed in 2020

According to the Nest thermostat, we had a day where it ran for 21 hours.
Debating if it’d be worth it to add a propane furnace. House is all electric now, I wouldn’t mind a gas range. Not sure what it’d take to add gas to an existing house.

 

[Hobby_Man22]

According to the Nest thermostat, we had a day where it ran for 21 hours.
Debating if it’d be worth it to add a propane furnace. House is all electric now, I wouldn’t mind a gas range. Not sure what it’d take to add gas to an existing house.

My electric bill made no sense though. We had two back to back freezes almost and the bill was nearly identical $150 for December and January. Technically they don't pull much power once running, so I guess you save a bit on startup loads.

 

[danski0224]

It's my understanding most are rated down to zero degrees now.

"Rating" does not equate to manufacturer performance data.

Mine is only good to 40 for some reason. It's a 5 ton goodman installed in 2020

Manufacturer heating performance chart, evaporator coil size and blower size will give you heating performance.

Then you need a load calculation to figure out the balance point.

That's based on charts.

If the house is built like ****, and if the ductwork is ****, the engineering charts also become ****.

There is no such thing as "some reason".

There is a reason.

You need the data.

 

[Notgrownup]

I’m no HVAC guru by no means but heat pump systems I believe are designed for different regions. when visiting my family in Canada their system keeps up pretty well but I think they use “Hyperheat” not sure what the difference is but mine doesn’t have that.. I’m pretty sure load calculations and styles of machines come into play.

 

[Notgrownup]

According to the Nest thermostat, we had a day where it ran for 21 hours.
Debating if it’d be worth it to add a propane furnace. House is all electric now, I wouldn’t mind a gas range. Not sure what it’d take to add gas to an existing house.

I have a gas wall heater and it really helps getting the house up to temp on colder days, I have a Gas Rinnai water heater and a gas range. The gas range I really love.

 

[TRITOON]

According to the Nest thermostat, we had a day where it ran for 21 hours.
Debating if it’d be worth it to add a propane furnace. House is all electric now, I wouldn’t mind a gas range. Not sure what it’d take to add gas to an existing house.

No, it wouldnt be worth it.

If you want to add some space heaters, wood stove, pellet stove, etc for the coldest days of the year to increase comfort, then go for it. But it running 21 hours on the coldest day of the year isnt horrible. If its running non stop for days/weeks then yeah, propane might be better.

Dont look for a 20k solution when a $100 will do. Getting a propane tank put in, having gas plumbing ran, rent/owning the tank, keeping and eye on it and paying a heck of alot more for someone to come fill the tank when its cold, and then upgrading your furnace...

I would go for an electric blanket and a few off season space heaters; i personally like the oil radiator style. Silent, safe and will heat the entire room.

 

[pembol]

According to the Nest thermostat, we had a day where it ran for 21 hours.
Debating if it’d be worth it to add a propane furnace. House is all electric now, I wouldn’t mind a gas range. Not sure what it’d take to add gas to an existing house.

I would also look into getting a more appropriate heat pump for your area installed, there are plenty that would work fine, and it would likely be cheaper than running gas and adding a gas furnace. It would likely also save you some money on AC, as the efficient units tend to be efficient on both cycles. Or just add some radiant heat as is suggested above.