Heat pump crossover temperature setting

[augustus]

Hello, thanks in advance for any help!
My girlfriend just upgraded her thermostat and the ecobee assistant had her set the crossover temperature to 35F (this is the temperature where it switches to using pure electric heat). I've never had a heat pump, so neither of us thought anything of this until she got a huge electric bill. So, she has a Payne PH14NB030 heat pump (manual and pic attached).

Here's the question, how do I determine the correct crossover temperature?
Thanks!

 

[Phuckin' Jim]

If it were me, I would try setting it to 0F, and see if the house becomes uncomfortably cold.
If it does, then increase the crossover setting a bit.

 

[American Locomotive]

According to the datasheet, that unit still provides a decent COP and heating performance down to at least 17°F. I'd set the crossover to much lower.

 

[P0234]

35 is a great place for an antique heat pump. As mentioned above looking at the COP is critical but you need to be aware of defrost cycles as well which can eat into that. The best way to balance is to be able to watch the energy usage of the unit or even keep tabs on the electric meter. If that's too much work set it to 15 and see how that works.

 

[bonneyman]

Some good advice here. Settings with HVAC - especially heat pumps - depend on ALOT of factors. House orientation, efficiency rating of the unit, duct system airflow, insulation rating of walls and ceiling, etc. Typical temp settings are a trial and error process till you find what works with your specific home. A 15-20 deg setting sounds like a good place to start.

 

[larry4406]

Is she using a temperature set back function with that smart thermostat?

For example, when away she lets the temperature fall and then has it come back to temperature prior to return?

If she does then she is likely throwing it into resistance heat mode each time and this is most likely the driver of the high bill.

On heat pumps, if the temperature change requested exceeds around 3 degrees, then the resistance heat kicks in. So in set back mode for example, she goes to work and lets the temperature fall from 70F to 60F and then at 4:30 pm she commands it to comeback to 70F prior to her arrival at 5 pm; well to achieve this, the resistance mode is running full tilt.

On our new homes that have dual fuel furnaces (heat pump with propane heat back up instead of resistance heat back up), the HVAC techs set the crossover temperature to around 35-40F. The propane heat still kicks on if the temperature differential request exceeds 3 degrees F.

 

[augustus]

Thank you all for the guidance! I think I'll start at 15 and work my way down. I'm not sure how many days are left this season to really figure out the best number, although the weather seems all over the place so who knows.

Larry, that's an interesting thought, i hadn't even thought of that, and that was the main reason for buying a new thermostat. I should be able to manually induce that scenario and see if this thermostat does the same thing.

Thanks!

 

[danski0224]

Instead of guessing, why not do a load calculation for the house?

Then that answer can be put into a formula to figure it out.

There is the "balance point" where the heat loss of the structure matches the capacity of the heat pump at a specific temperature.

There is the "economic balance point", which can be useful. It takes utility costs into account. This number will change with utility costs.

These answers may not be the same.

Infiltration, AKA "ACH50" from a blower door test, will significantly change the Manual J (load calculation) results.

Or, you can just guess.

 

[P0234]

Instead of guessing, why not do a load calculation for the house?

Then that answer can be put into a formula to figure it out.

There is the "balance point" where the heat loss of the structure matches the capacity of the heat pump at a specific temperature.

There is the "economic balance point", which can be useful. It takes utility costs into account. This number will change with utility costs.

These answers may not be the same.

Infiltration, AKA "ACH50" from a blower door test, will significantly change the Manual J (load calculation) results.

Or, you can just guess.

Load calculations are great for sizing a unit, but really it feels like some badge of honor for HVAC techs to be able to complete one. Very useful for a new house. My personal opinion is that nothing beats the actual measurements. Pick a 15 or 20 degree day, let your house temp drop 3 degrees, run it back up 3 degrees and watch your power meter on HP, then repeat with coils. My HP beats my coils down into the single digits easily but the defrost cycles eat in to that once you get into the teens. Single digits the coils start winning when factoring defrost.

 

[danski0224]

Load calculations are great for sizing a unit, but really it feels like some badge of honor for HVAC techs to be able to complete one. Very useful for a new house. My personal opinion is that nothing beats the actual measurements. Pick a 15 or 20 degree day, let your house temp drop 3 degrees, run it back up 3 degrees and watch your power meter on HP, then repeat with coils. My HP beats my coils down into the single digits easily but the defrost cycles eat in to that once you get into the teens. Single digits the coils start winning when factoring defrost.

The load calc is useful in an existing house.

Put all of the information into a balance point calculator, and it will give you the results.

If the location is heating dependent or cooling dependent makes a big difference, as a heat pump in a cooling dependent climate will have plenty of "excess capacity" for heating.

 

[P0234]

The load calc is useful in an existing house.

Put all of the information into a balance point calculator, and it will give you the results.

If the location is heating dependent or cooling dependent makes a big difference, as a heat pump in a cooling dependent climate will have plenty of "excess capacity" for heating.

It useful as an estimation , but not as good as actual measurement and not practical in the case of OP unless he wants to throw some money to a "HVAC Professional", a field riddled with crackpots.

 

[danski0224]

It useful as an estimation , but not as good as actual measurement and not practical in the case of OP unless he wants to throw some money to a "HVAC Professional", a field riddled with crackpots.

VS GJ crackpots?

There are many places on the so-called "Internet" that have things other than p0rn.

Hard to believe, I know.

 

[Davefr]

Why don't you simply characterize your HP systems performance across multiple outdoor ambients. Measure output temp at a heat register at various outdoor ambients (ex: 20, 25, 30, 35, 40 degrees after 10 minutes running at approx. the same indoor temp and humidity). This should give you a rough idea where the heat output really starts to diminish.

Also keep in mind that even though it might be producing a little heat at below freezing conditions, but to do so it might be running continuously with lots of defrost cycles causing wear and tear. To preserve the life of my HP I simply switch over to resistive heat once temp drops into the 20's. It's a balance between energy savings and life of the equipment.

 

[P0234]

VS GJ crackpots?

There are many places on the so-called "Internet" that have things other than p0rn.

Hard to believe, I know.

Get over yourself. I'm not calling YOU a crackpot, but I am absolutely saying your professio,n if you are an HVAC tech is one of the most full of absolute idiots in the trades area. Its as if the majority of them are huffing on the magic cans. Its a complex field and unfortunately people smart enough to fully understand it are lured away to better paying jobs. What's left is, pretty bad. I'm sorry if you can't see that, I really am.

 

[fitter30]

There is more than oa temp for defrost. RH and the length and frec
quency. The kw of the electric heat as those charts note. Even though hp even at 0° still get at least 3.41 btus per watt the watts fall off around 27°.

 

[danski0224]

Get over yourself. I'm not calling YOU a crackpot, but I am absolutely saying your professio,n if you are an HVAC tech is one of the most full of absolute idiots in the trades area. Its as if the majority of them are huffing on the magic cans. Its a complex field and unfortunately people smart enough to fully understand it are lured away to better paying jobs. What's left is, pretty bad. I'm sorry if you can't see that, I really am.

What I see is what used to be a trade is now a commodity.

Focusing on the residential market, very few shops have the capability to make truly custom sheet metal (as in the equipment/tools), much less the people to make it. That means they are limited to what the supply house has on the shelf for sale. Whether it is a factory made part, or a flat sheet of metal and some cleat and tape to hold it together.

The only way to make money is to have a box truck set up as a mobile fabrication shop, or limit yourself to about 20 minutes from "the shop" to make stuff. Yes, you can measure beforehand, fabricate and bring it with.

It's a business, which has to make money.

The hourly labor rate is high, regardless of what the "tech" makes. Kinda like automotive repair, but the HVAC tech isn't pocketing any extra "book time".

There IS a point where it is not economical to pay for repairs to a HVAC system when the rate can easily hit $200 an hour... and the hourly rate can be buried in something called "flat rate".

Someone has to pay for all of those fancy wrapped trucks, the advertising, the uniforms... on and on.

The Office is pushing sales numbers. Therefore, there's the incentive for the hard sell. The "tech" gets evaluated on these things... metrics... KPI's... sales goals... whatever BS you want to call it.

Don't make the target(s), you can be looking for a job.

It comes down to how much money did you make me today? I don't care about yesterday.

The people that "know how to do it" take too long. At least for most of the shops out there, in my experience.

On the opposite end of the spectrum, if the employer says "skate with 8" if the job is "done" early, you will cut yourself on all of the cut corners.

I have seen plenty of people with substance abuse issues any any of the traditional construction trades. I have been laid off for not "going to the bar" after work. Eff that noise. A lot of this HAS diminished a lot over the last 20 years, but it is still out there.

The residential HVAC install is a once every decade-and-a-half or so investment. The broken equipment service call is the opportunity to make that sale, not fix it. Who wants to fix it when there is more money in replacing it? Then you get sold on the "extended warranty" that requires biannual checks to make sure the stuff is working. The shops are going in for the kill, no mercy.

 

[PoorUB]

Thank you all for the guidance! I think I'll start at 15 and work my way down. I'm not sure how many days are left this season to really figure out the best number, although the weather seems all over the place so who knows.

Larry, that's an interesting thought, i hadn't even thought of that, and that was the main reason for buying a new thermostat. I should be able to manually induce that scenario and see if this thermostat does the same thing.

Set back thermostats and heat pumps are a bad deal as the backup heat is often run to catch up when the temp is bumped up.

There may be a setting for it, droop it might be called. I am not familiar with that thermostat. If you can set the droop for farther than the set back the back up heat will not come on.

You might give up on the set back completely and find the system runs cheaper than with the set back.

As for the change over temp, study the info and give a few settings a try. My guess is 10 or 15 degrees will be about right.

As for doing a load Calc, it would be an absolute waste of money as whom ever does it will probably half *** it, guess a lot of the parameters and garbage in equals garbage out.

I gave a customer a quote for a HVAC system. I looked the home over and gave them a quote. They asked if I was going to do a Manual J as the previous company did one. I asked what size equipment they quoted, same BTU, different brand.

I went on to ask if the other company stuck his head in the attic and asked about how the walls were inslated, windows and a few other points, nope! How long did the heat loss calculation take? Maybe 15 mi utes i was told. Well, he pretty much guessed to, but he forged a bunch of numbers on paper to look important!

I explained to the home owner a proper heat loss will take 4 to 8 hours. The more info, the better the accuracy. Drop in some guesses and that is what you get in the end, a guess.

By the way, I got the job, even though we were a bit higher priced. The customer told me he appreciated that I didn't try some song and dance on him and was frank about the sale.

 

[P0234]

What I see is what used to be a trade is now a commodity.

Focusing on the residential market, very few shops have the capability to make truly custom sheet metal (as in the equipment/tools), much less the people to make it. That means they are limited to what the supply house has on the shelf for sale. Whether it is a factory made part, or a flat sheet of metal and some cleat and tape to hold it together.

The only way to make money is to have a box truck set up as a mobile fabrication shop, or limit yourself to about 20 minutes from "the shop" to make stuff. Yes, you can measure beforehand, fabricate and bring it with.

It's a business, which has to make money.

The hourly labor rate is high, regardless of what the "tech" makes. Kinda like automotive repair, but the HVAC tech isn't pocketing any extra "book time".

There IS a point where it is not economical to pay for repairs to a HVAC system when the rate can easily hit $200 an hour... and the hourly rate can be buried in something called "flat rate".

Someone has to pay for all of those fancy wrapped trucks, the advertising, the uniforms... on and on.

The Office is pushing sales numbers. Therefore, there's the incentive for the hard sell. The "tech" gets evaluated on these things... metrics... KPI's... sales goals... whatever BS you want to call it.

Don't make the target(s), you can be looking for a job.

It comes down to how much money did you make me today? I don't care about yesterday.

The people that "know how to do it" take too long. At least for most of the shops out there, in my experience.

On the opposite end of the spectrum, if the employer says "skate with 8" if the job is "done" early, you will cut yourself on all of the cut corners.

I have seen plenty of people with substance abuse issues any any of the traditional construction trades. I have been laid off for not "going to the bar" after work. Eff that noise. A lot of this HAS diminished a lot over the last 20 years, but it is still out there.

The residential HVAC install is a once every decade-and-a-half or so investment. The broken equipment service call is the opportunity to make that sale, not fix it. Who wants to fix it when there is more money in replacing it? Then you get sold on the "extended warranty" that requires biannual checks to make sure the stuff is working. The shops are going in for the kill, no mercy.

Thanks for the inside view, certainly seems like the deck is stacked against the techs and the homeowners. Around here I constantly see 5-10 year old units replaced rather than fixed, because as you say, there is no money in simple repairs.

 

[danski0224]

Thanks for the inside view, certainly seems like the deck is stacked against the techs and the homeowners. Around here I constantly see 5-10 year old units replaced rather than fixed, because as you say, there is no money in simple repairs.

I worked at a place over 10 years ago.

Owner professed to be "Christian" and did missionary work in underdeveloped countries.

They ran a $99.00 A/C clean and check "special".

We were specifically told that the price covers about 30 minutes of travel, and that we had 15 minutes to either finish the job or find something to sell.

If one reads the charging instructions on an A/C unit, you are supposed to wait 15 minutes after adding or removing refrigerant. Even 15 minutes to just check it. This isn't new.

So tell me how that job is supposed to be "done right"?

That's what the employees are up against.

 

[Jeff F]

Follow up 3 years later… what did you find? Just moved into a house with this exact same unit, with oil aux heat. Probably about impossible to evaluate where the crossover should be unless I had a way to measure oil consumption vs electric use and do all the calculations. Heating season is (hopefully) close to over, but wondering how far down this unit continues to be efficient.