DIY Home Improvement Forum banner

1 - 20 of 50 Posts

·
Registered
Joined
·
108 Posts
Discussion Starter #1
Right now I am looking at the outside temperature of 33F and 100% humidity. On my smart thermostat screen, that gets the weather data over Internet.
The dew point is 33F (based on Accuweather).

My "smart" Honeywell TH 9320 WF5003 is insisting of using the heat pump, even is it "knows" that this will freeze the coils. But it doesn't care.
I have cold air blowing from my vents, a glop of ice on the condenser unit, but the t-stat thinks it's saving me money.
More stupidity, if I set the temperature two degrees higher, it will start "Auxiliary Heating". But will also spin the heat pump compressor, to "help" I guess.
I have to manually switch it to "Emergency Heat" to actually make the compressor to stop. Or raise the call temp with 4 degrees, to force the "Aux Heating" only.

Is there a true smart T-stat that will behave better in this situation? I have the "C" wire available.

PS: I used to own an older LuxPro PSPU732T that had the option to decouple the functionality of the heat pump from the electric heat. And programmable temperature when to switch between those. It stopper working correctly and I assumed that a new, WiFi connected T-stat will be at least as good.
 

·
Registered
Joined
·
13,181 Posts
I think you don't really understand how a air source heatpump is designed to operate.

Having the outdoor coil ice up is normal especially in cold, wet conditions and it's supposed to automatically defrost - either after a time delay (generally 60 or 90 minutes) or when it detects there's too much ice.

If yours does not defrost, there's a problem - call a tech.

Emergency heat mode should only be used for breakdowns and when the outdoor temperature is so cold the heatpump isn't putting out enough heat to make it worth running, generally between 0 and 10F outside.
(better thermostats can be set to automatically lock out the heatpump below x outdoor temp)

Running on 100% electric is extremely wasteful and costly.
When it's too cold for the heatpump to supply all the heat required, the electric elements should be cycled on and off while the heatpump runs continuously.

It's absolutely silly to shut down the heatpump when it's still picking up significant amounts of "free" heat from outside. (the energy used by the compressor gets absorbed by the refrigerant and is not wasted)

I don't know what your definition of "cool air" is.
In colder weather, the supply air without the aux heat on may be only 10 to 20F warmer than room, the colder it is outside the less heat you get out of it.
Considering body temperature is in the high 90s, the supply air can feel luke-warm, not hot.

At 33F outdoor, I would expect 85 to 90F supply air when the return is at 70. The efficiency should be in the range of 200 to 300%+ depending on your unit.
Now, if it's not heating at all under these conditions, there's a problem - switch to emergency heat and call a tech.

Lux t-stats are known to be junk, honeywell is far better.
I'm unsure if yours allows for a heatpump lockout.
 

·
Registered
Joined
·
450 Posts
I think you don't really understand how a air source heatpump is designed to operate.

Having the outdoor coil ice up is normal especially in cold, wet conditions and it's supposed to automatically defrost - either after a time delay (generally 60 or 90 minutes) or when it detects there's too much ice.

If yours does not defrost, there's a problem - call a tech.

Emergency heat mode should only be used for breakdowns and when the outdoor temperature is so cold the heatpump isn't putting out enough heat to make it worth running, generally between 0 and 10F outside.
(better thermostats can be set to automatically lock out the heatpump below x outdoor temp)

Running on 100% electric is extremely wasteful and costly.
When it's too cold for the heatpump to supply all the heat required, the electric elements should be cycled on and off while the heatpump runs continuously.

It's absolutely silly to shut down the heatpump when it's still picking up significant amounts of "free" heat from outside. (the energy used by the compressor gets absorbed by the refrigerant and is not wasted)

I don't know what your definition of "cool air" is.
In colder weather, the supply air without the aux heat on may be only 10 to 20F warmer than room, the colder it is outside the less heat you get out of it.
Considering body temperature is in the high 90s, the supply air can feel luke-warm, not hot.

At 33F outdoor, I would expect 85 to 90F supply air when the return is at 70. The efficiency should be in the range of 200 to 300%+ depending on your unit.
Now, if it's not heating at all under these conditions, there's a problem - switch to emergency heat and call a tech.

Lux t-stats are known to be junk, honeywell is far better.
I'm unsure if yours allows for a heatpump lockout.
What they said
 

·
Registered
Joined
·
450 Posts
What is the ACTUAL temperature being delivered from your registers when your heat pump is operating?

Please post both the return air temp and the supply air temp.
 

·
Registered
Joined
·
34 Posts
The heat pump is suppose to run when its 30 degrees out and it is suppose to freeze up. But the defrost system is suppose to come on to defrost it. If it is not defrosting or if you have excessive ice, then your defrost board is not working or you are low in refrigerant (we just went through this)
 

·
Registered
Joined
·
474 Posts
Hi, I would imagine that you have the stat setup for heat pump operation under system preference.
what happened to the old Stat that made you think it stopped working?
 

·
Registered
Joined
·
108 Posts
Discussion Starter #7 (Edited)
I think you don't really understand how a air source heatpump is designed to operate.
"The coil is supposed to frost up". That's the BS line that we are told since 1960's.
Yes, the condenser's control board will eventually defrost the coil, but WHY would we run the unit when we KNOW that will freeze?
In the time when all we had were mercury contact t-stats, that was the state of the art - if coil freezes, just use more energy to defrost it.

In Anno Domini 2021 the thermostats can connect to the Internet and KNOW the outside temperature and humidity, they proudly display it. So it's easy to know when the coils would freeze, why even try to run the compressor in that specific combination of temperature and humidity?

Looks like all this fancy new "intelligent" software is stuck in the 1960's control mindset. Yes I can control it with Alexa, but the basic operation remained the same.
My question was if there is out there a thermostat that is truly intelligent in this respect.
 

·
Registered
Joined
·
13,181 Posts
"The coil is supposed to frost up". That's the BS line that we are told since 1960's.
Yes, the condenser's control board will eventually defrost the coil, but WHY would we run the unit when we KNOW that will freeze?
In the time when all we had were mercury contact t-stats, that was the state of the art - if coil freezes, just use more energy to defrost it.

In Anno Domini 2021 the thermostats can connect to the Internet and KNOW the outside temperature and humidity, they proudly display it. So it's easy to know when the coils would freeze, why even try to run the compressor in that specific combination of temperature and humidity?

Looks like all this fancy new "intelligent" software is stuck in the 1960's control mindset. Yes I can control it with Alexa, but the basic operation remained the same.
My question was if there is out there a thermostat that is truly intelligent in this respect.
The thermostat is not the problem!

The coil will freeze when it's running below freezing and there's moisture in the area, just like the coil in your freezer also freezes up, but there's a defrost mechanism.

Your heatpump works the same way as a freezer -> pulls the heat out of the air and moves it into another space.
The freezer stays cold despite the defrost cycles.
 

·
Registered
Joined
·
34 Posts
The reason a heat pump continues to run in the cold and is allowed to freeze is because in most situations it is still more efficient and will cost you less than heating with just the secondary heat source (most have pure electric that is expensive to run).
 

·
Registered
Joined
·
108 Posts
Discussion Starter #10 (Edited)
For all that keep saying that is "normal": I am fully aware of the physics involved. Humidity from outside 100% humidity saturated air will condensate and freeze on colder than 32F coils, thank you Dr. Obvious!

What I want is a thermostat smart enough not to run my condenser in those conditions. It's probably not more efficient - manufacturer testing that says otherwise is done with coils free from ice. Sure at 25F, with 10% humidity, I believe them. But not in this specific case (33-34F, 96-100% humidity), where the unit will have run the defrost cycle very often. That puts wear and tear on my unit for no gain.
I just heard the unit outside my office window running for 20-30 minutes trying to keep the 71F temp in the house. It stopped for like a minute and now it just started a defrost cycle (compressor running with the unit fan off). That's just wasted energy and wear on my compressor.

And in any case, colder air (91F) blown from vents for longer periods, only to switch to Aux Heat (98F) after like 30 minutes, is not pleasant for me, as owner. Don't care if I have to pay extra.

LE: The defrost run like 5 minutes. Then another 1 min pause and condenser starts again in heating mode.
 

·
Registered
Joined
·
13,181 Posts
You don't know what you're talking about and there's no value in asking how to do something unwise from those who know more than you and than saying "you're wrong".

Suite yourself, if you want to shut your heatpump when it's still delivering 2 to 3 times as much heat as energy it's consuming, with maybe a 10 to 20% penality for defrosts, it's not our problem. It's supposed to run a lot under these conditions, and despite needing to defrost it's far more energy efficient than running on strip heat entirely.
The HSPF efficiency ratings do include the impact of defrost cycles, fiy.
 

·
Registered
Joined
·
108 Posts
Discussion Starter #12 (Edited)
You don't know what you're talking about
Don't insult me, I probably know more than you assume. Just repeating the mantra that "heatpump when it's still delivering 2 to 3 times as much heat as energy it's consuming" doesn't make you smarter than anyone else.
Defrost cycles wears out the equipment. Also HSPF is seasonal, and the power ratio can be 2.25 for a good unit (with a 7.7 HSPF rating). But in this specific instantaneous instance (frosted coils + defrost), the actual efficiency is way lower than the average for the season. I happen to think is unitary. No need for me to run the unit for no gains.
The manufacturers won't consider those specific conditions when publishing their efficiency numbers. They use an average season.

Anyway, back to my original question - does anyone know a thermostat that can programmed (intelligently or not) to avoid freezing the coils when the outside temps are in 31-34F range with humidity in 95-100% range?
 

·
Registered
Joined
·
108 Posts
Discussion Starter #13
Seems like what I want can be accomplished with am IFTTT-aware device.

Example: If weather is "this" then use "Aux Heat"... Will have to dig more into this.
 

·
Administrator
Joined
·
40,845 Posts
The heat pump is still more efficient with frost on the coils, than electric resistance heat. It won't provide the same amount of heat as non frosted coils. But still cheaper to use than just resistance heat.

That is why many manufacturers went to on demand defrost controls, instead of just time and temp.

Yes, sleet and freezing rain can and often will coat the coils more than the defrost can handle, and the unit should be switched to emergency heat during that time period.

But, there is not enough demand for a thermostat to do what you want. Perhaps if you write to Google/Nest about it, they will include that feature in an update to their firmware/software. Never know, might be a small reward in it for you if they use your idea.
 

·
Registered
Joined
·
450 Posts
I have always preferred the Honeywell Pro 8000 series thermostats.

Have a look at option 350 located on page 8 (heat pump compressor lockout) in the setup menu. This should achieve what you want. It provides a description of how the function works on page 11.

http://www.honeywellmanual.com/pdf/honeywell 8000 installation manual.pdf

When the coil freezes the efficiency drops. Just because it’s frozen doesn’t mean it has dropped below the efficiency rating of electric heat. It needs to get pretty cold for that to happen when the heat pump is operating correctly.

I service heat pumps that are beyond their 20th year in use. They preform defrost cycles as required. I don’t think it’s nearly as hard on the equipment as you think it is. I would be interested to see what data you have on this, perhaps I’m overlooking something. When compared to the starting of the compressor I would say that is far more hard on a motor and it’s components than the switching into a defrost cycle. On the equipment I primarily support we do everything we can to keep the compressor running. This means injecting hot gas into the suction line so we can continue to run the equipment even when the room load is low.

When a system enters defrost mode it is switching the hot gas and suction lines. Momentarily there is hot gas that runs through the compressor, but the compressor is running unloaded during this time so the increase in noise that you hear is the motor running at unloaded RPM and the scroll plates may separate to bypass the hot gas as it equalizes. Lubrication is maintained because the compressor continues to operate, compare that to starting the compressor from its off state and you will find the motor runs momentarily with no lubrication while it establishes oil pressure and stresses the motor windings because it draws lock rotor amperage while it is starting.

... I could go on for hours here but it’s Valentine’s Day, so I’m going to wrap this up.

Although the compressor runs longer while producing less heat in cold weather, the wear and tear on the compressor is probably less than what you get from running the heat for shorter run cycles in warmer weather, and cooling in hot weather, because you are starting the compressor less times with this longer run cycle. So although your energy efficiency may not be at its maximum, the wear and tear will be reduced. As long as the efficiency is above the 1:1 that you get from electric then you are probably better off to use the heat pump.

I’m happy to hear your thoughts on this. I agree, some factors are not going to be included in their ratings systems that are produced. Cost of maintenance, equipment wear, etc, will not be included so if we really want to microanalysis and compare them results will be quite variable since those factors that are not included are variable.

Typically when a system fails early or has a short life, there was a design condition that was not maintained within spec or that was completely ignored. If the system is operating correctly then it should provide many years of trouble free service. This is why the user that first responded to you was suggesting that you have the system checked out if it is failing to provide adequate temperatures. It should be able to provide some heat even when the coil starts to freeze outside, if it can’t do that then it indicates a problem that should be resolved because if it’s not then it may cause additional wear and tear that you aren’t noticing or aware of.

Anyway, back to the original point, have a look at the link to the manual that I posted. Be aware that buying a thermostat from Home Depot or other box store will not include all the features and functions that professional models will provide. This manual is for a professional model. There are other Honeywell models that have this function, I just prefer the 8000 series because I find users have the least amount of troubles using them.
 

·
Registered
Joined
·
108 Posts
Discussion Starter #17 (Edited)
Have a look at option 350 located on page 8 (heat pump compressor lockout) in the setup menu. This should achieve what you want. It provides a description of how the function works on page 11.
Thank you. A similar function was available also on my older LuxPro PSPU732T that I also had it with the optional temperature and humidity sensor. But, as you see, the logic is strictly related to temperature. Does not take outdoor humidity in calculations. It is silly to lock-out the compressor at 32-34 F when there is no (of just a bit) humidity present to freeze the coil...
And yes, in my Trane unit defrost is not time based, but a bit more advanced, pressure based. Slightly better, but still it pains me to hear it working for nothing.

As long as the efficiency is above the 1:1 that you get from electric then you are probably better off to use the heat pump.
The heat pump is still more efficient with frost on the coils, than electric resistance heat.
Thank you again for the posts, however I also know those are general marketing claims. And I know that seldom claims meet with reality.
I didn't see anywhere posted results of measurements of efficiency for exactly this situation (frost on coils). Since the average conditions yield like 2.25, why would we assume that we still get that efficiency without actual testing? What is we only get 1.00-1.25 with heavy frosted coils? Is then still economically feasible to wear the compressor?
I am really curious to know if there are measurements about this.

Like I said, I don't care that much to save a penny (I do in general like to save a dollar, that's why I have bought a heat pump), but most important is to have comfort. There are days when heat pump is a great tool to heat the house, but there are others when the strip heating is more pleasant. I suspect that manufacturers don't like to boast about features that would seem to make their thermostats less efficient... It's all angles and perceptions.

Looks like every manufacturer talks of how much energy their thermostat will save, nobody talks about actual user comfort.
I am maybe more upset that my Honeywell TH 9320 WF5003, very fancy looking, doesn't even have any of those options like older thermostats.

PS: While I was typing this (end editing) the unit outside started three times. First time at the end of the 20 minute run, when it frosted. Then it stopped and started one more time for defrost. Then it stopped and started again to get the heat up to 71 F. Yes, I like 71F, I know that putting the system at 68 or 65 would save a lot of this trouble. But again, I work for my money, I want to feel good in my house, so I am willing top pay for that comfort.
 

·
Registered
Joined
·
450 Posts
Thank you for the long post, however I also know those are general marketing claims. And I know that seldom claims meet with reality.
I didn't see anywhere posted results of measurements of efficiency for exactly this situation (frost on coils). Since the average conditions yield like 2.25, why would we assume that we still get that efficiency without actual testing?

Like I said, I don't care that much to save a penny, but mostly to have comfort. There are days when heat pump is a great tool to heat the house, but there are others when the strip heating is more pleasant. I suspect that manufacturers don't like to boast about features that would seem to make their thermostats less efficient... It's all angles and perceptions.
Looks like every manufacturer talks of how much energy their thermostat will save, nobody talks about actual user comfort.
The data I’ve seen shows the heat pumps start to become less efficient when outdoor temps reach 10 DegF. Above that temp and you are saving money. That is only based on The amount of watts used and not taking into consideration the other factors that I mentioned. It would have to include an average number of defrost cycles, they would not be able to omit them all, but environmental conditions are different on a day to day basis so it would be impossible to make a single number be true for everyone.

I’m a little surprised your thermostat doesn’t have provisions for outdoor temp but before you go changing that, depending on your location that may not really be a consideration you need to worry about. What is your design temp for your area?

I have reason to be skeptical of some posted numbers by manufactures but I don’t think you need to worry to the extent that your concerns appear to be. Typically people adjust their compressor lockout temp above the 10 Deg mark for comfort, not because they notice a difference in their energy bill. They just like to have warmer air blowing out of their vents.

Since it doesn’t appear to be an option in your thermostat then you may want to switch to the 8000 or other that will provide that user setting. An outdoor temp sensor will be required, or they may pull the data off the web. You’ll need to determine that.

If you are finding the comfort being different between the modes then you may want to consider a humidifier. The RH in your space will make change how a given temperature feels. If you maintain that constant then your temperature will feel more consistent.
 

·
Administrator
Joined
·
40,845 Posts
You can check the efficiency of your heat pump in those conditions.

Just switch it to emergency heat. Then measure the return temp, the supply temp in the trunk line 1 to 2 foot into the trunk. Then measure actual volt and amp draw. That will give you your watts, BTUs, and CFM. Then do same with only the heat pump running. You'll be able to see the difference in watts used to heat delivered.
 
  • Like
Reactions: azeotrope

·
Registered
Joined
·
108 Posts
Discussion Starter #20
The best test is that wife is saying that's a draft coming from the went, and she's right. For the first 5 minutes after the defrost cycle, the incoming air is cold. Then slowly gets luck warm.
It eventually gets to 98F (at the closest vent from furnace discharge, just a few ft of duct), but it takes a while compared to the Aux Heating that gets there in like 30 seconds...

PS: This is the weather here right now.

643394
 
1 - 20 of 50 Posts
Top