Balance point vs ambient lock


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Old 12-15-17, 12:23 PM
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Balance point vs ambient lock

Hey guys,

Just got a new Lennox SL18XP 3 tons heat pump & CBX40UHV air handler installed at my new place.

I'm from Montreal, so winter temperatures are between 20 and 0F.

The installer is the one having the best reputation in the area. I'm very satisfied.

The only minor concern is the low temperature ambient lock which is set at 10F. When I observe the system around the limit (10 to 12F), the heat pump easily keeps up and seems to be above its balance point. When I touch the refrigerant line, it is just as warn to the touch as when it is let's say 20F. Furthermore, the auxiliary heat never kicks in at 10 to 12F except when defrosting. Then, due to the ambient lock set in the thermostat, as soon as it reaches 9.9F, the heat pump totally shuts off and the air handler takes over.

My installer says this is normal and 10F is pretty much what the SL18XP can deliver in these circumstances.

My understanding was that when approaching the heat pump balance point, the auxiliary would slowly take over but they would still work together until it economically makes no sense to use the heat pump.

What do you think?
 
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Old 12-15-17, 12:36 PM
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The AUX/heat pump balance point is usually controlled by the thermostat. When the room temperature is 3f or more below the set temperature..... the AUX is activated and runs in addition to the heat pump. As the gap closes the AUX shuts off and the heat pump runs to maintain. If it can't maintain.... the AUX will again come on.

Now with your system... it may be able to be set up so that the AUX mode comes on automatically when the heat pump is locked out eliminating that 3f spread.

Since I'm not 100% sure/familiar with your setup.... I can't answer the lockout part. My forum partner, Houston, may have an answer.
 
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Old 12-15-17, 02:57 PM
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The logic that you describe is what I would expect as well.

On the Lennox i-Confort t-stat, there is this option that my installer enabled:


Mine is set at a low of 10F and a high of 50F.

It is funny because as we speak, it is currently exactly 10F and the heat pump is literally laughing at me. It's easily able to keep the temperature. The refrigerant line is extremely warm to the touch.

Unfortunately, it'll be totally shutting off in a few minutes once it gets to 9F.
 
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Old 12-15-17, 03:48 PM
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Ten degrees is pretty low and good for a heat pump.

It's not always can the system deliver heat inside when it's 10 outside ..... it's can the condensor un-thaw and at what cost since it requires the electric reheats and it removes heat from the house at the same time.

I would have to believe your installation pro should know his system and how they respond in your area. If you feel it should be able to run below 10 .... ask him.

Your picture shows 25- 50 but you're saying 10 - 50. That's a "before " picture ?
 
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Old 12-15-17, 05:10 PM
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While I have seen posts claiming that it is still more efficient to run the heat pump until it is 0 degrees I cannot say that I have any experience with outdoor temperatures this low.

I would also refer this question to your installing company.

The only time that I set lockout temperaures is when I have a dual fuel system with gas heat for aux heat.
 
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Old 12-16-17, 10:11 AM
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For me, the LowTemperatureCutoff and BalancePoint are set with jumpers in the outdoor unit because that's where the ambient temperature is measured. ...

After about 30 years of experimenting with a couple different heat pumps, I've found that the lowTemperatureCutoff of 4F works fine in the Boston area - where we go below that only a few days in a year but regularly experience single digits. One might expect that a well designed modern heat pump could operate down to -10F ... do the experiment ?

The Balance Point , which is the temperature above which the outdoor unit will not activate auxiliary heat, I have set at 40 - though 30F works just fine (but I choose comfort over cost ).

Depending on how the wall thermostat is configured (or configurable) the auxiliary heat can be independently activated by that based on runtime, temperature difference to suit your tastes ... and I guess if you have a new, clever system the LTCO and BP may be controllable from it rather than outdoor unit jumpers.
 
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Old 12-16-17, 12:17 PM
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Thanks for the results with your system for comparison.

Yes.... I'm sure it will actually run down to -10 but is it actually extracting any
heat at that temperature and if it is... is it enough to make it cost effective.
 
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Old 12-17-17, 12:32 PM
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Whether the heat pump is effective and desired at the lowest temperatures depends not only on the temperature but, importantly, on the weather , especially the dewpoint. If the weather is such that defrost cycles are needed, they are enough to spoil the performance that the manufacturers tout: if there's a defrost cycle of 5-10 minutes every hour or so, the gain from the heat pump disappears pretty quickly.
My system uses demand defrost so that when the air is dry the performance is good and I go many hours with no defrosts and the net gain is pretty close to the manufacturers boasts. But if there's wintery weather in the form of snow, freezing rain etc the heat pump spends its time condensing water to ice and then evaporating it with not so much net gain for heating - defrosting, in short.
I set my LowTemperatureCutOff very low so that I can take advantage of good weather (i.e. low dewpoint) and when the weather is unsuitable, or I just get tired of hearing the heat pump running, I find it easy enough to just switch to Emergency mode.
 
 

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