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Set heat pump at constant temp or Do setback?

Posted by pbx2 (My Page) on
Mon, Dec 16, 13 at 13:46

Is it better to setback our Trane XR17/Tam8 heat pump temps when we're away from home or to leave it at a constant temp all day?

What is the best practice: Running it all day or ramp it up quickly when needed?

I have emergency heat via aux electric strips.


Follow-Up Postings:

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RE: Set heat pump at constant temp or Do setback?

What thermostat do you have?


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RE: Set heat pump at constant temp or Do setback?

I agree, thermostat model matters. I prefer the models that have adaptive intelligence. You would want an outdoor sensor.

Also the amount of setback. No more than 2-3 degrees.

The lockout of heat strips down to a specific outside temperature.

And probably the most important is how close you are with excess capacity to your heating BTUs from condenser alone. Heat pumps are good at maintaining temperature but can be poor on setbacks if sized very close.

IMO

This post was edited by tigerdunes on Mon, Dec 16, 13 at 15:01


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RE: Set heat pump at constant temp or Do setback?

@ionized - we have the Trane ComfortLink2 - XL950.

@tigerdunes - We were setting back temp from comfort - by about ~ 3-5 degrees or 68-70 back to 64-65 - when out/away.

Noticed that the heat pump ran ~80% of the time 1st stage/15% 2nd stage/5% Aux heat.

These are visual estimates based on thermostat's supplied metrics.

I really think the aux heat only came into play @ dawn & right after work since the setback for both are greatest in the main living area & I guess the HP had to ramp up quickly to heat that area up from the overnight temp drops & its idlenss during the day.

My main concern with the OP was to see if I am gaining any $ benefits doing one or the other esp. since I am really still using mostly Stage 1 when using set-back.


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RE: Set heat pump at constant temp or Do setback?

I think the setback is too much.

You will need an outdoor sensor to lock out heat strips down to a specific temperature. You don't want heat strip on except when necessary.

I have my gas backup locked out down to 30 degrees except for defrost cycle if necessary which would be very few calls.

Just my ideas.

IMO


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RE: Set heat pump at constant temp or Do setback?

I don't know that much about adaptive thermostats, but I know that is important which is why I asked. I am just a joe homeowner and I've never had a heat pump with alternative-fuel heat back-up. I do know that there are a lot of myths coming out of heat pump set-backs. Particularly vexing is the application of dated heat pump recommendations for other heat sources. ("Setting back too far costs more than not setting back." That defies kindergarden physics.)

You obviously want to have as large a set-back as you can while not at home as long as you can avoid the electric back up activation. You don't want to hit the heat strips at all for morning warm-up either. The lower you can go and the longer you can go at the setback, the less you will spend. If the outside temp is too low and your capacity too low to bring you up, I would hope that the the adaptive thermostat should not allow the set-back to go too low. You see to have a pretty good handle on operation. You'll have to rely on others, and your manual about what is possible with your thermostat.

With simpler heating systems, your penalty for setting a set-back too low to recover in time is not increased cost, but cold feet, hands and maybe everything else. It won't cost you anything. With a heat pump you can be cold and your wallet hurt at the same time.


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RE: Set heat pump at constant temp or Do setback?

I think you both raise good points & I appreciate the feedback.

Intuitively, I would think that the ramp up is from the heat strips while coming back from being setback or idle for prolong periods of time thus causing the aux strip heat to be used.

However, I would think that the installer should have set-up some sort of smart recovery to slowly ramp-up from setback to comfort & limiting any heat strip usage under fairly normal condition (i.e. waking up in the AM).

In the XL950 installation guide page 13 - there are these entries: can you all tell me what it's saying?


Aggressive Recovery:

This option disables the 10 minute inhibit anytime the thermostat sees more than a 2° change (such as moving from one program time frame to another). This applies to heating & cooling modes.

Heating Aggressive Recovery: This option disables the 10 minute inhibit based on outdoor temperature in heating mode only. The value for outdoor temperature can be set from 0°-70°.

* Heating Aggressive Recovery can only be enabled if Aggressive Recovery is disabled.
* There is a 10 minute “inhibit” period between first and second stage compressor as well as compressor heating & supplemental strip heat.
* This 10 minute inhibit period minimizes the cycling of higher capacity request due to drafts around the thermostat or marginal load conditions.

Here is a link that might be useful: XL950 Comfort Link Installation Guide


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RE: Set heat pump at constant temp or Do setback?

I set up my set backs as follows;
Sleep 1030 pm 70
Wake 0930 am 71
Home 1100 am 72
away 70
Theory is that it will work less during the night when it is coldest out and make it comfortable when we are up around the house with one degree adjustments. This has worked better than a constant 72 degrees.


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RE: Set heat pump at constant temp or Do setback?

I have a smart-setback thermostat. I don't do setbacks any longer being as I work from home ... but some years ago when I did, it was a 10°F setback. I happened to come home early once and found that the auxiliary still triggered in the last few mins of the recovery because the room temp was maybe 2°F short of the target. I'd rather not have the auxiliary trigger to quickly make up the difference, so I had an outdoor temp sensor added to lock-out the auxiliary, which I set at 25°F. Much better.

IMO, a setback of less than 5°F is of no benefit.


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RE: Set heat pump at constant temp or Do setback?

Dadoes,

I respectfully disagree that a setback of less than 5 degrees is no benefit. I have a daily energy usage displayed and have seen tangible results from my settings. If my wife worked and no one was in the house for periods longer than 6 hours I would do a larger setback but with a heat pump I would still limit it to around 5 as max. But our small setback still saves and doesn't make it uncomfortable to be home all day. I have carriers infinity with greenspeed though.

Edit. The exception is long periods such as trips. Than setting it to more extreme set backs would be beneficial for periods longer than 48 hours.

This post was edited by Energywise.ks on Mon, Dec 16, 13 at 21:44


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RE: Set heat pump at constant temp or Do setback?

Aggressive Recovery/Heating Aggressive Recovery. What this is saying is that if you have a 2-stage heating/cooling system, there is a 10-minute waiting period before the unit will step up to the 2nd stage.

Turning ON the Aggressive Recovery will activate the 2nd stage immediately if there is an 'equal-to-or-greater-than' 2* temperature change requested. That is, if you walk up to the thermostat and change the temp from 65 to 70 it will jump into 2nd stage immediately rather than waiting 10 minutes. This would also apply to the end of a setback.

My thermostat has an Intelligent Recovery function. As it runs, it computes how well the unit is operating in degrees/minutes. It then can predict when to start doing the recovery so that it hits the correct temperature on time. For example, if I have a 5* setback and the heat pump is performing at 1*/15 minutes, it will start doing the recovery 1 hour and 15 minutes before the setback period ends. No aux heat required. Same idea for cooling.


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RE: Set heat pump at constant temp or Do setback?

Thanks Weedmeister for the careful explanation.

I'll have to check with my installer if my T-Stat has the intelligent recovery like your or do I have to plan a bit if I am setting things back.


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