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TOPIC: How long should furnace cool before applying heat?

Re: How long should furnace cool before applying heat? 6 years 4 months ago #53987

Do what you want, but your wasting your time...

The "lack of heat" when testing is worse than having the heat on before the switch over.

Have you watched what the pressures do when you turn on the heat? You are not heating 100% of the refrigerant circuit so the pressure is at saturation, not at the heat of the furnace discharge.

I have had to set HVAC equipment to run in heat and cooling at the same time to control moisture in places like museums.

Common sense tells you one thing, but common sense does not drive the HVAC field.
What you think should be, most often is not.

"Dang, thought about it again, Steve, and realized that in most furnaces, the evaporator coil is after the heat exchanger,"
If the evaperator coil was before the heat exchanger it would rot out the heat exchanger.

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Re: How long should furnace cool before applying heat? 6 years 4 months ago #53988

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Common sense tells you one thing, but common sense does not drive the HVAC field.
What you think should be, most often is not.


How true!  I've wanted to strangle a few engineers over the years for their ignorance of what actually happens in the field.  (Sorry if you're an engineer, David.  But maybe you can still laugh at that, too.)

The "lack of heat" when testing is worse than having the heat on before the switch over.


If you mean basically testing without a load, then, yes, I can see that.

Have you watched what the pressures do when you turn on the heat? You are not heating 100% of the refrigerant circuit so the pressure is at saturation, not at the heat of the furnace discharge.


Well, not having ever rigged the heat to come on while my gauge is on the suction line, I'm unfortunately dealing with theory.  So, assuming a properly charged residential system, are you saying the heat introduced by firing the burners while the AC is on is not enough to take the A-coil out of saturation?  If so, I agree, which is why I said:

I personally think that the heat "generated" by the heat exchanger can't be transferred fast enough to the A-coil to cause a harmful rise in pressure,


What Steve is talking about is so temporary an effect on the system to be apparently negligible, but I still would like to know what a manufacturer says about the instantaneous switchover practice (just going suddenly from AC to heat, not a half second round trip from AC to heat to AC).  After all, so often when it comes to liability, if we (those in the field) don't heed or at least acknowledge manufacturer recommendations - even when they defy what the real world demands - that kind of disregard can be costly.  I think I'll contact a few manufacturers to see what they have to say and get back to everyone here....

I have had to set HVAC equipment to run in heat and cooling at the same time to control moisture in places like museums.


You mean when charging only, to simulate a heavier load, right?  You don't mean you set the heat and AC to run together all the time when asking for cooling, right?  Just curious on that one.

By the way, thanks, David, for having this discussion.  I'll try to get some feedback from a manufacturer, sticking to Steve's original question, so it doesn't look like I hijacked this thread....

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Re: How long should furnace cool before applying heat? 6 years 4 months ago #53990

When you have to control humidity in a large building, you can not just turn down the thermostat. It will lower the IA temp down to the dew point and cause condensation (or absorption of moisture in some materials).

There are also temperature ranges that must be maintained in some cases.
A/C with re-heat is a practice to keep things where they need to be by turning on the heat when the temperature gets too low while the a/c keeps running and dehumidifying.

Air leaking into the building can not always be controlled ie. loading docks, high pedestrian traffic, required minimum ventilation etc.
So this application is required under outdoor ambient conditions that are not constantly present.

I can't ever remember hearing anything that a Mfg. Co. said about change-over.
If it is a problem, there is a built in time delay such as an anti-short cycle device to allow pressure neutralization. You can't can't hurt anything because the safety in built in.
No Mfg. is going to leave it up to the consumer Not to do something that could damage the equipment.

Some electronic thermostats have a built in time delay between heat and cool. There are also time delay relays (in case the thermostat is changed) to prevent t-stat jockeys from jacking the a/c on/off.


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Home Inspector
HVAC Systems Design
ITC Level III Thermography - Building Science Thermographer
Thermal Imaging
Serving Clarksville - Nashville TN and the Mid TN area
www.MidTnInspections.com
www.ThermalImagingScan.com
To link to my pages:
www.midtninspections.com/link-submission

Re: How long should furnace cool before applying heat? 6 years 4 months ago #54080

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When you have to control humidity in a large building, you can not just turn down the thermostat. It will lower the IA temp down to the dew point and cause condensation (or absorption of moisture in some materials).

A/C with re-heat is a practice to keep things where they need to be by turning on the heat when the temperature gets too low while the a/c keeps running and dehumidifying.


Hmmm...do they make a "smart t-stat" that "reads" latent heat?  Well, one that incorporates a hygrometer and that can run the AC with reheat?  That would be the same reheat that I forgot about from my Commercial classes.  Thanks for reminding me so patiently, David.

Anyway, over a week ago, I contacted via "email" 8 furnace/AC manufacturers (American Standard, Bryant, Carrier, Coleman, Goodman, Rheem, Trane, and York - actually I had to send a message from their website, since only one of those websites used my email client, so it's hard to know if that part of the website is working).  Only one responded with an email answer directly.  One called and gave me an "answer" but said that all technical inquiries are handled by local dealers and that I should call one of them. The other 2 who responded via email said that all technical inquiries are handled by local dealers and that I should contact one of them.  I ended up just calling local dealers who rep the 7 other manufacturers who didn't answer via email directly.  Here was the question:

"Assuming an outdoor ambient temperature of greater than 65 degrees F, can ********** residential systems, both R-22 and R-410a, be switched instantaneously from cooling to heat at the thermostat (but not instantaneously from cooling to heat to cooling) without damaging the unit? Or should there be any waiting period between turning the AC off and turning the furnace on? I am a home inspector."  I pointed out that as a home inspector, in the realm of possibilities I might come across an old T87 or rectangular mercury t-stat without a delay controlling a furnace/AC.  (Not all units have a built in delay to protect the compressor from t-stat jockeys in such a situation - also in the realm of possibilities.)

Here are the resulting answers....

Four of the local dealers said that it was OK to go instantaneously from cooling with the AC running to firing the burners and asking for heat.  (They all stressed not to make the round trip back to cooling before the high and low sides equalize pressures, though, which I think all of us know.)

One of the manufacturers who called me said that a 2 or 3 second delay was sufficient but that I could not quote that to put it in an online forum (which is why I'm not identifying any manufacturers) and that I should call their local dealer to get an answer.  That young-sounding tech at the local dealer said it was OK to do it...after I confirmed for them that a T87 was the same as a "yo-yo" thermostat.  

One local dealer said with a little caution in their voice that, yes, you could make the instantaneous switch from cooling to heat, but that it is better if you let the unit finish running the fan (if there's a timer - if not, run the fan for a minute or so to warm the coil up).  I did not prompt or coach this person.

And the only manufacturer to respond directly to me with an email answer said that their technical support department advises waiting at least 5 minutes before making the switch.

I gave up trying to get one local dealer on the line, so the sample size ended up being 7, not 8.

After all this, I think the likelihood of damaging something in the system by making the sudden switch from cooling to heat is pretty dang low.  However, I'll still be running the fan for about a minute to slowly warm things up, but that's me.

David, don't think by my saying that that I'm telling you (or anyone) how you should run your inspections, please.  I'm sure you have more experience than me in the field of HVAC.  But here's what I am telling you:  Thank you for your patience and your civilized responses.

And I'd like to think, Steve, if you are still reading this, that you can glean something from this thread that helps you....

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Re: How long should furnace cool before applying heat? 6 years 4 months ago #54108

Thanks for your efforts in getting this info from the "Horses Mouth" so to speak!  ;-)

I wish I considered this thread (and others going on elsewhere) when I was at a BBQ this past Saturday. It was at the home of a VP from Trane and there was a room full of design engineers at my table I could have discussed this with and acquired more opinions for you...

Sorry!

I even have an Electrical Engineer from Trane boarding her horse on my farm!
Just don't think about this subject!
I'll try harder.

BTW: I'm having this conversation elsewhere and I would like to add that changing between heat and cool "Does" stress the equipment!
If the equipment is about to fail (which we have no clue of this potentially happening) you could in fact damage the equipment.
However, it is about to fail anyway!
As an Inspector, I would rather it fail during testing than to have it fail on your client the day they are  moving in (in August)!
You know they will call and want to know why it failed when you were just there inspecting it!

If this is really an issue, and there are several devices that can be used to prevent things from happening, you know they would not leave it out and gamble that it won't happen during the warranty period (10 years). These devices are a heck of a lot cheaper than a compressor failure.  They are not going to rely on the "good sense" of the homeowner to do the right thing.

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Home Inspector
HVAC Systems Design
ITC Level III Thermography - Building Science Thermographer
Thermal Imaging
Serving Clarksville - Nashville TN and the Mid TN area
www.MidTnInspections.com
www.ThermalImagingScan.com
To link to my pages:
www.midtninspections.com/link-submission
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