Vision Pro, dual fuel, and aux switchover

14 years ago

I've offered a lot of VP programming advice to folks over the past couple of years. Now I have a question about how my own system is behaving.

We have a Rheem/Ruud HP with 80% gas furnace backup. It's run by a Honeywell Vision Pro (8321, IIRC) with an outdoor sensor and set to switch over at 30F. We've had a week solid of The stat is set to setback the temp from 69F to 62F during the day on weekdays and Intelligent Recovery is enabled. There are 2 things I've noticed and I'm wondering about:

First, yesterday with a high of 38F and dropping off after it got dark, the HP ran almost continuously and took probably 4-5 hours to get the temperature up to 67F. I'm wondering, if it's running that long without getting to the setpoint, why it doesn't kick on the Aux heat to get there? (FYI, it's set to 'standard' heating, as opposed to more aggressive). Also, FWIW, register temp was 78F dropping to 74F as the outdoor temp dropped from 38F to 30F.

The other thing that I noticed last weekend (no daytime setback) was that it didn't switch back over to the HP when the temp got above 30F.

I don't know if this complicates things, but I did make one change yesterday after scratching my head again on the stat's settings. The furnace has a variable-speed fan and, IIRC, low and high fire on the burner. I believe it runs strictly off of the control board in the furnace- the furnace runs at low speed and low fire and then kicks into high speed and high fire if the call to run from the stat doesn't go away after a certain time. Don't think the stat has any influence on how this works other than on-off.

The stat had been set as 3H/2C by the installer, but there isn't a 2nd compressor stage (Y2 terminal isn't even wired). I changed it to 2H/1C yesterday. The other thing I noticed is that, I thought the stat would show "Aux" somewhere when auxiliary heat is called, but it doesn't.

Any ideas on why the switch from HP to gas and back doesn't seem to be happening when I think it should? And why the stat doesn't show "Aux" when the gas is running? Cheers.

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Comments (2)

neohioheatpump
14 years ago
Hi,
I recently installed a heat pump on my house which currently has a gas furnace also. I have the honeywell focus pro wireless thermostat kit with the outdoor sensor and all.

You need to set the droop-control to maybe 2 degrees or something small. The heat pump will take way too long to bring the house from 62 to 67. The droop control basically says if the temperature difference in the house is more than the number of degrees you set, it uses auxilary heat.
You can also lockout the droop control on my thermostat. That means if its more than 2 degrees difference in the house but the outdoor temp is above 45, then it won't engage the auxilary heat.

2 degrees is what I use for droop control because the heat pump isn't so great at bringing up temperature, but it does a good job at maintaining temperature in the house. I have the auxilary lockout at 45. At 45 and above the heat loss isn't so bad on my house, also the heat pump puts out more heat. I can't imagine the house losing more than 4 degrees on a day like that anyway.

I have my switchover also at 30. To be honest, if I could do the switchover at 32 I would. But my thermostat has switchover's in 5 degree increments. The heat pump still seems to do ok even at 35. But once it hits 33 or 32, it seems like the heat cools down.

On my heat pump there is also a setting on how many hours to let it run before it hits the target temperature. I have it setup for 1 hour. So if the heatpump can't bring up the temperature in the house, it switches to auxilary. You can disable this feature and have it run endlessly to bring temperature back up if you like. I would do this maybe if gas were to get real expensive again. Right now its cheap.

I noticed, when it turns 30 degrees sometimes the gas still runs. Maybe after a while it switches to heat pump. I guess the thermostat doesn't just instantly switch from heat pump to auxiliary or vice-versa instantly when the temperature for the switch is hit. You would think it would be it doesn't on mine either.

You may just want to switch your switchover point to 35.

Go Duel FUEL!!!! Hope this helps.
thull
Original Author
14 years ago
I'm not sure what you mean by 'droop' control. I don't think the VP lets you control when the 2nd stage kicks on based on the difference between measurement and setpoint.

I've gone back and raised my setbacks from 62F to 65F. We'll see how that works. Last night, it was pumping out 76F air at 36F outside. And not keeping up with the 69F setpoint.

Back to my original questions:

- Should it show "Aux" or some other indication when the 2nd stage is running? The red LED does come on when emergency heat is turned on.

- Should it try to run the HP when the temp rises above 30F (the lockout temp)? Is it possible that the stat has 'learned' that the HP doesn't keep up in the low 30's?

Thanks.
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Vision Pro, dual fuel, and aux switchover

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neohioheatpump

thull
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Vision Pro, dual fuel, and aux switchover

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neohioheatpump

thullOriginal Author

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thull
Original Author