Suggest new thermostat for dual fuel system


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Old 10-17-14, 06:07 PM
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Suggest new thermostat for dual fuel system

Hi, I have a rather new heat pump and oil furnace with a honeywell visionpro TH8321R1001 thermostat. My first and main concern is that the new thermostat must have an adjustable temperature differential up to 2 to 3 degrees. I am looking to have the most efficient run times and energy savings $. The temp in my house will usually settle down about 1 to 1.5 degrees a short time after the heat turns off but then it will stay at that temp for quite a while. This does not work well with my current thermostat that tries to keep a constant temp within 1 degree causing too many on off cycles.
(btw I saw the sticky at the top about honeywell thermostats, and since I'm new here I'll hold my comment)

I may not be using exactly the correct terms here, but I also want to make sure the thermostat will have a large enough setting for temp droop before using aux heat so the heat pump will be able to run alittle longer if needed to meet the set temp and not turn on the aux heat.
Also another settting I read about but am not as familiar with is upstage timer. I want to make sure that would also be highly selectable or able to turn off so that it would not turn on the aux heat after a set amout of time if the heat pump has not met the set temp. I'm not sure but that my be overrode anyway by the outside temp cutoff for aux heat.
I want to make sure the aux oil heat is usually only on when really needed in cold weather. I am okay with the temp taking alittle longer to meet the set temp using the heat pump in milder weather.

I'm looking for thermostat suggestions, thank you

edit- would also perfer non touch screen and thermostat that would control heat pump and furnace directly without add on kits or relays,
and of course would obviously still need outdoor temp sensor ect. thanks
 

Last edited by ronjohnson; 10-17-14 at 06:17 PM. Reason: add info
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Old 10-17-14, 06:13 PM
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Welcome to the forums.

My specialty is repair and wiring. The forum pro, Houston, is the thermostat expert. Hang tight and he'll be by to make recommendations.
 
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Old 10-18-14, 03:40 AM
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I may not be using exactly the correct terms here, but I also want to make sure the thermostat will have a large enough setting for temp droop before using aux heat so the heat pump will be able to run a little longer if needed to meet the set temp and not turn on the aux heat.
See pages 29, 30, 76 and 77.

https://customer.honeywell.com/resou...0s/68-0312.pdf


Your current thermostat is capable of meeting much of your demands when you enter the advanced menu. It will be unlikely that a non touch screen stat will meet your criteria.

You can set the backup heat droop from 2 to 15 degrees. (ISU 309)

Looks like you can set your existing stat for up to a 3.5 degree stage 2 heating differential (ISU 305) and you can also set the backup heat to energize if you have not reached set point in 0, 30,45,60,75,90 minutes or up to 16 hours in (ISU 311).

You have already noted that you have ambient temperature lock out capability with your existing stat. (ISU 312)

Honeywell doesn't offer the stage 1 differential with your application (ISU 306)
You might look into a Lux or Ritetemp thermostat for this option.
 
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Old 10-18-14, 09:27 AM
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Thank you for the link, when I tried to find it all I kept comming up was the older 68-0280.pdf.

To be more clear from my original post, I have a basic single stage air to air heat pump. Also a touch screen would be okay if that is what I would need to make everything else work.

I was just looking over the ISU table to see if there was a different way to (hack) set up the thermostat to allow (ISU 306) and still work with my system. I'm not seeing that its possible though, and correct me if I'm wrong, but it looks like (ISU 201) would need to be geothermal radiant and if it is then the thermostat keeps the radiant heat on when it calls for backup heat which would not work me because I need the compressor to turn off before the backup heat comes on.

I think with all the advanced options on this honeywell thermostat they should have the option to change the temp differential, one of the most basic settings to have on a thermostat.

I'm going to look at the options from Lux and Rite temp as you suggested.

I was looking at a Robertshaw 9725i2, but then I read somewhere that it limits you to 40 min of stage 1 heat because of the upstage timer. I have not been able to find out any more hard info about it though other than what someone said on an internet forum.
 
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Old 10-18-14, 09:54 AM
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If you have a dual fuel system with an oil furnace it is not a basic single stage air to air heat pump.
A basic heat pump would have an air handler and electric heat strips. Many heat pump stats do not support your system.

I like the fact that my stat will heat to 72 degrees and not let the house hit 70 when I set it to 72 but I do see a lot of posters with your request.
 
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Old 10-18-14, 10:16 AM
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Sorry for the confusion, all I was trying to say is that the heat pump by itself is a single stage air to air unit.
I think of the heat pump and coil as one unit and then what ever type of "furnace" as the second unit whether that is an air handler with heat strips, or an oil or gas furnace.

What I am doing now is manually turning the heat off when the temp reaches the set point at 69 and then after 2 or 3 hours or however long as it takes to go back down to about 67 I'll turn it back on and let it go back up to 69 again. It does not take too long to run to go back up to temp in this milder weather. If I would have left it on as normal it would have cycled off and on many times within that 2 to 3 hour time.
I will not be able to do this when I am not home and when the winter comes and outside temps are changing alot.
 

Last edited by ronjohnson; 10-18-14 at 10:38 AM.
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Old 10-18-14, 10:27 AM
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If you buy a single stage heat pump stat it will not support a W2/Aux terminal when configured for heat pump. I have seen hundreds of posters make this mistake. Many multistage heat pump stats will not support a dual fuel system.

I have typed "Your new thermostat will not support your multistage heat pump." so many times I thought about making it part of my signature at one time. They only considered the single stage compressor in the outdoor unit as a stage of heat.


You might look into Ecobee and Venstar.
 
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Old 10-18-14, 10:37 AM
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What I am doing now is manually turning the heat off when the temp reaches the set point at 69 and then after 2 or 3 hours or however long as it takes to go back down to about 67
Are you doing this with your cell phone, tablet or the REM5000R 1001?
 
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Old 10-18-14, 10:49 AM
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The setting on the thermostat itself.
 
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Old 10-18-14, 11:09 AM
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Here's my actual equipment info

Johnson Controls Luxaire THJD24S41S5A heat pump
ECR Olsen BML80B2 oil furnace
Johnson Controls FC35C3XC1A coil
 
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Old 10-18-14, 11:11 AM
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Resetting the thermostat every 2 hours manually sounds like a lot of work.
I would avoid the hardware store thermostats.
 
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Old 10-18-14, 01:41 PM
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I was looking at the Robertshaw 9825i2 and here are a couple quotes from the manual,

"2nd Stage Delay
is a timed delay between 5 and 40 minutes. It is the maximum time 1st stage
equipment tries to satisfy demand before the second stage equipment is activated.
This feature is available only on the 9825i2 mode"

"Balance Points
– This choice only appears when the 1st stage is a heat pump.
Low and High balance points are selected. The low balance point prevents the heat pump from being used when it is least efficient. The high balance point prevents the second stage heat (furnace or strip) from activating when the heat pump is most efficient.
Outdoor setpoint temperature ranges are:
Low Setpoint : -20F to 75F (-29C to 24C)
High Setpoint : 0F to 75F (-18C to 24)"

I am wondering if the high balance point lock out for second stage would keep the 2nd stage timer from being active. It seems like one would have to override the other, that is unless the 2nd stage timer function is only active between the high and low balance points. I could not find where gave any information about this.
I like this thermostat other than I am concerned that the 2nd stage timer will be turning on the furnace after the timer has expired, instead of letting the heat pump keep running.
 
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Old 10-18-14, 05:22 PM
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That is a good looking thermostat.
Lennox rebranded one of those as their Signature Series thermostat about 10 years ago.

I have homeowners that find it difficult to navigate but this probably will not be the case with you.

I see the comparison chart shows the TH8321U as not compatible with fossil fuel applications. This is not true and they should correct this in their sell sheet.

http://www.robertshawtstats.com/imag.../150-2318B.pdf


I have never had a customer ask for a stage 1 differential with a residential thermostat. Did you see anything to indicate that this thermostat has this feature?

http://www.robertshawtstats.com/imag...%20English.pdf
 
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Old 10-18-14, 06:04 PM
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The last link you posted I have been studying,
Page 17 Comfort Settings
"1st Stage Differential
is the number of degrees away from the setpoint that the thermostat
allows before it calls for the 1st stage equipment to be activated. As this setting is lowered,
your system will cycle more frequently."

It did not see where it said the actual adjustment range on this model, other robertshaw thermostats with adjustable differential say: "Adjustable 1st stage temperature differential: 0.5 F to 3.0 F"
 
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Old 10-18-14, 08:13 PM
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I see that the Robertshaw gives you the ability to run the furnace and heat pump simultaneously. This would be like running an air conditioner on a 135 degree day. The heat pump would see very high head pressure.

I wouldn't enable this option.
 
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Old 10-18-14, 08:44 PM
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Thanks for the help. I did see that under the Equipment Setup menu, I would want the following option selected:
"The Comp. Type allows you to determine how 1st and 2nd stage heat work together.
Add-On– 1st stage heating compressor on the heat pump will be turned off when 2nd stage heating is activated."

I'm still not complely sold on this (9825i2) thermostat. I would like to find out if the limited time of the 40 min upstage timer would in fact be turning on the oil furnace or if it will stay off as long as the outside temp was higher than the high temp cutoff setting for aux backup.
 
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Old 10-19-14, 01:17 PM
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Well I emailed Robertshaw to see if they can answer my questions I have on that thermostat, will wait for thier reply.

Houston, do you think I might be better off with a separate type of dual fuel control to take care of that part of the system and then I would have more selection on other thermostats? I can't seem to find one that has the features I need and at the same time dosen't have features I don't want.
 
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Old 10-19-14, 09:12 PM
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I have been reading the manual for the Venstar T5800, it looks like it may have the settings I am interested in. I was trying to stay away from something with that type of design, but if it has the correct options that would be okay. The free cooling feature may make for something interesting to play with down the road as well.
 
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Old 11-02-14, 07:05 PM
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Just to update, I had a robertshaw rs6110 sitting around here, so I set it up to run the heat pump only (its a single stage thermostat) and I have used that for the last couple weeks. The outside temps have been mild enough that I wouldn't have been getting into the fossil fuel yet anyway.
I did buy a venstar T5800 that I mentioned in the last post and I got it setup this weekend. Its a nice thermostat and seems to be working good so far. I still have doubts about just how long the screen can last long term. I'll be keeping that rs6110 around as a backup incase there would be a sudden problem with the T5800.
 
 

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