Geothermal Heat Pump Doesn't Use Stage 2 Heat


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Old 12-15-15, 07:20 AM
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Geothermal Heat Pump Doesn't Use Stage 2 Heat

I'm new to geothermal systems after 35 years of living with gas fired forced air furnaces. We have a new house with a 3.5 ton Bosch ground source heat pump and a Honeywell Prestige RedLink IAQ thermostat with EIM (THX9421). This is our second winter in the house and in general, we've been very happy with the heating and cooling system. My only "issue" is the system doesn't enter stage 2 heat under normal circumstances. This is an "issue" because the discharge air temperature is about 70 degrees F. while running in stage 1. Everything is fine until the outside air temp falls below 10 degrees or so. Below 10 degrees, the furnace struggles to keep the house warm. Last year we simply ran the stat at 67 degrees and dressed more warmly.

While poking around in the stat's STATUS page I noticed that stage 1 is active when the stat calls for heat, but stage 2 remains OFF. I can force the system into stage 2 by setting the stat set point up 2 degrees or by manually selecting stage 2 in the installer test page which indicates the system is wired correctly.

I've reviewed the ISU settings numerous times and everything appears to be correct;

ISU 3050 Staging Control = COMFORT
ISU 3021 Finish with High Heat Stage = ON
ISU 3150 Heat Cycles/Hr Stage 1 = 3
ISU 3150 Heat Cycles/Hr Stage 2 = 3

In Honeywell's installation manual (Page 97) under the heading STAGING CONTROL there's one statement I don't understand;

Heat Differential
The indoor temperature must drop to the selected differential
setting before the thermostat will turn on the stage of heating.
For example, if stage 2 is set to 2 F (1.0 C), the indoor
temperature must be 2 F (1.0 C) away from the setpoint
before stage 2 turns on. When set to Comfort, the thermostat
will use the stage of heating as needed to keep the indoor
temperature within 1 F (0.5 C) degree of the setpoint. The
thermostat turns on stage 2 when the capacity on stage 1
reaches 90%.


In this description, what is "capacity" and how does the stat determine capacity?

At this point, I think what I want is for the furnace to start in Stage 1 for a few minutes and then shift to stage 2 to finish the run. This may result in more cycles, but should allow us to keep the house at 70 degrees under normal circumstances. We can stand more hysteresis in the temperature control, but I'd be satisfied to understand the stage 2 "issue" at this point.

Thanks in advance for any information folks can offer.
 
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Old 12-15-15, 04:16 PM
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Well first, if the system is working correctly then you will rarely see 2nd stage kinking in. These systems are built to run as constantly as possible on stage one and only use stage 2 once the stat figures out that stage one simply isn't enough.

Secondly, you don't want to increase cycles on a heat pump system (air or water source). They run much more efficiently if they're always running. I have an air source heat pump and I have my cph all the way down to 1 so I can try and run it as constantly as possible with little cycling... and it's almost always operating on stage one. I almost never see stage 2 until it gets really cold.

Third, I would check that output temperature again because it seems kind of low. Right now the outdoor temp here is about 21 and with that my air source heat pump's output is about 85F (stage one). Water source heat pumps are even more efficient than air source so I would suggest you should be doing better than 70
 

Last edited by Bob Sanders; 12-15-15 at 04:46 PM.
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Old 12-15-15, 05:00 PM
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What is the model number of your Bosch Geothermal heat pump?

Is stage 2 heat controlling the compressor (Y2) or electric heat strips?

Are you measuring the temperature rise at the unit or at supply and return air grills?

Does your unit have any trouble codes displaying on the door?
 
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Old 12-15-15, 05:18 PM
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I have seen 2 stage compressors fail to energize the stage 2 solenoid because of a wire that wasn't connected more than once. ( both factory and field installed)
 
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Old 12-15-15, 06:40 PM
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Model # SM036-1VTC-SLT-AA

Stage 2 controls the compressor

Temperature rise measured at the supply grill

No trouble codes displayed.

Thanks for the fast response!
 
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Old 12-15-15, 06:51 PM
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Thanks for the response Bob!

I think I could live with the system running a high percentage of the time if it would keep the house at 70 degrees. If it can't...well, I may have to call the installer and see what his story is. Comments here provide an independent source of ideas and are very valuable to me.

I will remeasure the output temperature and verify what I remember from last year.

My other concern with high duty cycle operation is the effect it has on the field temperatures. If the system shuts down every now and the the field may have a better chance of maintaining a useful input temperature. It's a complicated topic and I'm sure I don't understand a fraction of what's going on below ground!

Steve.
 
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Old 12-15-15, 06:54 PM
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Do you think there could be a problem even though I'm able to force stage 2 to energize through the stat?
 
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Old 12-15-15, 07:09 PM
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Does this unit also have electric heat strips?

There should be 7 more digits in that model number. If the next number is an X you do not have electric heat strips. If you have heat strips it is possible that you are energizing them instead of stage 2 compressor.




Is the thermostat configured for 2 stage compressor with heat strips?


You may also have duct work problems. Measuring the temperature rise at the unit with a digital "meat" is preferred. Infrared temperature guns will not measure the delta tee accurately.

I believe that I have installed that Geo unit before but never with electric heat strips.
 
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Old 12-15-15, 07:21 PM
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Forcing stage 2 does not mean problems. You can force the 2nd stage on any thermostat with a relatively high increase in the thermostat. The thermostat is simply doing its job when it reacts that way to a large heat demand increase. It's normal. I don't know if you have back up heat but if you do you need to be careful in resetting the thermostat too high because you could cause the back up heat to come on.

I have the same thermostat you do and there is an option to LOCK OUT the back up heat till a certain temperature and I suggest you use that option. I have mine set to lock out the backup heat until 5F but there is a timer on it as well which will override the lock out if the thermostat continues to see a drastic fall in temp.

The point is that you shouldn't have to force 2nd stage though. Stage one should be quite satisfactory in most circumstances except when it gets very cold. At 10F you may see stage 2 kick in every so often depending on the insulation value of your house bu that's about it. This is why they make 2 stage devices. Stage one carries most of the load while stage 2 only comes in for short periods if it's too cold for stage one. Also it is best that you do not finish with high stage heat. Allow the system to back to stage one. This allows for a more constant run with less cycling.

If you are correct in your output temperature being 70 then there is something else wrong with your system. It should be higher than that and subsidizing the issue with stage 2 is just a band aid solution. You should be getting AT LEAST a 10 degree delta (check spec sheet for exact delta). If your intake temp is 70 then your output should be 80 or slightly better. If not then you need service.
 
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Old 12-16-15, 07:23 AM
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The system includes backup electric heat. The full model number is:

SM036-1VTC-SLT-AA-B1XXXX

Referring to the stat's STATUS page the electric backup does not come on under normal circumstances. If I set the STAT 4 degrees above the current temperature the electric backup kicks in. The ISU is set for 2 stages of heat pump and 1 stage of backup heat.

I mentioned I would verify the input and output temperatures with a bit more fidelity;

This morning the outdoor temp is 44 degrees F (!), the discharge air temp is 81 and the return air temp is 70. The discharge and return temps were measured on the respective plenums with a digital instant read meat thermometer.

As Bob mentioned, checking the Bosch spec suggests a 15 - 22 degree rise depending on the input water temp. Seems like it might be a "little light in the loafers".

Any speculation on the snip from Honeywell's spec?

(The thermostat turns on stage 2 when the capacity on stage 1
reaches 90%.)

Looking at the interconnect diagram there isn't an obvious signal from the heat pump to the stat that 90% capacity has been reached. Thinking about it (probably over-thinking it!) I don't even understand how 90% capacity would be determined (run time, internal temperature, internal pressure...?)
 
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Old 12-16-15, 10:36 AM
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I can try to contact my Honeywell Rep on your question. What is the ISU number at the top of the screen when you read this option?

Do you have duct sensors installed in the supply and return plenums?
 
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Old 12-16-15, 11:09 AM
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The question about staging based on 90% capacity was copied from Honeywell's technical specification for the stat. The document number is 68-0311-02 (page 97).

I have a pair of duct temperature sensors sitting on top of the heat pump, but they are not installed. Not sure why they weren't installed, but I have them. If it would make a significant difference it looks like something I could install in an hour or two.

I called Honeywell's tech support number to ask about how things work. After taking the stat's information the Honeywell rep said the Prestige stat is a professional device to be installed and maintained by a licensed professional. I understand their position, but I was disappointed in the response. I don't know squat about heating a cooling, but I'm pretty good with "gadgets"!

Thanks again for your help.

Steve.
 
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Old 12-16-15, 12:06 PM
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Change stage 2 compressor operation to "Comfort" at the thermostat.
 
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Old 12-16-15, 03:39 PM
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(The thermostat turns on stage 2 when the capacity on stage 1
reaches 90%.)
I believe it is time x temperature rise

The stat calculates the time required to raise the temperature in a given period of time based a certain standard plus previous records. If it comes in below 90% the 2nd stage kicks in.

Again I believe you're overthinking stage 2. It should NOT be kicking on unless it gets really cold. You can force if you wish, but cycling your system off/on up/down will simply wear it down prematurely.

As Bob mentioned, checking the Bosch spec suggests a 15 - 22 degree rise depending on the input water temp. Seems like it might be a "little light in the loafers".
And you're only getting 11 degrees. I don't know where you are located but if it's 44F in your area then you're probably live in a pretty warm climate (compared to mine) and your ground temp I'll bet is warm enough so that you SHOULD be putting out at least a 18 to 20 degree delta.
 
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Old 12-16-15, 06:42 PM
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That is a low delta tee. Something is wrong with the system with an 11 degree rise. (possibly low charge)

If you set the stage 2 to comfort it will energize sooner but that unit should be checked.

What was the delta tee with the compressor stage 2 running?
 
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Old 12-17-15, 04:40 AM
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Good catch! Made the change to COMFORT for stage 2 and found it had been set to 2 degrees. Given the way things are working it may be the best I can do. I plan to call the installer and see if he has any insight into the system's performance. I agree the system should spend as much time in stage 1 as possible. It just has to work well enough to let that happen.

I'm much better prepared to talk with him after everyone's assistance. Thank you all again!

We live in extreme northeast Iowa and winter low temperatures are typically in the -25 to -20 degree F range. We simply haven't had much winter to this point. I'm not complaining though.

Steve
 
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Old 12-17-15, 05:01 PM
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I wish I actually made that catch.

I texted my Honeywell Rep.

I wonder what the water pressure and temperature read at the 2 Pete's plugs that should be present.

The static pressure and compressor amperage reading in low and high stage should also be measured before refrigerant gauges are used.

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