I've had my geo heat pump for close to 7 years now and it has never been able to maintain the desired temp on the first floor of my house. I keep the tstat around 72 but it never gets warmer than 67 to 69 and the HP runs all the time. I don't know anything about HVAC so I always assumed this was normal but this year I decided to research it a bit more. I have a set of gas logs that I use when I can't bear the chill any longer. I probably use around 80 gallons of propane during the winter.

Using cheap probe thermometers I've found that my return air temp is around 67 and my supply air temp is between 75 (stage 2, high fan) and 85 (stage 1, low fan). LAT is around 87-ish when aux heat strips come on and fan is on highest. The heat strips seem to come on several times per day. I contacted the installer but their gshp expert no longer works for them and the service people they sent seem a little lost. I'm not trying to be disparaging, they admitted that they don't know much about it.

Here is some information about my system:
1) Located in West Virginia.
2) Not sure about heat loss/gain calcs, sorry. I have the original WaterFurnace Energy Analysis but I'm not sure what to look at.
3) WaterFurnace E072 - 6 ton (with hot water assist but currently turned off), installed in early 2004. Intellizone panel with 4 zones (1200sqf, 1200sqf, 500sqf, 500sqf). The two large zones are turned on all the time, and one of the small zones is turned on about half the time. The fourth zone is rarely turned on.
4) Horizontal loop, 3x 300' trenches, total of 3600' of pipe, 3/4" pipe (I think), pipes at 6' deep and 4' deep, 2 Grundfos UP26-116F pumps.
5) From my electric bills it looks like during Dec-Feb I usually use 2700 - 3500+ khw per month. April and October are my lowest usage months with around 700 kwh per month. I do use around 80 gallons of propane in winter.
6) EAT 65 to 67, LAT 73 to 85 (not including aux heat).
7) EWT ~37, LWT ~35 (around 1.5 to 2 degrees difference).
8) Not sure about the percent of load. On the Energy Analysis I see "heating load: 101.6 million btu, WF unit: 10,096 kwh" and "cooling load: 37.9 million btu, WF unit: 1,857 kwh". I also see a line that says "max system balance point 10 degrees F and average system balance point 3 degrees F".
9) Not sure what the installer's assessment is. They want to try turning off one of the pumps thinking that the flow is too high. I assume this suggestion came from WF tech support but I don't know that for sure.
10) I don't have any previous heating/cooling costs since this was installed in a new house.

When the service people checked the pressures they said suction pressure was around 105 and discharge pressure was around 235 when the unit was running in stage 2. They decided to add a little refrigerant to see what would happen and the discharge pressure came up to around 260. According to the numbers I could find in my WF manual the suction pressure should have been somewhere around the 70s - 80s and the discharge pressure should have been around 300 - 330. But again, I know very little and I'm just looking at the numbers in the book.
The idea of turning off one of the pumps seemed a little odd to me but when I tried to research it I saw mention of Reynolds numbers and such and it quickly went over my head.

Am I correct in assuming that my LAT should be much higher? From my WF manual it looks like I should expect 90 - 100+ degrees, even without the aux heat strips.
Should I be concerned about this idea of turning off one of the pumps to see what happens?
Does anything else stand out as being a problem?

Any insight you can provide would be greatly appreciated. I only have a few years left on the warranty and I'd like to make sure my unit is working correctly before it expires.

Thanks in advance.
Glenn

There is something wrong with your refrigerant circuit, your delta T on the loop side is too low. While (2) 26-116 sound a lot of pumping power (although I do not know your pressure drop calculations) and could be a reason for a lower delta T, 2 degrees is too low. Also if your pump was running 24/7, your loopfield should be much colder by now, not 37 degrees EWT. So techs need to determine the flow and calculate the heat extraction.
If Heat extraction is confirmed to be low, chances are that you need even more refrigerant, or you have an issue with your compressor.

Yeah, surprised they didn't take pressure readings across the in/out ports so they could calculate the flow rate.

Posted By glenn.campbell on 07 Feb 2011 10:20 AM
Using cheap probe thermometers I've found that my return air temp is around 67 and my supply air temp is between 75 (stage 2, high fan) and 85 (stage 1, low fan). LAT is around 87-ish when aux heat strips come on and fan is on highest.

LAT is 85 in stage 1, but goes down to only 75 in stage 2? That seems kinda wrong.

The low LAT in stage 2 and low water side delta T cause me to suspect 2nd stage may not be fully active - blower speeds up but unit compressor stays in low stage. Any tech familiar with two stage sytems - regular heat pumps using Copeland Ultratech compressors (most brands except Trane) ought to be able to diagnose that (compressor current rises about 50% in high stage)

Low heat extraction is another observation supporting this hypothesis

There is an easy way to check approximate water flow through the unit assuming it has "Pete's Ports" - measure pressure in and out of unit using same guage for both readings and subtract.

Are you measuring return temp at the unit or at a return grill in the conditioned space. Leaky return ductwork or uninsulated return ductwork passing through a cold space will depress return air temp by the time it gets to the unit.

I found my manual that has the table of pressure drops and corresponding flow rates so when the techs come back I'll see if they can tell me what the pressure drop is and try to figure out the flow rate.

I'm measuring the return temp at the unit, next to the filter. I tried to take the measurements as close to the coil as was practical. So the probe for the return temp is in the duct before the air goes through the filter and the probe for the supply air is in the duct several inches from the unit.

As far as the difference in LAT in stage 1 vs. the LAT in stage 2, I assumed the lower LAT in stage 2 was because the blower was running at 1850 cfm and when in stage 1 the blower was running at 1000 cfm. I thought the higher air flow might account for the lower LAT. But that was just an assumption on my part.

No, usually stage 2 results in 2-3 degrees higher LAT. You are right about the higher airflow, but stage 2 usually more than compensates for it. Again, get the flow and the heat extraction in both stages, and go from there.

Posted By glenn.campbell on 07 Feb 2011 02:21 PM
As far as the difference in LAT in stage 1 vs. the LAT in stage 2, I assumed the lower LAT in stage 2 was because the blower was running at 1850 cfm and when in stage 1 the blower was running at 1000 cfm. I thought the higher air flow might account for the lower LAT.

Doesn't the cfm settings seem to be a bit wrong?  6 ton unit with only 1000 cfm in stage 1 and a delta T of around 20ish...  Is stage 1 really only running with 1000 cfms?

Posted By dgbair on 07 Feb 2011 07:28 PM
Doesn't the cfm settings seem to be a bit wrong?

Seemed low to me, too -- but the specs say it's OK.

http://waterfurnace.ca/Engineer/E%20Series/E%20Series%20Specification%202%20to%206%20Ton.pdf

Try WF site for a local tech.
You need more help than we can do remotely.
(although you might be on the Custis summer tour route.......)

Thanks for all of your insights so far. I realize this is getting too deep to expect any resolution remotely but I do very much appreciate the suggestions you've given. I'll keep posting information as I get it in case there are any red flags that I should be aware of. And hopefully it can be helpful (or at least moderately interesting) to others.

The service tech stopped by yesterday. He thought maybe the flow was too high and causing the problem so he wanted to try disconnecting one of the pumps to see what would happen. I had him take some readings before disconnecting the pump. These readings were when the unit was in stage 2.
EWT: 34
LWT: 31.9
delta T of the loop: 2.1
pressure drop: 8 (60 - 52)

The chart in my manual didn't go up that high so I had to extrapolate a little but it looks like that would be around 19 GPM. Assuming the formula I found for calculating heat extraction is correct (HE = delta T * GPM * 485) I get around 19K.

After he disconnected power to one of the pumps and waited about half an hour the readings were:
EWT: 34.2
LWT: 31.1
delta T of the loop: 3.1
pressure drop: 4.7 (51.9 - 47.2)
From the manual it looks like that pressure drop equates to around 13 GPM. Plugging those numbers into the heat extraction formula still gives around 19K.

I forgot to ask for readings in stage 1 and I'm kicking myself for it. I'll try to get those when the service tech comes back. He said he was going to take these number and get back to me later.

Again, thanks for all of the information you have provided.

The unit should use about 3 gmp/ton, so your flow with 2 pumps on is ok. 19 kbtu/h Heatextraction is way too low for a 6 ton unit, it should be about twice as much in 2nd stage. Likely compressor or refrigerant issue.

I continue to wonder if the compressor is actually entering stage 2. There is a perceptible noise change upon stage change, although I find it impossible to tell by ear alone what stage a unit is in when I walk up on it. A clamp amp meter reading taken at the compressor should tell which stage. Any tech should be able to check this in seconds.

Here's the latest update on the situation with my heat pump. I had another service tech take a look at my system. He was kind enough to make a quick stop over the weekend to get some initial information about my system, including the sizes of my ducts. I didn't think to post that information earlier so here it is now:

The two large zones each have 8x16 ducts.
The two small zones each have 8x12 ducts.
The return duct is 10x24.

He called back today and said that after running the numbers my ducts seem to be undersized and that inadequate air flow in stage 2 is likely the cause of my low supply air temperature. This was very disheartening news for me since my house is a 2.5 story and I can get to only the first floor duct and the return duct to make any changes, without tearing into walls and ceilings.

I'll post more information as the situation develops. Hopefully the information will be valuable to others in the future.

WRONG answer.

Either tech is flailing and failing or you've incorrectly relayed what you were told...it happens.

Low air flow INCREASES rather than decreases supply air temp.

Those duct sizes are woefully small for a 6 ton unit. A 6 ton unit needs to flow 4 tons of air in low stage. Forcing that amount through an 8x12 or even 8x16 is ludicrous.

I wouldn't size a 2 ton return at 10x24, never mind 6 tons.

Ducts definately not a good match/design is suspect. Hopefully the techs you are employing are not from the firm that installed this system.
I agree with Curt. I'd like confirmation that stage 2 is energized and make sure all zones are open when this thing runs.
Code in my area suggest 6sq" of return for every 1,000 btu's so you have about 1/2 the indicated return.
Good luck,
Joe

Posted By engineer on 16 Feb 2011 12:41 AM

.... 

Those duct sizes are woefully small for a 6 ton unit. A 6 ton unit needs to flow 4 tons of air in low stage. Forcing that amount through an 8x12 or even 8x16 is ludicrous.

I wouldn't size a 2 ton return at 10x24, never mind 6 tons.

Hmm, I wonder if my duct work is sufficient.  What is the ideal duct sizes for a WaterFurnace Premier Unit Model PO46T Deluxe Geothermal Unit (46,000 BTU)?


> Code in my area suggest 6sq" of return for every 1,000 btu's so you have about 1/2 the indicated return.

So I would need 276 sq. inches of supply and return? So that's 16.5 x 16.5 inches? Another thing I never quite understood was that air ducts are huge, but the supply vents into the rooms are very small, perhaps 4 x 10 inches at the most. I would think this would cause a bottle neck. The return vent it a lot bigger though.

I would want more than 16x16 return on a 4T - Joe's figure may be a code minimum and more return is nearly always better

Necking area down at a register increases air velocity and throw (distance air travels into room) this is desireable, up to a point, to ensure whole room is conditioned and air does not stratify.

Returns are supposed to be designed for lower velocities so end up larger.

Posted By engineer on 16 Feb 2011 08:02 PM
I would want more than 16x16 return on a 4T - Joe's figure may be a code minimum and more return is nearly always better

Necking area down at a register increases air velocity and throw (distance air travels into room) this is desireable, up to a point, to ensure whole room is conditioned and air does not stratify.

Returns are supposed to be designed for lower velocities so end up larger.

The main supply coming out of the unit is 17x17, this splits into two 20x12 branches, the vents get progressively smaller the farther from the unit until they are 12x12 to the supply the vents. The return is mostly 20x12 until it reaches the main return, which is 26x14. The return doesn't line up perfectly, so there an offset from the 12x20 returns to the 26x14 of about 4 inches, kinda of like an small h connecting into a square.

After all that ductwork discussion, could we get back to the point that the heat extraction is about half of what it should be!!!! A 6 Ton unit in second stage should extract about 35-40 KBTU/H, but it does less than 20 KBTU/H. That is the key question you need to ask your tech!