Ohio extreme loop pressure issues

I have a 4 ton water furnace with open canister/ non pressurized. Loop field is 4x 150 loop or 600 ft .

I used to have a pressurized unit however in winter of 2015 my system would build extreme pressure blow the hose off the unit. I had the system flushed to among other things, remove any possible air in system. Same issue. I had an open canister installed. Fast forward a couple of winters/heating seasons and I don't have the pressure issue per se however water leaks over the top of my open canister but only when the system has been running in stage 2 heat for extended periods. Generally not an issue as I just add H2o once the levels settle back down however recently the water levels in my open canister dropped to a point where the pumps tripped off and I was on Aux heat all night.

I should mention that we eliminated the possibility of the problem being a Freon or other issues with the geo unit as I literally had a new unit installed in 2015, had the same issue, and then had my old unit thoroughly inspected and reinstalled.

Here's what I think is happening. My system is undersized- both the unit but perhaps more importantly the loop field. As such my unit is extracting every bit of heat possible, longer than normal run times etc. and the ground doesn't have time to recover resulting in loop contraction / shrinkage. It wouldn't take much to cause back pressure in that scenario. I've also heard that there are different types of pipe (some may be more prone to contraction than others). Really believe this has to be the issue as we've accounted for every other possible variable. Thoughts?

 

Thoughts:
- freezing the system solid (in coil)
- air pushing back at random times (can check for this when purging to see if water levels change when system off). A kinked circuit or such could theoretically make it very difficult to purge properly.

 

Sort of follow. The looop had been flushed multiple times by different contractors- one of which is Yoder, here in Ohio and this co, knows what they are doing. Not saying you aren’t correct but does the theory about an excesssivly cold loop field (due in large part to undersized unit and loop field) -that during heavy run times this creates excessive pressure on the loop, causing loop to slightly contract,thereby creating uncharacteristic loop pressure seem feasible?

 

PE Pipe has a thermal expansion coefficient of 83 x 10^-6 in/in per degree F.
If you have say 3,000 feet that gives
3,000 * 12 * .000083 = 3 inches per degree F

If your temp swings from 90 to 30 over a season, that 60 degree change would allow a change of 180 inches pipe length if the pipe is unconstrained, as in a pond. There will also be contraction of the diameter. That is more tricky to figure.

3/4" pipe holds about 0.023 gallons per foot, so a 15 foot change would change your system pipe volume by about 0.34 gallons per season with some simple assumptions. 3000 feet would hold a total of 69 gallons. Water specific density is 0.12043 gallon per lb at 90 and .11984 gallon per pound at 32 degrees. This would be a change of 0.5%, so 69 gallons of summer water would be about 68.65 or about 0.34 gallons. The change in water volume and change due to length of pipe volume would about equal out.

What is a variable is pipe volume change due to contraction in pipe diameter. Running through some PI r squared and 2 PI r math makes me think radius would change by about .995 and volume by .995 squared. SO pipe volume due to diameter contraction could be 99% of warm season. With my made up assumptions, that would be a change of about 0.69 gallons seasonally. SO I would think it feasible that a system change by up to a gallon a year with a 60 degree temperature swing.

 

This pressure/ canister spill over only occurs during very long stage 1& 2 run cycles. Rather than add extra loop and an upgrade to a 5 ton, I’m seriously considering retro fitting unit with a natural gas backup to act as stage 3/ aux., or even to kick in when stage 2 required or maybe even have it run during stage 1&2. This would give loop field time to recover, potentially eliminate the canister overflow issue, cetrainly lower energy costs as I wouldn’t rely on electric aux heat and it would introduce redundancy into system . Thoughts?

 

What is your entering water temperature? If it is well below freezing and the soil around your loop pipes is saturated with water, the earth around your loop pipes is likely freezing. As the saturated soil freezes, it takes up more volume than the unfrozen earth and puts a squeeze on your loop pipes.

 

It’s coming in above freezing at roughly 33 and leaving unit around 28 at the heaviest run cycles. Just had system flushed on Friday with new glycol , good to 15 degrees. Point being, water not freezing in loop but do believe loop field / ground heat being depleted beyond normal given undersized loop field. So to restate everything, geo unit fine, loops been flushed multiple times over past three years and alway have the same issue (pressure) but only when system running heavy - meaning long stage 2 with some aux back up. In my mind, if water isn’t icing inside the loop and I’m confident it’s not, the only way pressure could build is outside /exterb pressure against the loop

 

Arlie, yes that’s what I’m thinking - external pressure but why do I only get back pressure/ canister overflow during long run times?

 

During long run times you are putting 28 deg F water out into the loop and the longer it runs the more ice forms around the loops exposing more and more of the loop pipe to ice. When the unit is allowed to shut down periodically, the ground temperature will melt some or all of the ice that was formed during the previous run cycle.

This ice formation external to the loop pipes is what is holding your entering water temperature up around 32 deg F due to latent heat of fusion. Similar to the latent heat of vaporization, it takes additional energy to make the liquid water go through a phase change from liquid to solid. This indicates that part of your loop field has not yet changed from saturated soil to frozen soil. If your entire loop area freezes, then you will see a temperature drop on your entering water temperature below 32 deg F.

 

The million dollar question. Is this the source of my back pressure during long run times? I assume it has to be. And if so my remedies are 1) add more Loop in ground & possibly a 5 ton vs existing 4 ton unit, 2) get rid of geo and install natural gas and an outside a/c unit or 3) retro fit geo with a natural gas back up to kick in during any of the stages ( giving thought to which ones) as this would give my loop/ground more time to recover. Leaning heavily toward option 3. Also, if it was just that my system was undersized / not as efficient as it should be = higher energy costs I wouldn’t do anything. It’s more about eliminating the back pressure . Thoughts?

 

There doesn't appear to be anything significantly wrong with your loop, especially since you are maintaining >32F entering water temperature. Going to a 5 ton unit vs. existing 4 ton unit wouldn't make any appreciable change. If you still had a pressurized system, you could have installed a bladder type pressure expansion tank that would have been able to absorb the loop contraction without appreciable changing system pressure (as the loop contracts due to cold/ice, the rubber bladder in the tank expands to hold the pressure relatively constant).

With your existing non-pressurized system, you could do a couple of things: 1) with a hand pump of some sort, draw down the fluid level in your flow canister to the minimum level at the start of the heating season after there is any chance of running AC and store the fluid in a jug (s), then in the spring before you run your AC, pour the removed loop fluid back into your canister, or 2) you could build a non-pressurized volume expansion tank similar to a car radiator overflow tank as I am not aware of any commercially available volume expansion tanks for water loops. You could build the expansion tank out of 1-1/2" and 4" pvc pipe to absorb the excess fluid without overflowing your tank. The 1-1/2" PVC pipe would Tee into your loop return line to your canister. This 1-1/2" PVC pipe would go up and Tee into the 4" PVC pipe running horizontal. The top of the 4" PVC pipe would need to be roughly at or slightly below the level where your canister overflows. The length of the 4" PVC pipe would need to be enough to accommodate the volume that you are seeing expelled. 4" PVC pipe holds approximately 0.5 gallons per foot, so a 2' horizontal piece of pipe would hold roughly a gallon. The 4" PVC pipe needs to be vented to atmosphere to allow water to enter. At the top of this 4" you could Tee in another short piece of 1-1/2" PVC pipe with a couple of 90 deg bends at the top to form a turned down vent path. The 4" PVC pipe will need caps on each end. Another simple way to to create a vent for the 4" PVC pipe overflow tank would be to drill a 1/4" hole at the very top of one of the pipe end caps. That is probably all that is needed for the vent, but you could RTV glue in a short piece of 1/4" plastic tubing to form a turned down vent opening.

 

If I’m following, I’ve done some of what you suggested already. In other words, when we get into the heating season and the water starts to rise in the canister I take water out and put in a jug however the problem with that is that when the system stops running and or the loop expands back to its normal size , occasionally that water will drop well below the pump levels in the pumps will kick off and the System will go into emergency aux mode. I Then have to add about 3 gallons of water and flip the breakers to get the system working again. In other words, it’s incredibly manually intensive -I have to babysit the system. As far as the pressurize system with expansion tank, I get so much pressure that quite literally it would turn itself into a bomb because pressure well over 200 psi . System performs as expected 85% of the time . It’s only during extreme cold spells or less than 15% of the time that I have pressure issues. Again, to me , the only viable solution is to have less stage 2 and aux and the way to accomplish this is a cost effective back up like natural gas. This will materially lower what I spend on 3rd stage :aux heat (gas vs electric) and give loop field time to recover

 

Also, I respectfully disagree that there isn’t an issue with the loop field. No reason I should be blowing hoses when I had a pressurized system, or having fluids constantly leaking out of canister during heavy sun times - so much so that I have to add gallons of water back after the loop presumably expands back after the long run times. I’ve had multiple hvac folks out along with drillers multiple times and non of them has seen anything even close to what I’m experiencing. ThiS is why I’ve reached our, etc

 

ALSo, loop temp coming in was 44 degrees Friday morning but down to 29 or so today after a very cold weekend. And now system having trouble maintaining 68 degree (66 degrees for a while now) Not sure what to read into that or if that’s expected but in any event, seems to strongly suggest undersized loop field which is why leaning toward the gas retrofit, etc.

 

If you are at 29 degrees F entering water temperature, then you have essentially frozen all of the earth surrounding your loop pipes. This is different information than was posted previously.

Note that it doesn't take very much expansion/contraction volume wise in a closed system to cause large swings in pressure since water is essentially non-compressible. If there had been a properly sized expansion tank installed with your pressurized loop system, say a 5 gallon bladder tank with the capability to absorb ~2.5 gallons of expansion, then pressure could have been controlled without exceeding system limits.

Is your loop field in a water saturated / soggy area in the winter? That might explain the large amount of contraction present during extreme cold conditions.

Do you actually have 4 loops with 150' of pipe each or do you have 4 trenches 150' long with 300' of pipe in each trench (150' down and 150' back) or is it a slinky system where the trench is typically 3' or more wide and 600+ feet of pipe is put in a 150' long trench? If you only have 4 loops 150' long = 600' of pipe, then you are definitely short looped for 4 tons of load. Evan at 4 loops x 300' of pipe each, you would be marginal. But a lot depends on the loop spacing, depth, type of soil, and amount of moisture present. In my area, for each ton of capacity, a typical loop of pipe is ~600' long (300' trench at 6' to 8' deep) and each trench is at least 10' away from the adjacent trench.

 

Do you have the space available to install an additional 3' wide by 150' long trench where a 600' long slinky of 3/4" loop could be installed at 6' or more depth? If so, I would get pricing on that option to see how it compares to your other options.

 

4x150 or 600 verticale. Not sure how they configured and in otherwise “dry” soil to my knowledge. Given that we’ve established its undersized , thoughts on my solution? Leery of any expansion tanks as I’m done dealing with bleeding tanks, checking fluid levels constantly etc. I either want to try the option I laid out and have gas back up or I’ll remove geo and install conventional natural gas furnace. And ev n if there is an expansion tank solution, with an undersized loop system I’m paying far too much on expensive electric aux. RAther rely on natural gas

 

Before abandoning Geo, if you have natural gas available, you might consider adding a small natural gas boiler and add a hot water coil into the ductwork. You could sequence the hot water to come on with stage 2 heat.

 

That’s exactly what I was thinking . Have you done that before and or know of anyone who has? I have an hvac contractor who comes highly recommended and that’s what he and I have been discussing. I had his Service guys at the house today just to check the coil to make sure there’s no Freon leak. Everything checked out good with the unit. Therefore I’m ready to proceed with the retro