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Author Topic: Ground loop pressure drop  (Read 4756 times)
Whimbley
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« on: April 30, 2020, 07:50:35 PM »

We had a few teething troubles with our GSHP after install 3 years ago. First the valve caps on the air relief valves cracked and ultimately failed, which were replaced with stronger versions. Second, a leak appeared from within the unit itself, which was diagnosed as the elbow joints from the ground loop being tightened too much on install. All replaced and after that we seemed to be trouble free!

Or so I thought. I've noticed a drop in pressure, from about 1.2 bar to 0.8 bar at rest. I can't see any leaks fortunately, but it sounds like air is starting to get in the system as the external brine pump is getting noisier. I have read that the pressure can drop as the temperature increases, is that correct? The pressure drop has coincided with the warmer weather, and seemed to be stable over winter after the joints were replaced.

We have a long ground loop (1200m) and the incoming temperature has gone from 4 degrees over winter to 8.5 currently. Does that sound about right or should I get the installers to take a look?
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benseb
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« Reply #1 on: May 16, 2020, 08:07:41 AM »

I would have thought pressure would increase with higher temp

But 4c difference would be negligible. Iím not an expert on GSHP. If the drop is stable then perhaps there was a small leak on a joint thatís not stopped as the pressure is lower. Is there an expansion tank, perhaps thatís playing up

Have you tried to increase pressure back to 1.2 bar?
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titan
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« Reply #2 on: May 16, 2020, 01:10:19 PM »


 I've noticed a drop in pressure, from about 1.2 bar to 0.8 bar at rest. I can't see any leaks fortunately, but it sounds like air is starting to get in the system

That should read unfortunately, as if you have a leak on a ground loop it would be nice to be in a place to be able to see it. Difficult to see how air is getting into a pressurised system. I would put the pressure back up to it's design pressure and check for leaks, the leak may be small enough to stop at 0.8 bar but may show at a higher pressure.

I should have added it may be the expansion vessel, I would check the air pressure in that before restoring the loop pressure
« Last Edit: May 16, 2020, 01:13:34 PM by titan » Logged
Countrypaul
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« Reply #3 on: May 16, 2020, 01:30:42 PM »

Water is at its densest at 4C. The thermal expansion of water with increasing temperature from 4C is about 0.0002 per degree C, so you 1200m rising 4C would expand by about 24cm, assuming your brine expands the same as water. What diameter is your ground loop and do you have an expansion vessel, if so how big?

I would be tempted to do as suggested as a first step repressurise back to 1.2bar and see how it goes. If the ground loop temperature rises another 4C and the pressure drops again then that might be normal. If the ground loop temeprature remains constant and the presure drops at least you could rule out the ground loop temperature as the cause of lower pressure.

If you leave it at the lower pressure (is it with spec for the GSHP?) and nothing changes until the ground loop temperature starts to drop and teh pressure increases then you may want to consider whether it is just a seasonal effect.

I take it from your description that you have no previous years to compare to as there were problems with the system then anyway.
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JohnS
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« Reply #4 on: May 16, 2020, 04:54:34 PM »

Water is at its densest at 4C. The thermal expansion of water with increasing temperature from 4C is about 0.0002 per degree C, so you 1200m rising 4C would expand by about 24cm, assuming your brine expands the same as water. What diameter is your ground loop and do you have an expansion vessel, if so how big?

I would be tempted to do as suggested as a first step repressurise back to 1.2bar and see how it goes. If the ground loop temperature rises another 4C and the pressure drops again then that might be normal. If the ground loop temeprature remains constant and the presure drops at least you could rule out the ground loop temperature as the cause of lower pressure.

If you leave it at the lower pressure (is it with spec for the GSHP?) and nothing changes until the ground loop temperature starts to drop and teh pressure increases then you may want to consider whether it is just a seasonal effect.

I take it from your description that you have no previous years to compare to as there were problems with the system then anyway.

Paul

I am a bit confused. 

With water at its greatest density at 4C, when the temperature increases, it becomes less dense and expands and thus takes up more volume.  This increase in volume would squeeze the air in the pressure vessel, increasing the pressure.

Or have I got it all wrong?  Or is the ground loop pipe stretching/expanding at a greater rate or something like that?
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Countrypaul
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« Reply #5 on: May 16, 2020, 07:13:39 PM »

Water is at its densest at 4C. The thermal expansion of water with increasing temperature from 4C is about 0.0002 per degree C, so you 1200m rising 4C would expand by about 24cm, assuming your brine expands the same as water. What diameter is your ground loop and do you have an expansion vessel, if so how big?

I would be tempted to do as suggested as a first step repressurise back to 1.2bar and see how it goes. If the ground loop temperature rises another 4C and the pressure drops again then that might be normal. If the ground loop temeprature remains constant and the presure drops at least you could rule out the ground loop temperature as the cause of lower pressure.

If you leave it at the lower pressure (is it with spec for the GSHP?) and nothing changes until the ground loop temperature starts to drop and teh pressure increases then you may want to consider whether it is just a seasonal effect.

I take it from your description that you have no previous years to compare to as there were problems with the system then anyway.

Paul

I am a bit confused. 

With water at its greatest density at 4C, when the temperature increases, it becomes less dense and expands and thus takes up more volume.  This increase in volume would squeeze the air in the pressure vessel, increasing the pressure.

Or have I got it all wrong?  Or is the ground loop pipe stretching/expanding at a greater rate or something like that?


John,

I think you have it right.

Given that you indicate others have found the pressure can drop as it warms up, I assume the ground loop and other things expand reducing the pressure (as I say, assume), however if you have a leak in, say, the expansion vessel bladder (but could be elsewhere) then that would also result in a pressure drop, extremely irritating if once the pressure drops to 0.8bar it then seals itself.

Could it be that the wamer weather results in less water in the ground, so less external pressure on the ground loop, so it can expand a little? I don't know, but we can probably come up with all sorts of theories, it is tying those down with real evidence that is difficult.
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rogeriko
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« Reply #6 on: May 16, 2020, 11:29:45 PM »

If you have 0.8 bar pressure there is no way air can be getting in. That can only happen when you have 0 bar or minus pressure. Just repressurise it and it will be fine.
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Iain
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« Reply #7 on: May 17, 2020, 07:15:16 AM »

Hi
If you have an EV fitted on the loop. While there is no pressure or low pressure it might be worth checking if the pre charge pressure is correct in the EV. Just to see if it has dropped, would cause system pressure to drop. Or if it is set too high, can also cause pressure to drop by causing the PRV to operate as the EV can't accept any rising pressure.
A lot of EV's come new with 3 bar precharge and need dropping to reflect the loop pressure or if they leak after installation the pressure will drop

Worth a check if you have one.

Iain
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« Reply #8 on: May 17, 2020, 10:44:36 AM »

My system has 1200m of ground loops.  Pressure was initially set to 1.0 bar.  Over the course of 4 years the pressure slowly dropped to zero and you could hear air "gurgling" when the brine pump started. The level in the expansion tank also rose a little.  Repressurising to 1.0 bar (just pumped some air in) took the level back to original and stopped the gurgling.  I suspect the drop in pressure is air leaking out around the top of the tank.

Over the course of a season my ground loop temperature varies from a high of 15 deg C at the very start of the season to a low of around 8 deg C.  The level in the expansion tank varies very little, the pressure also changes little  (0.2 bar) at most.

The tank is 120mm diameter and the level changes by a maximum of 40mm (I have marked the tank!)  So that is a volume change in the tank of around 0.5 litre.  The capacity of the loops is around 1200 litres and the expansion coefficient of pure water is 0.000088 / deg C at 10 deg C   which is roughly 0.1 litre / deg C   so over the range of 7 deg C that is 0.7 litre - close to the 0.5 litre change I see in the tank. 

There is no expansion vessel with the rubber diaphragm on the brine side of my system it is just a sealed pressurise clear tank.

To be clear,  in my system higher brine temperature = higher level in tank and higher pressure. Which seem logical to me as water expands with increasing temperature (above 4 deg C).  The coefficient of expansion of HDPE pipe is very, very small compared with that of water. Not sure how the anti freeze affects the coefficient of expansion.

Roger
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Whimbley
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« Reply #9 on: May 20, 2020, 09:29:48 PM »

Just catching up with all the replies! Will hopefully answer everyone who asked questions:

We have 1200m of ground loop, it's 40mm diameter, 2.3mm thick. There is also about 100m of insulated pipe running from the manifold to the house, as the ground loop is in a paddock behind the garden.

There is an expansion tank, though it has no separate pressure gauge, not sure if it should? I did wonder if there could be a bit of residual air in the system that has expanded with the warmer weather. When the temperature dropped in early May, the pump was much quieter again.

The pressure on the ground loop gauge hasn't dropped any more, so I'm tempted to think it is a seasonal effect. And it has been difficult to see if this would be normal due to the previous problems. The pressure is within spec (min 0.2 bar according to manual) so think I will keep an eye on it for now and mention it when it is due for the service.
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TT
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« Reply #10 on: May 21, 2020, 11:50:41 AM »

https://www.kensaheatpumps.com/accessory/antifreeze/

my service company did this, they have an old wheely bin that they pump the system through the wheely(just used to move it easily about) bin, letting air out and filling it back up with the supply being at the bottom of the glycol level.

maybe worth checking the glycol level too?
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