Do we have a leak?
I'm house-sitting for a friend who's away for extended time, but we're in frequent contact. We're trying to determine whether or not his slab-heat residential boiler system has a leak (approx. 1,800 square ft).
He recently had the zones drained (they were nearly or completely pure water) and refilled with a glycol mix. This was the first time doing this after twenty years of successful service. We're not sure, but presumably the system was stocked with glycol at the outset.
When considering the question of "Where did the original glycol go?," we're entertaining conflicting theories (not between he and I; between theories).
- There's a leak, and the water was introduced via the makeup valve.
- There's no leak, and the water was introduced via various maintenance projects over the years, where the system was partially drained each time.
(Note that he doesn't have a glycol makeup tank installed—the present configuration allows clear water to flow in.)
To attempt to isolate all of this, we've closed the line valves before and after the makeup valve and undertaken a twice-daily monitoring effort. We started in mid-November of last year.
We've noticed a ten-pound drop from thirty to twenty since that time, but we don't know whether this is expected/normal. In the case of a leak, one might suppose that the pressure would drop faster than this—even with the tiniest of pinholes.
Amid all of this is the question: "If we don't anticipate pressure loss, why the concept of a makeup valve in the first place?"
Do we have a leak?
Comments
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if the pressure doesn’t drop to 0 ever, you probably do not have a leak. Assuming there is no water make up system connected?
The pressure will vary depending on the temperature of the system and the size and condition of the expansion tank.
If the boiler has a common 30 psi relief , you can run up to 25 psi if needed
Why is there glycol in the system?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
Its possible you have an expansion tank issue (low air pressure in the tank)
Its possible you have a leak but since you have been watching it for 3 months it would have to be a small pin hole.
My understanding with glycol is it should not in most cases be connected to the water MU system for several reasons.
- if you have a leak and your MU is diluting the glycol you will never know
- You don't want glycol to back up into the water system
We used to leave the make-up water end near the system with a hose end valve. The put a hose end valve on the boiler. Connect these with a female x female washing machine hose.
That way the only way the glycol gets diluted iis if someone knowingly connects the hose and opens 2 valves
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if the pressure doesn’t drop to 0 ever
It hasn't yet, but of course it would if we let it, given the current drop rate. Granted that'd take some time, but eventually it would.
The pressure will vary depending on the temperature of the system and the size and condition of the expansion tank
The system hits +180°F max; the pressure tank is new and is, as far as I know, the normal size for this type of system (about the size of a 20lb propane bottle).
Why is there glycol in the system?
That's standard fare here in the Alaskan Interior. Winter temps hit -40°F regularly. In fact, before the glycol replacement, the garage slab froze twice due to small air leaks under the main door. Thankfully neither incident was a hard freeze, and I was able to get the zone flowing again with an all-night floor heating session each time.
Thanks for your reply. In your opinion, if we don't have a leak in one of the zones, what could be causing the drop? Are there other avenues in a typical home boiler system for pressure to escape?
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new water has air in it that comes out of solution and gets vented by the air elimination. the precharge in the expansion tank usually slowly leaks out the shcrader and possibly permeates the membrane so it tends to be gone after a couple years and that will drop the pressure.
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Its possible you have an expansion tank issue (low air pressure in the tank)
That's one thing I forgot to mention: it's a new pressure tank as of summer last year. Given this, would you still possibly suspect a tank issue?
since you have been watching it for 3 months it would have to be a small pin hole
That's the part that's difficult to gauge, given our collective inexperience. At what rate would one expect the pressure to drop with the smallest of pinholes? Is our drop excessive, normal or ideal?
glycol … should not … be connected to the water MU system
Agreed. He's aware of the option of having a glycol makeup tank installed, but that's an expense he's still mulling over. Of course, that expense would pale in comparison to that of replacing a zone with a different heat source, but we're not quite there yet. We're still trying to pin down the cause of this pressure drop.
And we hadn't even considered the problem you point out, that of the possibility of glycol backing up into the home water system. We'll add that one into the mix, thank you.
the only way the glycol gets diluted iis if someone knowingly connects the hose and opens 2 valves
So do I hear you saying that you've been able to leave a system without a makeup for some time without any pressure drop?
Overall, what do you think about our rate of drop? Is it something to be concerned about?
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That's very interesting! The one thing I forgot to mention in my original post is that the pressure tank is brand new as of late summer last year.
So all of that might explain our drop, do you think?
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I have heard from a manufacturer's rep for Amtrol, the folks that make Extrol expansion tanks, that the membrane can have as much as a 1 PSI air pressure drop over a year by gas migration thru the membrane. That air then can vent out of the closed system by way of an automatic air vent. When that air leaves the tank the tank has more room for water in it. As the air is vented into the atmosphere out of the closed system, the pressure will drop. I don't think that the pressure drop is linear though so you can have enough air leaving the tank to cause more than a 1 PSI water pressure drop on the boiler gauge (but not 10 PSI).
Another way air is lost from the expansion tank is by the schrader valve. Have you ever “Tested” the tank by letting a short burst of air out of that tank? That is not the correct way to test the tank, all that does is let air out, it does not tell you the amount of air that is in the tank. You need a tire pressure gauge and you must make the boiler pressure lower than the 12 PSI that is supposed to be in the tank. (That is a problem for your system since you need to let water/Glycol mixture out of the boiler system in order to reduce the pressure.)
You see, if the boiler water pressure is at 20 PSI, then the water in the closed system will fill the tank until the air in the tank is compressed to 20 PSI. If the empty tank air pressure is say 6 PSI, then it takes more water in the tank to compress that small amount of air to 20 PSI, but if the empty tank has 15 PSI in it, then it takes less water to compress that air to 20 PSI. but the tire pressure gauge will still measure 20 PSI regardless of the amount of air that is in the tank.
See this discussion for details on that topic
Also, another way to lose pressure on the boiler gauge is if the water pressure exceeds 30 PSI and the relief valve releases the over-pressure into the drain or floor. Since you have had the boiler pressure near 30 PSI, there is a strong possibility that the relief valve has discharged some water/glycol mixture at some point.
Finally, (although there are other things that might account for the pressure change, this is the last one I will offer) if you take the pressure reading when the boiler and system are at a high temperature, when the water is expanded to its maximum temperature you will get a higher pressure reading the very same day, if you measure the pressure after the boiler has been off for sometime and all the water temperatures in the radiators, pipes and the boiler are at their coldest where the water contracts to its smallest volume, then the pressure will be lower. So, take that into consideration when you take the pressure readings. What is the system (not just the boiler) temperature.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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See this discussion for details on that topic
Yes, I'd like to get my head around that. Do you have a link?
So, take that into consideration when you take the pressure readings. What is the system (not just the boiler) temperature.
Good point! I'll consider that with my readings. But even still, that wouldn't seem to explain the 10lb drop over the ~three months.
You need a tire pressure gauge and you must make the boiler pressure lower than the 12 PSI that is supposed to be in the tank. (That is a problem for your system since you need to let water/Glycol mixture out of the boiler system in order to reduce the pressure.)
I suppose I might wait until it gets down to 12psi and try that then.
Another thought I had, especially since warmer outside temps are on the horizon, is to try this pressure monitoring project against the individual zones, i.e. open them one at a time and run my checks then.
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the zones are all open all the time, the return is still open to the system pressure even when a zone valve is closed.
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Here's a snapshot view of the system. The smaller valves on the left can be used along with the main zone values to isolate a given zone.
Is that what you were referring to?
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@jeffbowman Said: "Good point! I'll consider that with my readings. But even still, that wouldn't seem to explain the 10lb drop over the ~three months."
Yes it can. This can explain the pressure difference from one hour to the next. As the call for heat is ongoing, the entire system with the circulator operating with all zones open, will have more water in it by volume than the entire system has when it is cold, and there is no call for heat.
Here is a fact: 100 gallons of water at 70°F will increase in volume to 104 gallons of water at 210° just by the water expanding. that is a 4% increase in volume. Now water can not be compressed, so in a closed system where there is no air to compress, where does the 4 gallons of water go? If the closes system was a 100 gallon drum. the hydraulic pressure would cause the drum to expand and balloon out of shape to fit the larger water volume until the tank would no longer hold the pressure (that could go over 500 PSI). then the tank would burst and explode with a very violent result. Look at this video about a water heater with no relief valve
So your system can change from 20 PSI to 30 PSI in less than one hour from a cold start to a full system reaching the limit temperature.
The expansion tank is the place reserved for that expanding water. There is an air cushion in the tank to allow the water to compress that air in the tank as water enters the expansion tank
Here is the link about the empty air pressure compared to the tank when it is connected to the boiler pressure.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Yes it can. This can explain the pressure difference from one hour to the next. As the call for heat is ongoing, the entire system with the circulator operating with all zones open, will have more water in it by volume than the entire system has when it is cold, and there is no call for heat.
So in other words… you don't think we have a leak.
Correct?
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So in other words… you don't think we have a leak.
Correct?
Well actually, I was just about to put an EDIT on my last comment.
the edit was going to be short and sweet:
"But that does not rule out a possible leak"
EDIT: You need to have a more consistent test criteria to prove the leak theory.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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