Radiant system losing pressure

WarmJames

hello everyone, looks like a great site, glad I found it.
So I have an in the floor thin slab radiant system I installed about 25 years ago when I built the house, and it has been generally a great system functionally and quite reliable (less a couple of pumps and zone valve motor failures).

last spring after I shut the system down for the season, I decided to close the domestic water supply valve and watch the pressure gauge in the system and to my amazement after about a week it went from 10 to 0 psi. so I opened the valve again and pressurized the system then repeated the test with same results. I left the supply valve closed all summer long.

so the question is where did the water go?
I have access to visually inspecting a lot of the copper supply piping, and where I don't (like between first and second floors) I see no evidence of leaking down to the lower story. I have also visually inspected all of the zone valves and tubing manifolds, all dry as a bone.

To be honest after I did the test in the spring and the subsequent visual inspection I basically forgot about the problem all summer long, please don't ask me why..

This fall when I started the system and opened the water supply valve I could hear air rushing out of the air separator for about 10 seconds or so, and since this is a 1 1/2 inch main circulation loop that sounds like a fair amount of water missing in the system to me...

I know I have not given a lot of system details, but I thought I would just pose the question and let people ask what might be relevant.

Thanks in advance, James

Zman

Some systems just leak very slightly. The big concern is that the fresh water added is oxygenated and will cause corrosion. I would try to quantify the leak. Does it take a whole week to drop? Are you just adding <1 gallon?
You can put the system under high pressure to try to find it if it is an issue. Little leaks like that can be very hard to find.

WarmJames

Thanks Zman,
Yes it takes the full week to drop to 0, maybe a little longer and I would say I am adding less than 1 gal to bring back to 10psi. Since the added water is from the utility I would say it is oxygenated, but isnt that the function of the air seperator to remove any gas?

Zman

It is hard to explain but the oxygen is entrained in the water and is only depleted when it oxidizes something. This can corrode ferrous components in your system. I don't think I would be too excited if I was losing such a small amount, although it would be nice to know where it is going.

WarmJames

yes I agree been nice to know where it's going..
I did some other reading and one post suggested taking a water sample from the Why strainer and see if there are any black bits in there to see if they're attracted to a magnet telling you that it's corrosion. I did this and there were a few very small black bits but not magnetic, so I assume hose or gasket fragments. I also checked the water pH and it looks like it's sitting at about 6.8. The only ferrous metal in the system is the boiler jacket itself.

GroundUp

6.8 is very low, likely due to your excessive makeup water, and will speed up the corrosion process. This is subjective, but I like to keep pH in the 8.5-9 neighborhood. How many loops is the slab system and does each loop have its own isolation valves? For the system to be losing anywhere near 1 gallon of water over the summer, you have a fairly substantial leak and if it were above the floor I'm fairly certain you'd have noticed. If it were mine, I'd be isolating 1 loop at a time and keeping the makeup water closed while letting the system run. If you check it every day, you'd notice a drop in pressure or lack thereof and should be able to locate the problem loop.