Testing and Pressurizing old 1950s in-slab black pipe radiant floor system?

coolfeet

When we moved into our house ~10 years ago it had been converted to oil-fired hydronic baseboard. Our neighbor is an original owner (his parents bought the house new). He told us our whole neighborhood was built with black-pipe in-slab, electrically heated, radiant floor heating. He said that the lady who used to live here converted the house over to oil-fired hydronic baseboard in the 1980's and the in-floor radiant heating system was fine. She never had a leak or any problems, she just wanted baseboard and oil-fired after the oil embargo because "everyone was converting" back then...

I've got five pipes sticking out of the slab next to our boiler. The two outside pipes are 1.25" OD (NPT thread), and the three inside/middle pipes are 1" OD (NPT thread). I assume the three little pipes are supply and the two bigger diameter pipes are return??? Who knows.

I have a vacuum pump, an air compressor and I have air pressure gauges.

I already cleaned up the threads. All the pipes are magnetic so they are either galvanized or straight black pipe. What amount of pressure do I want to put on the pipes? I'm just trying to see if it's airtight. Also, I'd like to see what is supply/return because after ~40-years of not being capped off and empty/dry, I doubt that I'll get air flow through the whole system.

Any ideas how to proceed?

I'm assuming if I have air flow through the system and if I can pressurize the supply or three little pipes, and I get pressure back out of the two bigger return side pipes, then next step would be to try and flush it out, measure the flow, maybe pressurize the system and let is sit pressurized for a few days/weeks to see what happens??

Is there a fluid/flush I can use to clean the system out? I've seen "stop-leak" type products that you can flush through systems like this, similar to trenchless sewer patch systems. (flush a glue fluid through the system then drain & let dry)

My goal: Only when it's really the coldest months of the year I'd let the in-slab hydronic run. House has no insulation around the slab, I'd dig down and do that too. Thinking about doing work to our house so that parents can live here as they get older. Thought in-slab heat might be nice in combination with the baseboard hydronic slant-fin.

Also: Yes, I know it'll probably require glycol and a heat-exchange system of some sort because you can't just add-on an in-slab hydronic heat loop into a non-condensing oil-fired burnham boiler as a second zone. Trying to see if the heat loop is still good before I worry about running fluid through it.

Thanks.

Paul Pollets

I would build a manifold and pressurize the piping to 60PSI. If it holds pressure for 15 minutes, it will work at boiler pressure (12-15PSI). There are various chemicals that can be used to clean systems. Usually they're designed to sit in the pipes for 12-24 hrs, then flushed carefully so no residue remains.

hot_rod

30 psi would be my suggestion. A boiler typically has a 30 psi relief valve, so you would never exceed that. With an air compressor you can figure out the loops S&R.

If it holds air for 24 hours build a manifold and circulate a hydronic cleaner and see how it flushes. If no large rust particles come out, you may be good to go.

It may not have underslab or perimeter insulation, so operating costs could be a bit higher than the current system

If flowers grow around the foundation all winter…😳

EBEBRATT-Ed

If it's black or galvanized and its 40 years old (I think it's likely older) I think you're wasting your time. But if you want to spend the time on it go for it.

coolfeet

Thank you. Umm, it's losing pressure. I did the manifold thing. My tig welding is terrible so that was my first mistake but then I got that leak sorted. It held ~25-28psi for about 16-18 hours and then it started ticking down (dial gauge) about one notch every 12 hours. It eventually got down below 10psi and I'm pretty confident that it's not at the manifold. I'm pretty confident that somewhere in the system there is a very small pinhole.

Any chance I could flush it, then run a slime sealer fluid through it and revive the system?

retiredguy

I saw a few of these radiant floor heating systems back in the early 1970's that used black steel piping. After about 20 years or so they all started to leak and had to abandoned. It was suspected that there was a reaction between the concrete and the steel piping but how would you know for sure. After that, a few of the houses that were built with slab heating, used copper for the" in floor heat". Did that system last, I do not know. Today, there are a few contractors using plastic or vinyl for the slab heating. I saw this in a new plan of houses that are for senior living. Nice idea if it lasts.

hot_rod

If it i9s a pin hole a hydronic sealer may work. Fernox makes a sealer for that.

When you chop up those old steel pipe systems you often find the bottom of the pipe where it lays on the ground rusts away. So it would be a had leak to seal away.
But it might be worth the cost of a can of sealer? Buys you some time perhaps.

Soap the gauge and schrader valve, that is often the source of a small leak.