Radiant floor heat boiler overpressuring and blowing steam

rypkr937

I have a radiant floor heat system that runs off of a gas boiler.

I purged all of the air from the floor loop and closed the shutoffs assuring the floor loop is tight. I then hooked a drain hose to the boiler drain valve and flushed the boiler side of the system assuring it was water tight.

I left the boiler at 20psi with no air in the system, opened the shutoffs to the loop and turned it on. After it started heating up it blew the pressure relief valve and a mix of air and steam blasted out until it tripped the lower water cutoff and killed the system.

My initial thoughts were I had too much water PSI in the boiler so I repeated the above process, but this time turned the boiler back on with only 5psi in the boiler. I also left the garden hose connected to boiler drain valve and when the pressure began to rise I throttled it open letting some water out. This worked as it heated up for a few degrees and then suddenly a sharp rise in pressure and it blew the valve open again.

Why is my boiler getting overpressure? I'm guessing the second time that I may have let out too much water and created an air pocket that quickly heated and created a dangerous pressure situation? Yikes if so.

But the first time I fired it up what could have been the issue?

Thanks for any help.

Jamie Hall

Expansion tank. Where is it piped in? Is there a valve from the system to it? Is the valve open? Does the tank have the proper air charge (match the system pressure -- probably around 15 psi). Then -- aquastat on the boiler. At what temperature is it set to turn the boiler off? Does it?

STEVEusaPA

Are you sure all the valves are opened that need to be opened?
Some pictures, far enough away to see all the components would help. And I agree with @Jamie Hall about an expansion tank issue. If it's steel it's probably flooded.

rypkr937

I replaced the pressure tank last year because it failed. I briefly tapped the Schrader valve to assure it wasn't water logged and got no water. It's oriented with the Schrader on the bottom so if it had failed I would expect to get water.

There is no valve between the expansion tank and the system to close.

I'm not sure what the aquastat is. But this might be my issue. Is this my Taco brand controller? The only setting I'm aware that I'm able to set on it are the Warm Weather Shutdown, Heating Curve, Differential, and Farenheight/Celcius. Is this component also what is going to tell the boiler to shut off at a certain temperature, or am I looking for a different component?

STEVEusaPA - The only valve I have shut in the system is the bypass for the mixing valve which I typically leave shut.

Jamie Hall

Ah.... um. If the pressure tank is a bladder type tank -- most are, these days -- you won't get water from the air valve anyway. The only really reliable way to make sure the air charge is correct is with the system off and depressurized, check (use a reliable tire gauge!) the air pressure in the tank at the Schrader valve. It should be -- for most houses -- around 15 psi. Then bring the system pressure, after bleeding, up to about the same pressure.

Zman

What is happening is not normal. You could be airlocked, failed circulator or bad expansion tank. Can you post a picture of the entire system? Your method of troubleshooting is scary, this would be a good time to take a timeout and evaluate.

rypkr937

Zman said:

What is happening is not normal. You could be airlocked, failed circulator or bad expansion tank. Can you post a picture of the entire system? Your method of troubleshooting is scary, this would be a good time to take a timeout and evaluate.

I don't think it's airlocked because I purged it quite extensively and had no bubbles in my bucket. Though, I guess, anything is possible. The circulator did kick when I turned the thermostat on. I'd be surprised if the expansion tank is bad, I replaced it 11 months ago, but again anything is possible. I will drain the water side and check the precharge tomorrow when I get home from work.

I'm not a professional HVAC - I don't have the money to spend $80/hr if it's a relatively simple fix. I don't mean to be dangerous and please intervene if something I'm doing/have done I should know better to not have.

I will get pictures tomorrow.

Thank you for the help so far.

JUGHNE

Is your pressure gauge accurate?
Does it read "0" with no pressure applied....that is with relief valve held open? (cold of course).
Then does your auto fill valve only charge it to 12 PSI?
Do you have a by-pass lever on the fill valve that might be partially open?

Gordy

I’m with @Zman. Sounds like there is no flow. The circulator will still kick on even if it is air locked. Any gurgling?

However the boiler should shut down on high limit before popping the relief valve.

Double check x tank pressure. The system should be at zero psi when checking the tank. The tank should be around 12-15 psi for a two story home. Water,temp should be ambient.