High pressure on hot water boiler

michael123

Hello. I have a Laars JVS hot water boiler with cast iron radiators. It was installed about 3 years ago. When it runs and the temperature rises, the pressure rises and hits 30 PSI. The pressure reducing valve is working and expansion tank is not leaking water. About half of the tank is water and half air.
The bypass valve was half sized so the plumber put in full size bypass near the boiler. What got me thinking is that maybe the bypass is supposed to be after the expansion tank and pressure reducing valve. Currently it’s right next to the boiler.
Please see the picture attached. It shows the old bypass and the new bypass. The second picture shows the pressure reducing valve and expansion tank.

Jamie Hall

Well, the expansion tank and all may or may not be in the right place, but...

If the pressure rises that much, the expansion tank simply isn't doing its job. Assuming that it isn't actually somehow valved off from the system, either it's failed, it's too small, or it wasn't pressurized properly (or has lost its pressure -- same thing as failed). Unhappily, I don't see any good way to isolate the tank from the system and drain it -- which is the only correct way to set its operating pressure.

EBEBRATT-Ed

What @Jamie Hall

said. Expansion tank likely failed

JUGHNE

If you press the air valve on the bottom of the tank what comes out?
If water....tank is bad. If some air, it might needing topping off.
With system cold, tank should be empty. As the water expands it will push itself into the tank.


What is the cold pressure on the system?
The tank should be set at 12-15 PSI when it is not connected to the system.

Also, if you have cast iron radiators and what looks to be large piping in the basement, that tank may be borderline for size. Air tank size is based on the water content of the system.
You may need the next size up for tank.

Is there only one pump on your system?
Is that bypass valve always fully open?

michael123

Hi. If I press the air valve on the tank, only air comes out, no water. When the boiler has been off for a while, the pressure is about 15psi. 
Yes, there is only one pump and the bypass valve is fully open.

Jamie Hall

michael123 said:

Hi. If I press the air valve on the tank, only air comes out, no water. When the boiler has been off for a while, the pressure is about 15psi. 
Yes, there is only one pump and the bypass valve is fully open.

15 psi in the system? Or in the tank. Check the tank air pressure with a reliable tire gauge -- although that won't tell you if it was properly precharged.

And I tend to agree with the folks above -- that tank looks pretty small for a system with pipes that size.

Alan (California Radiant) Forbes

The expansion tank is screwed into a black tee, the perfect place for sediment to build up. I would remove the tank and stick a long screwdriver up through the tee and into the microbubble air eliminator to make sure it's all clear.

When you replace the expansion tank, add a ball valve so that you don't have to drain the entire system to service the tank.

bucksnort

A bigger tank might not fit with that lower bypass setup as is. My new system going in this Summer is on the borderline on needing a 60 instead of a 30. The old bladderless tank is in good shape and i might just keep it. But I'll make provisions to easily add a 60 should the bladderless decide to sheit the bed down the line.

JUGHNE

A larger tank can be piped with fittings to a different location.
IDK about the 60, but the 90 has a 3/8" female spud on the bottom.
I used it to provide floor support with 3/8" all thread rod.

bucksnort

JUGHNE said:

A larger tank can be piped with fittings to a different location.
IDK about the 60, but the 90 has a 3/8" female spud on the bottom.
I used it to provide floor support with 3/8" all thread rod.

I agree 100%. But looking at the pics a new 60 wouldn't be a simple spin and replace with those existing pipes in the way. OP still needs to verify if there's even air pressure in the tank now and then go from there. Spinning on a new 30 might be a quick fix until he can repipe.
Just looking at the pics it looks like big supply and return pipes. Only after adding it all up can a choice be made. It never hurts to go bigger on a tank.

JUGHNE

He mentioned cast iron radiators, looks like an old gravity system.
If a 30 made it for 3 years, a 60 would not be overworked like the 30 might have been.

michael123

Hello. Thank you all for your response. My boiler output is 63k Btus. My average pipe size is about 1.9 inch. The supply side length of pipe along the wall is about 46 feet and the same for return. The unit is in the basement and only heats the first floor radiators. 
I don’t know the current pressure as I don’t have a gauge but the factory install pressure was 12 psi.  

dopey27177

Here is a possibility.
The cold pressure on the system should only be 2 psi above the static head of the system pressure,
Assume you are in a building where the the highest radiator or convector is 10 feet above the radiator the static pressure is 10 X .434, that means the the static head is 4.34 psi add 4 psi that that number and you will have 8.34 psi cold pressure. when the water heats up the pressure will go up.
If the pressure goes up to 30 ten the compression tank is to small.

I am a little dopey here what are the by passes for. Me thinks both bypasses should be closed when you re in the heating mode.

Jake

Henry

The proper way to check the tank is to remove it. Then check the pressure of the tank. Pump it to operating pressure. Wait 30 minutes and then check the air pressure in the tank. You will be surprised that it has probably not dropped. I have seen in 44 years very very few tanks that have failed. The tanks loose 4% of the air through the bladder. It is a great $$$ for an inexperienced plumber or a scammer to replace commercial expansion tanks without a proper test. I saw one replaced with an ASME 150 gallon expansion tank for a four floor 32 apartment building last week. A PX 90 would of done the job.