Boiler Relief Problem

dgr68 · 02-21-2010, 09:39 PM

I assume that the water pressure on the DW side of the PRV is about 50 PSI and the boiler side is 15 PSI. I assume that the "feed" function of the valve is engaged when the boiler system pressure drops below a set point, and then a valve will open to add water to the system until it gets back to a certain PSI and then closes. Given that system pressure will go up and down by 3-5 PSI depending on water temp, does the PRV take that into account and not feed water unless there is a greater deviation than normal- i.e. there is a leak in a radiator pipe?

Quote:

Originally Posted by beenthere

Ok.

From original fill up. Water pressure in system while cold is 12 PSIG.
Water is then heated up to 180. Pressure on boiler side of BFP is now 18PSIG, pressure on PRV side is still 12PSIG. BFP now vents out excess pressure to keep entering and exiting pressures the same.
Boiler temp lowers and pressure on boiler side of BFP now drops below 12PSIG, auto feed now lets more water in. Boiler gets a call for heat. Process starts over. And over and over and over.

**beenthere** · 02-21-2010, 10:02 PM

The PRV doesn't allowed for temp. It only knows pressure.

Some might need a 3 to 5 PSIG difference before adding more water, some 2PSIG.

NHMaster · 02-22-2010, 08:25 AM

Quote:

Originally Posted by dgr68

I assume that the water pressure on the DW side of the PRV is about 50 PSI and the boiler side is 15 PSI. I assume that the "feed" function of the valve is engaged when the boiler system pressure drops below a set point, and then a valve will open to add water to the system until it gets back to a certain PSI and then closes. Given that system pressure will go up and down by 3-5 PSI depending on water temp, does the PRV take that into account and not feed water unless there is a greater deviation than normal- i.e. there is a leak in a radiator pipe?

I just want to say that no system that I have ever installed has had pressure go up or down or any way at all that was measurable on the boiler gauge.

dgr68 · 02-22-2010, 11:19 AM

Sunday morning (after boiler produced water from relief) I shut off the make up valve. Early Mon morning during recovery there was water in the bucket and boiler was at 27 psi at 190 deg.

NHMaster · 02-22-2010, 01:09 PM

I would be looking at the expansion tank then. It's either got a bad diaphragm, not enough air pressure in it or it ain't big enough. The only other thing that would cause it would be if you have an indirect and there is a hole in the coil.

**beenthere** · 02-22-2010, 02:22 PM

Quote:

Originally Posted by NHMaster

I just want to say that no system that I have ever installed has had pressure go up or down or any way at all that was measurable on the boiler gauge.

If you put a big enough expansion tank on. You won't see it on the gage. But then your using a tank larger then needed.

NHMaster · 02-22-2010, 02:32 PM

Beg to differ. You are using the right size for the total head pressure on the system with thermal expansion compensated for. Why use a tank that is too small. There is a 15 dollar difference in price.

dgr68 · 02-22-2010, 02:56 PM

Any reason why I shouldn't try a 60?

dgr68 · 02-22-2010, 03:02 PM

Quote:

Originally Posted by beenthere

If you put a big enough expansion tank on. You won't see it on the gage. But then your using a tank larger then needed.

Not sure what you mean when you say "wouldn't see it on the gage with bigger tank" Any reason I shouldn't go with a 60? Also, how do you raise the system pressure? Do you change the setting in the PRV and watch the pressure reading at the boiler?

**beenthere** · 02-22-2010, 03:28 PM

Quote:

Originally Posted by dgr68

Any reason why I shouldn't try a 60?

With your circ set up, you should use a 60, and its air charge should be brought up to 20PSIG before you install it.

**beenthere** · 02-22-2010, 03:33 PM

Quote:

Originally Posted by NHMaster

Beg to differ. You are using the right size for the total head pressure on the system with thermal expansion compensated for. Why use a tank that is too small. There is a 15 dollar difference in price.

Sole purpose of an expansion tank is a place for the water to go to when it expands. To prevent excess pressure build up. It isn't suppose to prevent the pressure from rising. A 3 PSIG rise is ok.

Nothing wrong with using a 60 on a system that only needs a 30.
Or a 90 on a system that only needs a 30.

Even on the old open vent systems. A 1 or 2 PSIG rise was normal.

dgr68 · 02-22-2010, 05:38 PM

Quote:

Originally Posted by beenthere

Sole purpose of an expansion tank is a place for the water to go to when it expands. To prevent excess pressure build up. It isn't suppose to prevent the pressure from rising. A 3 PSIG rise is ok.

Nothing wrong with using a 60 on a system that only needs a 30.
Or a 90 on a system that only needs a 30.

Even on the old open vent systems. A 1 or 2 PSIG rise was normal.

Got it. Thanks again beenthere. Do I let system fill with water and while boiler is still off adjust the PRV so that the pressure reads 22?

**beenthere** · 02-22-2010, 10:08 PM

First you put a 20PSIG air charge in the tank. Then you install the tank to the system. Then you adjust the PRV to 20 or 22PSIG.

dgr68 · 02-23-2010, 12:02 AM

Quote:

Originally Posted by beenthere

Ok.

From original fill up. Water pressure in system while cold is 12 PSIG.
Water is then heated up to 180. Pressure on boiler side of BFP is now 18PSIG, pressure on PRV side is still 12PSIG. BFP now vents out excess pressure to keep entering and exiting pressures the same.
Boiler temp lowers and pressure on boiler side of BFP now drops below 12PSIG, auto feed now lets more water in. Boiler gets a call for heat. Process starts over. And over and over and over.

Beenthere- is what you're describing here specific to my orientation of the BFP being behind the PRV? How does the vent work on that backflow? Does it separate air from water?

**beenthere** · 02-23-2010, 12:08 AM

Quote:

Originally Posted by dgr68

Beenthere- is what you're describing here specific to my orientation of the BFP being behind the PRV? How does the vent work on that backflow? Does it separate air from water?

Yes. if it was installed before the PRV. It wouldn't sense the increased pressure on the boiler side of the PRV.

The vent. Is to release the excess pressure, so it doesn't build up on the boiler side/load side of the BFP. has no involvement with air.