Relief valve blow-off.

[NCPABill]

Thanks to all of the help in the past. New problem has arisen. I have a Biomass 40, with a 500 gallon buffer tank. I put a T&P valve on the tank. The expansion capacity is 4.5% of the estimated total volume of hte system, actual capacity, bladder tanks.

If I run my temperature on the boiler to 175, my T&P valve will release. Now that number is down to 162psi. The only T&P to blow off is the one on the tank. There is also one on the boiler. When the tank T&P blows off, the pressure on the boiler reads 20 psi or so (1.5 on the gauge).

Any ideas what could be causing this? Thanks for your input.

Bill

 

[Floydian]

Hey Bill,

You want remove that T&P valve. No need to limit your tank to 175. I believe one pressure relief valve is standard, located on the boiler. It's probably a 30 psi valve.

good luck,

Noah

 

[PassionForFire&Water]

A standard T&P valve is set 210F/150psi.
Watts has one that is set 210/75psi
My guess is that your expansion tank volume is to small.
4.5% of 500 gallons + boiler water content = 20 to 25 gallons.
A water supply set pressure of 12.5 psi and 550 gallons would require around 38 gallon expansion tank.
Google Watts and expansion tank calculation or check out this link http://www.watts.com/pages/support/sizing_ET.asp

It's ALWAYS wise to have a T&P valve, no matter what. The cheapest insurance out there for $15

 

[woodsmaster]

Unless you have a heater in your storage tank get rid of the t & p valve on it and just keep the one on the boiler. Nothing
should happen at 20 psi. You must have the wrong t&p valve or a faulty valve or gauge.

 

[maple1]

I would replace the T & P valve on the tank (there's a chance it could be faulty), with a T&P guage. I would always have at least two T&P guages on my system, they seem to go wonky at times from my experience (redundancy is good). Then you could verify your pressures - what are they at system cold, and system hot? I am also thinking you might not have enough expansion - what do you have for an expansion tank? If the pressures do seem OK after verification with two guages, you could always T a new second valve back in on the tank and have both valve & guage there.

Do most systems have a T&P valve on storage? I was thinking not, but don't know.

 

[dogwood]

Maple1, there was a previous post where having a pressure relief valve on storage was discouraged. The concern was if there was any water blown off it would get into the insulation and make for a mess. I suppose this might depend on what type insulation you had.

Mike

 

[maple1]

Yes, I could see that happening - although I suppose it could be alleviated by running a blow off pipe out of the storage enclosure, I think? I just can't see the need for one on storage with one on the boiler - or if a second one is desired (redundancy again), put it somewhere else on the system easily accessible.

 

[dogwood]

That's what I'm going to do with a second prv.

 

[ALASKAPF185]

T&P's are for domestic hot water heaters only, not for storage / buffer tanks. The tank has to have a protection device if there are any isolation valves. Expansion tank and boiler relief valve , which is different from a temperature and pressure relief valve. 150-160 degree and 125 psi. If that is what the actual problem is, seen it many times.

 

[NCPABill]

Thanks!

I sincerely appreciate all of the advice. I will change this to a gauge for now. I cannot valve this out by accident (handles removed) and I have one near my boiler (at top pf riser pipe). I also have one on my gas boiler, which sits idly by, but is connected to the entire system.

Thanks again. I read and read, but rarely have any real input. If you ever think I can help, pm me!

Bill

 

[woodsmaster]

T&P's are for domestic hot water heaters only, not for storage / buffer tanks. The tank has to have a protection device if there are any isolation valves. Expansion tank and boiler relief valve , which is different from a temperature and pressure relief valve. 150-160 degree and 125 psi. If that is what the actual problem is, seen it many times.

the tank don't need a protection device even if there are isolation valves. There is no way the water in the tank is going to get hotter if it is shut off.

 

[ALASKAPF185]

T&P's are for domestic hot water heaters only, not for storage / buffer tanks. The tank has to have a protection device if there are any isolation valves. Expansion tank and boiler relief valve , which is different from a temperature and pressure relief valve. 150-160 degree and 125 psi. If that is what the actual problem is, seen it many times.

the tank don't need a protection device even if there are isolation valves. There is no way the water in the tank is going to get hotter if it is shut off.

its code for a reason, but the one he's got on there now is blowing off........hmmmm

 

[RobC]

If you want some extra protection on your storage tank use just a pressure relief valve. They look just like the T & P but don't dump with high temps.
Code.... I'm not a plumber, but, you should NOT install a ball valve or shutoff in line for any type of safety device like relief valves just because.... that why they call them accidents....

Rob

 

[woodsmaster]

T&P's are for domestic hot water heaters only, not for storage / buffer tanks. The tank has to have a protection device if there are any isolation valves. Expansion tank and boiler relief valve , which is different from a temperature and pressure relief valve. 150-160 degree and 125 psi. If that is what the actual problem is, seen it many times.

the tank don't need a protection device even if there are isolation valves. There is no way the water in the tank is going to get hotter if it is shut off.

its code for a reason, but the one he's got on there now is blowing off........hmmmm

I'd like to see where it says it's code to have a relief valve on a storage tank.

 

[bigburner]

If you use a T &P valve for storage it's wrong [boiler side][use pressure only 30psi or 50 psi if rated] it should be same as the boiler. [usually one per system, boiler codes or an interpretation about tanks may require more then one] T=temperature & P= pressure when this valve gest weak on the temperature side and it's installedin a boiler it going to dump, the real problem is the pressure may not trip until a catastrophic failure. Check the boiler valve make sure it is not a T&P

 

[ALASKAPF185]

well if you were a licensed contractor, you'd have a code book to go by, its in there. All devices are supposed to be valved off and some require a bypass like the fill( PRV & CV) and the boiler itself, isolation valves are another code requirement. Even if its not valved off it still requires a PRV to meet 2010 boiler safety code. This obviously does not apply to open loop systems. The new upcoming code requires all boilers to be sold with low water cutoff and ODR's, how many of you have those ? There are alot of overlooked and required items that are not specifically mentioned in manuals, doesn't mean its code. Taking handles off doesn't remedy anything, just like the homeowner that went up hunting and his wife called a handyman/neighbor out to repair/help out. Only to make a big mess after he left. Good intentions gone wrong.

 

[thecontrolguy]

well if you were a licensed contractor, you'd have a code book to go by, its in there. All devices are supposed to be valved off and some require a bypass like the fill( PRV & CV) and the boiler itself, isolation valves are another code requirement. Even if its not valved off it still requires a PRV to meet 2010 boiler safety code. This obviously does not apply to open loop systems. The new upcoming code requires all boilers to be sold with low water cutoff and ODR's, how many of you have those ? There are alot of overlooked and required items that are not specifically mentioned in manuals, doesn't mean its code. Taking handles off doesn't remedy anything, just like the homeowner that went up hunting and his wife called a handyman/neighbor out to repair/help out. Only to make a big mess after he left. Good intentions gone wrong.

I also agree from an engineering perspective, that the relief is required at or near the device that is providing the heat or otherwise causing the pressure rise. Hence, the boiler should have a place for a relief that is SIZED to blow down the pressure rise and not allow the pressure to exceed the system safety rated pressure while the heat source is at high fire rate. The ONLY times I see more than one relief is if the design engineer spec'd more than one of a smaller size relief, then they are installed in parallel at the same location. Also, on the secondary side of a heat exchanger, there MUST be a relief that is sized for the BTU-HR capability of the heat exchanger and the pressure relief trip of the rated pressure of the system that it is protecting. A heat storage is not required to have a relief unless perhaps the storage has two or more unrelated heat inputs, such as wood, solar, and natural gas, and perhaps the storage tank is rated at less pressure than the supplying sources THEN the storage tank might get a relief. This, however is BAD engineering practice and all common-source system components should have similar ratings.

Above all, the reliefs need to be piped at least to the floor, preferably a drain, as think of what happens when it relieves from the top of the tank when you and the neighbor are standing there with some frosty cold ones and admiring your clever money saving devices. I have been burned by steam, and don't recommend it.