getting conflicting reports. do heating boilers ever have T&P Valves?

ElderGreen

Depending who you ask they do or they don't. Even inspectapedia devotes a lot to Temperature & Pressure relief valves on Heating boilers. So do they ever have them?

Alan (California Radiant) Forbes

Most, if not all

domestic heating boilers come with 30 psi relief valves (pressure only); some are already piped in, others come in an accessory kit for you to pipe in. 



I believe only water heaters come with T & P (temperature and pressure) relief valves.

Paul48

I believe

They may have been refering to putting the T&P on the domestic because of the 250* water from the wood boiler.

pipeking

PRESURE ONLY

 PRESURE ONLY FOR BOILERS, btu/h pertain to the size, to make sure it can displace water faster or as fast as the boiler can make it.

billtwocase

ditto

pressure only for boilers. Pressure/temp valve for domestic water heaters

Paul48

Question

His question is the result of advice given on another thread, and I think they may have been referring to putting the t&p on the domestic hot water side.Go back and ask for a clarification.

Jamie Hall

OK... now I'm confused

and I plead guilty to being the one who mentioned T&P valves on a wood fired boiler in that other thread.



So... am I to understand that one would not need a temperature relief valve on a hot water boiler?  The unit in question was a wood fired boiler, thus there was no way to shut it off on a high temperature limit -- it would be able to go to whatever the wood fire would take it to.  Therefore, if the circulation was not sufficient -- or the return water too hot -- to absorb all the heat from the fire, it would just keep going up.  And up.  In this case, the orginal poster mentioned a water temperature of 250 which considering that the system is under pressure, isn't outragiously high.



However, if a leak should happen -- or the pressure relief valve open -- one would get boiling throughout the system with, it would seem to me, potentially interesting, if not catastrophic, results (try opening the radiator cap on a car's engine when it's hot, to see what I mean...)



I would appreciate illumination... I'm basically a steam and automotive person, not a hydronics person.

SWEI

The temp can't get that high

Without increasing the pressure.

Jamie Hall

Optimist

What you say is quite true.  As the temperature rises, the boiling point rises; in a closed system, as the temperature rises the pressure also rises.



Last I looked, at 15 psig, the boiling point of water is 250 Fahrenheit.  That's where your pressure relief valve is supposed to crack.  Problem is, if it cracks, it releases the pressure -- and if you release the pressure, you have a system full of water at close to atmospheric -- 0 psig -- which will boil quite happily until enough steam is formed to bring the pressure back up to 15 psig -- and continue to do that.



At best you will get one heck of a fountain of steam at the pressure relief valve.  At worst?  I don't want to be around.



Perhaps it isn't required on a hydronic heating system.  In my humble opinion, it should be.

icesailor

Well, actually,

Well, actually, the system pressure sets the boiling point.  (Square Root of the pressure X 14 X 198 = boiling point of water).

The steam boiler makes steam at 212 degrees. As the steam pressure goes up, so does the boiling point. If it is a vapor/vacuum system, the boiling point goes down as the pressure goes negative. Or, steam, boils at a lower temperature in Denver, CO at 5.000 feet. 194 degrees I think.

Low pressure steam boilers have 15# relief valves on them. That's good for keeping water in liquid state up to around 250 degrees. But that wood heater and boiler get bad because with a 30# relief valve, for the valve to blow, the temperature will have to really get to a high level to make steam. The problem becomes serious on two fronts. If the temperature and pressure get high enough to blow the relief valve, there is really going to be a serious release of steam expansion. But worse, there probably isn't any make up water to cool down the water in the system and there is nothing to stop the fire. There is nothing more frightening than coming upon a hot water boiler that is bypassing the high limit and the boiler is making really strange noises. My rule is to shut off the power and leave. And come back in the morning. To see if anything is still there.

I once saw a side arm water heater blow up. I never saw the tank, but all four walls of the house and roof were on the ground.

Steam boilers have high pressure limit controllers.

Hot water boilers have high temperature controllers. But not pressure limiting controllers. Both types use a pressure relief valve to control excess pressure.

Its not something to triffle with.

SWEI

Pragmatist

PV = NRT

It's not just a good idea, it's the LAW.



I find that my belief in the laws of physics has served me well over the years.

Jamie Hall

Perhaps the real question

on this sysem -- and, perhaps, other wood fired hydronic systems -- is not so much the last ditch safety systems -- that is releif valves -- but what are or is the normal mechanism for high limit control?



Consider.  On steam systems, you have two or three pressure controls -- one of which should be manual reset -- which are supposed to shut the system off if the pressure rises too far.  Assuming that they are done right (that is, each on a separate pigtail, and wired correctly) the odds on all three of them failing to trip are pretty small.  You also have one or preferably two low water cutouts, so if the pressure goes too high and you lose water through the relief valve, or you get a leak, the boiler goes off -- and again, assuming they really are independent and at least vaguely maintained, the odds of their not doing their thing are pretty small.  OK.  On an hydronic system, I would assume (I am no expert on these things) that there is at least one and, I would hope, too high limit controls which shut off the burner if the temperature goes too high.



But what similar control is there on a wood burning boiler?  I do know on old coal boilers that there was a draught regulator hooked up to be pressure sensitive, which reduced the fire as the pressure rose.  Is there a similar contraption on wodd burning boilers?  Then also, is there a low water cutoff system, so if you get a leak or a pressure relief opens, there is a way to shut off the fire and not dry fire the boiler?  If so, how does it work?



Just curious...

Gordan

What I've seen

Clearly, with a solid fuel boiler there's no such thing as cutting off fuel. You can only throttle draft so much without making all this nice explosive wood gas. But I've seen boilers with safety water coils that are connected to domestic cold supply on one end and discharge into a drain on the other. I'm sure these are inside the water jacket so that they don't themselves create a hazard during normal operation, when nothing is flowing through them.

icesailor

Wood Powered Bombs:

Which is why I have always considered these home made wood burners as potential steam powered wood bombs.

People buy them, and read the first part of the installation, the part where they state to put the appliance on a level surface. After that, its whatever goes.

Some of these things are connected with potable water in direct contact with the heat generating source of the building. Some of them, pipe the potable water through the water heater and boiler and they drink the stuff. Because they may be "off grid", they have no reliable make up water system to cool them down.

The Pressure Relief Valve is supposed to meet some ASME rating as far as the input of the fuel being consumed by the appliance. I have never seen any published figures as to the input to a wood or coal heater so how does one choose what PRV to use? That's important because like a lot of things, less is less and more is more. Given a choice between a cheap low rated valve and a more expensive valve, some will always choose the lesser.

Or like a big gas/oil steam boiler I have been working on that is LPG and oil, the gas installer told the owner that you didn't need all that junk controls on the gas train on a 2.7 million BTU burner. So they just left them out. And the burner was set up for Nat. Gas. The first (and only) time it was fired on gas, it was (reportedly) very exciting.  After being brought up to code and conversion, it should be fired next week.

Does anyone understand how these things turn into bombs?

Some of you, but the rest?