Hydronic pressure reducing valves

Dave Hicks

I once read an article written by someone probably in P & M magazine about the proper procedure for the status of the manual valve before the PRV. I recall the author stating that after filling a system and purging air from the system that the manual valve should be closed. I believe that the reason for this is so that if there is a hydronic system leak and supply water is lost for any reason this would prevent a big boom as cold water is restored and the boiler is hot it will flash to steam. I have been doing this and teaching others this way for years and recently a situation presented itself that claims this is a wrong approach.
We had a service tech go out to a property and replace a circulator. After fill and purge he left the valve closed. The system developed a leak, or it existed when we were there and the tech was not aware of it, and there being no LWC, there was an explosion 2 months later when the system got low on water with a call for heat and the relief valve failed to open. Apparently the leak had existed for a long time and calcified the relief valve closed, and possibly even caused the hi limit to fail to operate properly. The explosion did 100K worth of damage. No one hurt, Thank God.

I also recall going to a manufacturers install instructions and it stating that the valve should also be closed. Today I checked a well known mfg install instructions and they call for the hand valve to remain open.

This situation begs an answer to the question? How many contractors out there require the customer to pay for a comprehensive safety check, when doing such perfunctory repairs as a pump replacement? Does anyone out there also shut the valve off after fill and purge? How many of you manually operate the relief valve and run the boiler and check the LWC and the hi limit? And for that matter the condition of the chimney and combustion efficiency? If you are not doing this, perhaps you should start, we will now. On annual clean and checks this stuff is all in our repertoire, but never have we done it, or required the customer to have to do this as a requirement to us doing repairs. We teach our techs not to leave a boiler or furnace that they know to have a life safety issue without fixing it. ie: rotted smoke pipe or chimney not drafting etc that are not related to our purpose for being there. If the customer will not do the needed repairs we instruct our guys to disconnect the appliance and get the client to sign off on the issue, that they refused repairs. Of course this only refers to the obvious problems readily seen by eye.

Which is it? open or closed?
Some feedback please.

Ken_8

Classic answer...

That depends.

If the manufacturer says to close the feed stop, close it.

If the manufacturer says to leave it open, leave it open.

If the manufacturer doesn't say, leave it half way open.

I'm kidding of course.

Or am I?

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hr

Plan on callbacks

if you close them right after a new install or drain down. Systems can burp air for a few days, especially radiant systems with 1000's of feet of tube.

And it's really an issue with P/S piping and injection mixing! Takes awhile for air to cross that bridge often times. Seem it just goes round and round in the secondaries. Always best to throw a second purger in those injection mix systems.

If the fill valve is not able to refill with water you will often get a no heat call.

If the system has a boiler with a flow switch these may not "make" if the system gets low on water from large air bubbles purged, same with high mounted LWCs.

It's a tough call.

If you plan on leaving them shut off after you install them, why bother even installing them?? They just slow down the purge ritual anways, unless you have a fast fill lever

Add a LWC, pressurize, and purge and let it run.

Saves on backflow preventor costs also

hot rod

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Unknown

Been leaving them open

since time out of mind . Haven't heard of a damage claim because of it yet . But it's sure to happen sooner or later with these aging systems and piping .

Glenn Harrison_2

Bell & Gossett says in their instructions...

to install a Low Water Cut Off, and close the supply valve. The primary reason for this (so I've been told) is a they were sued over a system that developed a leak, and the PRV filled a very hot boiler with cold water and caused it to explode. Amazing how a lawsuit will change someone's line of thinking.

The key is you HAVE to have a L.W.C.O. if your going to shut of the water supply valve.

To answer your other question, I do look at the Relief valve on every service call, and give the customer the option to test the valve with the understanding that it may leak after the test and need replacement. I also have them sign off if they don't want it tested. I also automatically recommend replacing the relief if it's over ten years old as a precaution.

I also do a combustion analysis on every cleaning, and at least a CO check on every call.

hr

Yeah, I've heard that reasoning also

although in the 30 plus years in the heating business I have never seen it happen. Most fill valves start the refill process as the pressure drops about 5 PSI. i'm not sure exactly how you would get a boiler hot enough (empty I suppose) to set up those conditions.

But I have had more problems with boilers that didn't have a fill valve do it's intended job when needed

Shake the die, I suppose, and pick the lesser of the two evils

hot rod

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Unknown

David

I think the article you are referencing to is one that I wrote for P&M last year on Low Water Cutoffs on hot water boilers.In that article I stated that one of the primary reasons for the LWCO was the event of a boiler overheating condition whereby a secondary zone control relay may be wired to the wrong side of the high limit. This would cause the boiler to overheat and the pressure to rise until the relief valve eventually discharges. When the relief valve discharges the superheated water in the boiler may flash to steam due to the pressure loss and form a large pocket of steam in the top of the boiler and supply piping. The LWCO will now be in this pocket and should shut the burner down. The likelyhood of the PR valve from feeding at this point though is slim as the pressure from the expanding steam will be well over 12 psi.

The other reason for the LWCO has to do with a possible system break. In the event of this happening the water will be replenished by the PR valve and eventually the burner will shut down because it may end up under water. If a LWCO is used on a hot water boiler, it is a good practice to shut off the feed water. here is a notation out of B&G's installation guide. Hope this helps.

Glenn Stanton

Manager of Training

Burnham Hydronics

www.burnham.com