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multiphrenic 11-29-2011 05:24 PM

Replacing a Pressure Relief Valve for a Boiler

This is my first post..but am very excited to join this forum. I have an older house from the 30s that runs on a water boiler to heat the house. When I first bought the house I didn't know the difference between a wrench and a ratchet but I've learnt a lot since then. I'd like to do whatever feasible myself, but if something is way over my head I will defer to the pros. Here's my recent dilema:

The water discharge pipe for the boiler (if that's the correct term) occasionally discharges water. At first, I thought this was because of the expansion tank. I have the smaller unit with the expanding diaphragm. I drained enough water to reduce the pressure to 0 and the tank was very low, so I used a bike pump to bring it back up to 12psi and the started the system up again. The pressure gauge on the side shows that the system is running a touch under 20 psi.

However, I still experience leaking from the discharge pipe. When I open the pressure relief valve, water comes out quickly. The water at first had a lot of brown sediment, though now it comes out cleaner. When I let go of the valve, the water trickles to a stop, taking a few minutes to completely stop.

I think at this point it's the valve to blame, even though it looks like the newest piece of this furnace.

Here is the valve:

And here is the boiler:
From what I've read and researched here's what I need to do:

Drain the system until the pressure is down to 0. Turn off power to the furnace. Use a pipe wrench to remove the drain pipe and the old pressure relief valve. Replace it with an identical valve (which I've found online for 40 dollars!), add new teflon tape to all the joints, and replace the pipes.

Anything else I should be watching out for or that I've missed? I've never worked with these types of iron pipes, only copper pipes for plumbing..and I've been fairly successful with those. The long extension going to the ground seems to be very tight, and I wasn't able to loosen it at first go, while the elbow and shorter straight pipe are already pretty loose. Before I proceeded further thought I'd ask for some tips.

Thanks a bunch!

harleyrider 11-29-2011 06:49 PM

show a picture of the pressure gauge please, while the unit is running if you can.

oh'mike 11-29-2011 06:53 PM

I'm going to leave this for a boiler guy---only offer you a tip ---put TFE paste on the threads of the new valve and teflon tape---have it ready to go in the moment the old one comes out--

This will save you from loosing much water---and might save time bleeding air out of the system.

Hope that tip helps while you wait for a boiler guy to show up here.---Mike---

diyorpay 11-29-2011 06:56 PM

I am not a plumber so what I offer is my experience from an earlier home I owned.

From the pics, your boiler is not as old as the house. Systems like that were typically sized for heat requirement for the house but one thing often overlooked was the quantity of system water. Newer boilers are designed for newer houses and assume copper 1/2 inch pipe and baseboard heat but are used for retrofits too.

So, if you do not have baseboard heat but have radiators and large diameter pipes (like 1 inch or greater) running throughout the house, you have a lot more water in the system than the boiler was designed for. It's ok, not end of world or anything, but pressure will be greater and pressure valve will 'blow' on cold days. It just means you need a larger expansion tank to absorb more pressure. Your relief valve may be doing its job.

If you confirm this is the case, and you are interested, I can dig out the formula for sizing a tank but it means draining all the water in your system into a marked bucket to get an accurate number of gallons (and then getting air out when done). Otherwise it's trial and error to try bigger tanks.

multiphrenic 11-29-2011 07:40 PM

The boiler is from 1992. I do have radiators, the pipes are 1.5" leaving the furnance but then are reduced to 1" when going up to the radiators. It's a smaller home, 7 radiators on two floors.

It didn't take me more than a few mins to bleed the entire system.

The relief valve is set to 30 psi but the pressure on the system shows it is about 20.

Here's a shot, sorry about the blurriness.

I didn't want to put too much money into this furnace this year, as next year we'll be looking to replace it with a higher efficiency model. Something tells me in 1992 efficiency was not much of a concern.

how 11-29-2011 07:46 PM

I drained enough water to reduce the pressure to 0 and the tank was very low, so I used a bike pump to bring it back up to 12psi and the started the system up again.
I've got a few questions Multiphrenic
Where did you drain the water from? How do you know the tank was very low?
You are right that the exp tank should be charged at 12 psi. If you tap the side of the tank, 1/2 should sound hollow and 1/2 should be sound solid as that half is filled with water. Tanks almost never need to be recharged since if the tank is really leaking then it will probably need to be changed.
If your boiler guage is correct and your boiler is running at 20 psi (which is fine) then tell us what is the temp of the guage reading when the boiler is operating. What are your aquastat controls set for? It should keep that unit under 180 F or 100 C. The relief valve that is venting will release with boiler pressure in excess of 30 psi but also from too high a temp. (usually higher than norm temps will creat higher pressures.)
Does the pressure on the guage jump around at all when the boiler is running hot?

multiphrenic 11-29-2011 07:57 PM

I drained the water from the drain valve. You can see it in my first post, it is the blue handle on the bottom right. There's a connection for a standard hose. After turning the supply valve off, I drained enough until the pressure on the gauge was 0. I checked the pressure in the expansion tank and it was in the single digits. That's when I used a bike pump to bring it back up to 12psi. This was a few months ago..and yet I experience occasional water leaking from the relief discharge.

The temperature of the gauge is around 160F right now. I don't see the pressure on the gauge jump around a lot. I do occasionally see water dripping (albeit very little) from the pipe attached to the pressure relief valve. After a few weeks, I empty the bowl I have placed underneath the discharge pipe..but I couldn't tell you if the water is a result of constant but small dripping or occasional bursts. I've never seen a burst but do occasionally see dripping.

ben's plumbing 11-29-2011 08:12 PM

agree with how...however still think boiler at cold temp pressure is alittle high i like to see them around 12-15 lbs of pressure since when they are running they tend to rise 8-10 lbs ....if that is true in your case thats pushing your 30lb relief which explains why it only drips..........maybe guage is not quite right?????? Ben

harleyrider 11-29-2011 08:15 PM

I agree with Ben....lower that running pressure and see if it stops the dripping....let us know

multiphrenic 11-29-2011 08:20 PM

sorry, how do i reduce running pressure?

beenthere 11-29-2011 08:55 PM

If your bladder tank had very little pressure in it after you drained the boiler to zero pressure. Probably need a new tank. Sometimes its just the schrader core, but not too often.

harleyrider 11-29-2011 09:33 PM


Originally Posted by multiphrenic (Post 781715)
sorry, how do i reduce running pressure?

By letting water out of the boiler.....

ben's plumbing 11-29-2011 09:37 PM


Originally Posted by multiphrenic (Post 781715)
sorry, how do i reduce running pressure?

open blue valve you open before and watch pressure guage stop at 12-15 lbs and run boiler see if dripping stops..

DannyT 11-29-2011 10:43 PM

looks like you are at 20 pounds at around 116 degrees. most boilers i used to see ran between 180-190 degrees. by the time your boiler gets up to operating temperature you are gonna be way above 20 pounds.

how 11-29-2011 11:10 PM

If you are showing a 20 psi average, especially after recently draining the boiler down to zero, then the only way you are going to be able to lower it is by draining some water out and only if the water feed valve to your system is shut off.
I have only ever seen a couple of residential systems with the water feeds left off out of thousands left on. I recently read here at DIY about differing opinions as to whether the water feed should be left on or off.
If one subscribes to the water feed shut off opinion then draining a little water will drop the water pressure. One to three cups should do it. Drain it too low and you'll start to have circulation difficulties on the top floor.
If one leaves the water feed open like just about every boiler system I've ever seen, then one would have to adjust the boiler pressure reducing valve, and then drain out some water, which is trickier.
Perhaps the water feed off and the a dropping of the pressure to 15psi by some draining when the boiler is hot, would be the easiest way to test if the relief valve will behave at a lower psi.

P.S. About your upgrading plans! Very few of my customers see the efficiency improvements on their gas bills that they were promised after switching to a new boiler. These are comparing entry level older boilers with entry level new boilers. Everyone sees it with insulation or window upgrades, but I for one look at such efficiency promises that don't translate as reduced gas bills as just more sales bunk!

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