Boiler Over-pressured after Mixing Valve

[Yanks02026]

Hello,

A buddy and I replaced the mixing valve on my furnace (combo for baseboard heating and hot water) the other day. For some reason the prior owners had removed the old mixing valve (found it laying in the basement and it was pretty crusty but maybe now the problems I'm having that is why it was removed), so i was tired of hot water in the house coming out around 170. So we installed the new mixing valve and everything seemed to be fine. But later that evening I noticed water on the ground. So now the pressure relief valve releases a small amount of water sometimes. Some water is also released from the feed water pressure regulator/backflow preventer. I just watched the furnace during the heating of the house and the pressure did get up to around 32 on the gauge but didnt release water even though the pressure relief valve is 30 PSI. But I'm guessing this is the cause for the water being released. I have set the temperature to a high of 180 and a low of 160 with a 20 DIFF. Before adding the mixing valve I never had problems with the water being released, so im sure there was never problems of high pressure.

My boiler is a Weil McLain gold oil and probably from the mid 2000's. So its on the end of its life probably but never had problems in the year that I've lived in the house. Also encase it matters I did install a Smart Nest but it worked fine since changing that.

 

[Reach4]

I think you are saying that you run heating water thru a tank that provides hot water for showering etc. You will need a thermal expansion tank, and that usually goes on the cold water inlet to that tank. The air precharge will be set to that of your city water, or the output of the PRV if you have a PRV.

You also will have a thermal expansion tank for the boiler water. That air precharge is set to the cold-water pressure for your heating system.

 

[Sponsor]

 

[rural_engineer]

Hello,

A buddy and I replaced the mixing valve on my furnace (combo for baseboard heating and hot water) the other day. For some reason the prior owners had removed the old mixing valve (found it laying in the basement and it was pretty crusty but maybe now the problems I'm having that is why it was removed), so i was tired of hot water in the house coming out around 170. So we installed the new mixing valve and everything seemed to be fine. But later that evening I noticed water on the ground. So now the pressure relief valve releases a small amount of water sometimes. Some water is also released from the feed water pressure regulator/backflow preventer. I just watched the furnace during the heating of the house and the pressure did get up to around 32 on the gauge but didnt release water even though the pressure relief valve is 30 PSI. But I'm guessing this is the cause for the water being released. I have set the temperature to a high of 180 and a low of 160 with a 20 DIFF. Before adding the mixing valve I never had problems with the water being released, so im sure there was never problems of high pressure.

My boiler is a Weil McLain gold oil and probably from the mid 2000's. So its on the end of its life probably but never had problems in the year that I've lived in the house. Also encase it matters I did install a Smart Nest but it worked fine since changing that.

I'm operating under the assumption that your boiler is also a tankless water heater, and the mixing valve is near the boiler. I suspect your expansion tank is bad, missing, or mislocated. I'm also guessing that the T&P valve that's venting is for domestic hot water, not for space heat. I'm also guessing that you have something like a PRV on the cold side that acts as a check valve.

Here's my theory. When the boiler fires, after water demand stops, the water continues to heat up for a bit and expands. The mixing valve closes the hot side, because the water heater is pushing really hot water through it. So now the water in the boiler is expanding with no where to go, and the pressure spikes.

The reason it didn't vent before is because without the tempering valve, the rest of your hot water pipes could flex a little and act as an expansion tank.

Anyway, there are a lot of assumption in this guess, but it's what I've got.

 

[Yanks02026]

I'm operating under the assumption that your boiler is also a tankless water heater, and the mixing valve is near the boiler. I suspect your expansion tank is bad, missing, or mislocated. I'm also guessing that the T&P valve that's venting is for domestic hot water, not for space heat. I'm also guessing that you have something like a PRV on the cold side that acts as a check valve.

Here's my theory. When the boiler fires, after water demand stops, the water continues to heat up for a bit and expands. The mixing valve closes the hot side, because the water heater is pushing really hot water through it. So now the water in the boiler is expanding with no where to go, and the pressure spikes.

The reason it didn't vent before is because without the tempering valve, the rest of your hot water pipes could flex a little and act as an expansion tank.

Anyway, there are a lot of assumption in this guess, but it's what I've got.

You’re correct that it’s a tankless water heater. I will inspect the expansion tank more today.

Also wanted to add that this doesn’t happen any other time furnace turns on. Only happens around 8 when the nest turns on the heat from 57 to 63. So I guess for now I’ll change the heat closer to 63 so it doesn’t have to work that much (even though 6 degrees ain’t that much).

Thank you

 

[rural_engineer]

Couple questions:

Do you have a T&P valve on your domestic hot water?

Do you have two expansion tanks, one for space heating and one for domestic hot water? You might not have an expansion tank on the domestic hot water side with a tankless water heater.

Are you sure your boiler is operating at 32psi and not 32 feet of water (13psi)?

 

[Fitter30]

Take a few pics of the boiler ,related piping and the gauge.

 

[Yanks02026]

Couple questions:

Do you have a T&P valve on your domestic hot water?

Do you have two expansion tanks, one for space heating and one for domestic hot water? You might not have an expansion tank on the domestic hot water side with a tankless water heater.

Are you sure your boiler is operating at 32psi and not 32 feet of water (13psi)?

There is no T&P valve on the domestic hot water. The only T&P valve is coming out of the top left of the furnace.

I only have one expansion tank and its attached pipes for the baseboard heating.

I double checked the gauge and I was looking at the PSI not the H2O line. I have attached some photos of the set up.

 

[Yanks02026]

Take a few pics of the boiler ,related piping and the gauge.

Have attached couple photos to other reply.

 

[John Gayewski]

There is no T&P valve on the domestic hot water. The only T&P valve is coming out of the top left of the furnace.

I only have one expansion tank and its attached pipes for the baseboard heating.

I double checked the gauge and I was looking at the PSI not the H2O line. I have attached some photos of the set up.

This is some really bad boiler piping. Whomever installed this might be a moron.

You need to check your expansion tank.

 

[Yanks02026]

This is some really bad boiler piping. Whomever installed this might be a moron.

You need to check your expansion tank.

Lol doesn't surprise based on some of the other stuff done in the house.

But the crazy piping for the mixing valve was me/coworker because it was the only way to make it work with that valve instead of totally changing everything.

 

[rural_engineer]

Your auto filler appears to be directly connected to your cold water, so it can't be disconnected. Have you verified that it's set to the right pressure? You can check this with the boiler turned off. Drain water from the boiler slowly. Watch the pressure gauge. It should drop, and then level off. Close the boiler drain. The pressure should stay level or go up slightly. Tell us this pressure.

 

[rural_engineer]

Your mixing valve is not installed correctly according to current manufacturer guidelines. It's supposed to be 12" below the hot outlet. I'd have to think what problems this could cause other than convection loss.

Here's the drawing from their WTGO series.