A couple issues with my combi boiler system

Ccranmer07

Hi! Ive been trying to troubleshoot my home heating system. I have a few issues currently.

1. When my boiler fires for DHW, some of my hydronic baseboards also get warm
2. The boilers PRV needs replaced about every 8 months

My system consists of a weil mclain gold oil boiler(series3) and a weil mclain indirect fired water heater.

Jamie Hall

Zone valves or zone pumps?

And by PRV valve do you mean the pressure reducing valve from the domestic water to feed the boiler, or do you mean the pressure relief valve protecting the boiler? Neither one should bail that often, but the possible problems are totally different.

EBEBRATT-Ed

If you have zone valves the one for the heating system is probably leaking by or is not closing due to sticking or a wiring issue.

If each zone has a circulator it could be the flow check valve on the heating zone sticking open.

Ccranmer07

this is my poor excuse of a diagram. The guy I bought my house from installed the system himself. I appreciate your expertise, and will be looking into those possible causes this week. Im sharing the drawing because ive also wondered if maybe there is a flaw in the design

EBEBRATT-Ed

The PRV and the expansion tank should be connected to the system in the same location or at the very least on the same side of the circulator.

I would cap where the PRV ties into the return and reconnect the PRV between the expansion tank and the air purger.

If you getting heat in the radiation on a DHW call and the heating circ is not running then the flow check needs cleaning or replacement.

Ccranmer07

Thank you for your help

Mad Dog_2

Thats actually a nicely drawn sketch, otherwise. Also, just so we speak the same vernacular, this is not what we call a "Combi boiler" system. Mad Dog

Ccranmer07

Thank you for the correction, although now im a little more worried because that was how the installer/former owner had explained it to me

psb75

I think you are talking about a Pressure Reducing Valve (in your diagram). It has a screen that can get clogged with rust and debris. It should also be paired with a Backflow Preventer.

Ccranmer07

The valve that i have been needing to change every so often is the pressure relief valve that exits the boiler and terminates just off of the floor

psb75

Ok. That is referred to as a TPRV or "temperature/pressure relief valve. They sometimes get debris stuck in the valve seat. That can (sometimes) be fixed by "blasting" water through it—by quickly opening the lever on top and letting it 'snap' closed. BUT…it is best to have a new replacement valve handy (nearby) to replace the leaking valve IF….it doesn't stop leaking.

clammy

Seems like you may have a expansion tank issue due to your safety valve blowing off .It would seem that your service provider may not be as well rounded as they state being you do not replace a safety valve every 6 months .You find the root of the issue bad expansion tank ,leaking pressure reducing valve or possible system leaks causing water to be taken on due to a drop in pressure . Personally i would chk the expansion pressure ,system pressure w a secondary gauge and then close the boiler water feed valve . If your system gauge drop within a day or so you have a leak and it should be located and repair .The constant addition of fresh water to any boiler is not some thing you and will cause issue like kettling and popping and shorten the life span of the boiler block . If you are using the same service providers who installed the boiler who as you stated have just been replacing the safety you need to find a better service provider being hey are not cutting it in my book .

peace and good luck clammy

EdTheHeaterMan

Here is a slide from my Hydronics seminar I used to teach for EH-CC.org, slightly modified to match your system.

If you look at this illustration, there are 3 pressure gauges.  One is at the boiler inlet near the auto feed valve. (PRV)  The next is at the circulator inlet and the third is on the circulator discharge.  Now to fill the system to 12 PSI the auto feed will stop adding water when the system pressure is at 12 PSI and all is good.   All three gauges will be at 12 PSI.  

When the pump turns on there will be a pressure difference between the inlet and the outlet of the pump.  For your system lets say that the pump pressure difference between the inlet and the outlet is about 6 PSI.  That means whatever the pressure at the inlet of the pump is, the outlet will be 6 PSI more.  So if the inlet pressure is 12 PSI the outlet pressure must be 18 PSI.   But the expansion tank is on the discharge side of the pump.  That is a point of no pressure change in your system.   That means the air pressure in the expansion tank is 12 PSI and unless you add water or subtract water from the tank. That air pressure will stay at 12 PSI. Some guy named Gill Carlson wrote a paper about this over 60 years ago. But believe me that is the point of no pressure change.  

Since you can’t change the pressure at that tank, the discharge side of the pump must stay at 12 PSI.  So if that is the case then the suction side of the pump must drop to 6 PSI in order to make the math work.   12 PSI - 6 PSI = 6 PSI.   Now there is some friction loss thru the boiler so maybe the gauge on the inlet of the boiler might read 7 PSI.   See what is happening here.  The pump is making the place where the PRV is located lower than 12 PSI.  Like maybe 7 PSI, right?  So the PRV is set to 12 PSI and the pressure in that pipe is 7 PSI so what is the PRV going to do?  

You guessed it…. Add Water!

Now if that adds water to the system when the pump is operating, what will happen if the water pressure on the pump inlet is increased from 6 PSI to maybe 11 or 12 PSI?   That’s right, the outlet of the pump will add 6 PSI and make the pressure near the expansion tank (the point of no pressure change) 17 PSI.  You see the added water in the system does not have anywhere else to go.  There is no extra space in the closed system because there is water in all the radiators and the boiler and the pipes are already full of water.  

The only place left is the expansion tank air space and that air will compress as the extra water is added by the PRV when the pump operates for the first time.  Then the system pressure at rest will be 17 PSI and that might be too high for some systems to start at with all the water cold.  As the water heats up it will expand and maybe the smaller air space in the tank will not be enough to accept that expansion without making the system pressure increase to maybe 28 or 29 PSI.  And that's too close to the relief valve release pressure of 30 PSI.   

EdTheHeaterMan

This is how you might set this up for better air control on your system.

psb75

clammy on filed X-tank: 👍️

psb75

correction: "filled or failed" X-tank.

EBEBRATT-Ed

@EdTheHeaterMan

Your correct.

Many miss this but the water feed PRV and the expansion tank connections have to be on the same side of the pump, preferably on the pump discharge. They don't have to be piped together but need to be connected to the system in the same vicinity with little to no pressure drop between them.

EdTheHeaterMan

Correcting Typo @EBEBRATT-Ed

Many miss this but the water feed PRV and the expansion tank connections have to be on the same side of the pump, preferably on the pump discharge Inlet. so that it is pumping away. They don't have to be piped together but need to be connected to the system in the same vicinity with little to no pressure drop between them.

EBEBRATT-Ed

@EdTheHeaterMan

I had a brain zap