This problem has just started recently. I run my shower and the hot water becomes cold just after 2-3mins. I've got endless hot water in the rest of the house. I have read through this dicussion board and have noticed much talk about water heaters however have seen little about Tankless Boiler. I've got a Tankless Forced Hot Water Boiler(Weil McLain Gold Series). I have since flushed the boiler, turned up the mixing valve, thermostat set to 170 and still have the same problem. I have slowed down the cold feed to the boiler and have seemed to get more hot water but it doesn't last. The hot pipe out from the boiler is scalding hot (coil OK), the pipe after the mixing valve is hot (mixing valve OK?). Could it be the pressure balance valve being stuck? Any help would be greatly appreciated?

It seems it's not the water heater, but it could easily be the shower valve itself especially if it's temperature regulating. Another possibility is debris in the line.

You may need to turn the temperature down. Check the specs. Your heater has a minimum flow. You crank the valve all the way to hot, and run it until the water in the line is all hot. You then adjust it to a moderate temperature reducing the flow of hot water below the minimum. The heater shuts off, and when all the hot water runs out of the pipe, berrrrrr.

The more I learn about those things, the happier I am that I didn't blow a fortune on one.

Seems that it is likely your mixing valve if you have good hot water everywhere else in the house. KISS asked if it was a temperature regulating valve, is it? Is your valve an anti-scald valve?

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Originally Posted by mklays

This problem has just started recently. I run my shower and the hot water becomes cold just after 2-3mins. I've got endless hot water in the rest of the house. I have read through this dicussion board and have noticed much talk about water heaters however have seen little about Tankless Boiler. I've got a Tankless Forced Hot Water Boiler(Weil McLain Gold Series). I have since flushed the boiler, turned up the mixing valve, thermostat set to 170 and still have the same problem. i have slowed down the cold feed to the boiler and have seemed to get more hot water but it doesn't last. The hot pipe out from the boiler is scalding hot (coil OK), the pipe after the mixing valve is hot (mixing valve OK?). Could it be the pressure balance valve being stuck? Any help would be greatly appreciated?

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It is probably in your shower on the mixing vavle inside of it if this only happens in the shower

Guys, thanks for the responses. It has definitely got to be in the mixing valve at the shower. What kind of work am I looking at to change over or clean this mixing valve? Should I just be calling a plumber? What is the general charge for a job like this?

This ended up being the answer to the problem.

A plumber would likely clean the valve so there would be a service call plus time. Could run about $100 but the job isn't too difficult so give it a try. There are serveral people here who can walk you through the small project. Basically, shut off your main water and open a faucet somewhere to blow pressure. Remove the handle and escutcheon ( plate thing). Now depending on your valve you should see a round plate with 2-3 screws holding the plate to the valve or you may see a nut like ring about the size of a 50 cent piece, remove this to pull the valve or cartridge. Once it comes out look at the bottom for the inlet ports, debris builds up here so pick it clean with a soft small object like a tooth pick. Flush the valve with water and before reinstalling hold a towel over the opening of the valve and have someone open and then close your main to flush an immediate debris from the lines. Most debris is found on the hot side primarily coming from your water heater. Flushed the heater lately, you should.

Just replace the cartridge all together! That's what I do when I go to a house call works 9/10 times the other times cleaning the ports work

Don't know Plumbjames, isn't that a bit like throughing out those dirty dishes and buying new? Cleaning the inlets doesn't cost mklays any money and one less trip to the hardware store. I think we are here to assist DIY projects and help homeowners understand how to keep their homes in good repair.

I checked the cartridge and the pressure balance barrel sounded good but I figured for $46 I'd just replace it with a TA10-3hole and I've still got the same problem. There was no debris in the lines. My question now is -- what else could it be? I've exhausted my efforts and have done everything I can think of, and have become frustrated from doing all this work and now I've got to give it to a plumber to fix what I know will be something I've easily overlooked.

Try running the hot water at a sink until it is hot, and cut back to a trickle, and see if the heater cuts out.

We may just be using different terms but is this a water heater or a combi boiler and what does the tempering valve do and where is it located? I am not familiar with that term.
Thanks

Out of consideration for your help, I'll let you know that a plumber came by the house to diagnose the problem and spent approx. 25 minutes checking the new valve in the shower, the lines and came to the conclusion it was NOT a plumbing issue and that it had to be the boiler. I then got on the phone to who services my boiler and after a tech. spent approx. 1.5hrs. Checking the aquastat, the mixing valve(boiler) and flushed the system. Result was slightly hotter water to the shower and was told there was nothing wrong with the boiler. This is where I currently stand on the issue.

OK I guess U should have asked this question long ago, how is the hot water at other fixtures? If the issue is just at the shower and no where else it is not the boiler and is a plumbing problem. I'm sure the plumber did this checking when he was out to determine if it is the shower or whole house.

It is a Weil McClain Tankless Forced Hot Water Boiler. Everything checked OK on the boiler. We ran hot water for several minutes and the aquastat on the boiler read the water was heated to 180 degrees coming out of the boiler and then it is the tempering/mixing valve which cools the water to its setting of 140 degrees before going through the lines to the house. After bringing the water to a trickle, the heater does cut out. At the shower, instant hot water after running hot water at the sink for a few minutes and then the water becomes lukewarm/hot but not 140 degrees which is the setting on the mixing valve at the boiler. It seems like a volume issue at the shower with the hot water.

I think I just need to know how the hot water is at all of the other fixtures in the house. Does your kitchen sink run hot water continuously, same for you bathroom sink? I think you are saying the hot water is fine at the sink so I still suspect a plumbing problem at the shower. Back to you.

I am sorry, but I was thinking of how the free standing tankless heaters work, not the combiboilers. With it seeming to be a low flow problem, I was almost right for the wrong reason.

Have you tried removing the cartridge at the shower and flushing the hot line out good?

If that doesn't do it, you will need to gather some more information, how does the system respond to low flows elsewhere? It must not respond if you turn the temperature back up in the shower.

Yes, I did tell the plumber it was ONLY THE SHOWER. The plumber had checked the pressure balance valve, checked for any debris in the lines and said they were fine as I had already known since I had replaced the valve.

I think you need another plumber. If the problem is only at the shower IT IS a plumbing problem. I think Labman and I are reaching the same conclusion, it is in the cartridge or the line feeding the hot side of the mixer. The fact that you replaced the valve makes me think there is something in the line, something big.