Weil-McLain Boiler - Reco for Indirect Tank for DHW

Greetings -

I have a 2013 Weil-McLain oil boiler WTGO 3 or 3R. Our DHW is coil-tankless. Our heat is also baseboard hot water.

Over the past few years, we have developed a problem regulating the water temperature in our 2nd floor shower. It will either be scalding hot or cool/cold. It's progressively become more difficult to set the fixture to hold a hot but not scalding temperature. When I say, "progressively", I mean, in the past 2 years this has steadily gotten worse.

I changed the Kohler mixing and balancing valves which did almost nothing to improve the issue. It was not an issue with the scald stop or anything like that and the valves were a little scaled up. The first floor shower doesn't have this issue but it has a different fixture and possibly a different stem (we're not sure).

Our plumber, who has done great work for us in the past, cautioned that based on his temp readings in the shower and other faucets in the house, that we likely had a coil lime scaling issue at this point. We do generally have a lime issue with our town water.

The plumber determined that the water coming from the boiler master mixing valve was much cooler than the water coming from the boiler, even when turned all the way up to 160 degrees. The boiler temp is 180 degrees and the water coming out of the faucets was 120 even with the valve turned to max.

He suggested that the mixing valve could be scaled up and preventing hot water from the boiler to mix properly and recommended replacing that as a first step.

He did that, at a cost of $1,200 and that seems to have provided little improvement. We have to set the temperature near 140 on the new valube and he said that is not ideal because of course, it will be scalding hot if turned all hot in any faucet, and it really should be set at 120.

So now it's back to the idea that the lime scaled coil is not heating the DHW enough and recommending bypassing it and installing an indirect tank as another heating zone.

Of course, the cost of this will be much higher than the master mixing valve. The number that was thrown out was essentially the cost of an entirely new boiler.

I asked if the coil could be cleaned and I was told it couldn't be at this boiler age. I asked if the boiler could be opened to confirm this was the issue and was told it couldn't be.

We can manage with this for some time. It is not urgent to address, but at some point it will make sense to pay now instead of pay more later.

I've only had one other tankless oil boiler in the past (different property) and it did not have this issue.

I was told that the tank would have a lifetime warranty and could be hooked into a new boiler when that needs to be replaced in 12-15 years. And that it would be far more efficient that what we are doing today.

Is this common and reasonable to do with these boilers?

Thanks in advance -