Radiators staying warm even when thermostat asks for no heat.

Shahrdad

My next door neighbor had his 1950s boiler replaced in 2016 with two Buderus cast iron gas boilers (Model G234X-45/5) each with an input rating of 186,000 BTU/h and an output rating of 153,000 BTU/h. The house is about about 4,000 sq feet and had an original gravity hot water system installed in 1891.
My neighbor was saying that on mild days when the outside temperature is in the 50s and the thermostat is not asking for any heat, his radiators remain lukewarm, and the house gets too warm. He then has to go into the basement and switch both boilers off.
The boiler piping appears very "creative" to me. There is a circulator pump on the return, which is then split evenly into each boiler. The boiler supply pipes then join each other in a T and get connected to the old gravity supply pipes. I will include a photo.
I am no expert, but this looked wrong to me. I told my neighbor that I thought perhaps the boilers keep themselves somewhat warm even when no heat is demanded in order to minimize condensation upon a "cold" startup, and with this piping system, the warm water in the boiler gently rises out of the boilers and circulates through the old gravity system.
Have you all seen a two cast-iron boiler system piped this way or experienced warm radiators when no heat is required?

Here is a photo. The pipe coming in from the right side with the pump on it is the cold water return. The pipe coming off and heading to the floor goes to the compression tank. The pipe on top goes to the supply side of the system.

Jamie Hall

There are a number of things wrong with the way that's piped, not the least of which is there is no way to isolate one boiler if you wanted to. But I won't get into that...

On the radiators staying warm. The first question is... do the boilers run from time to time? If they do, there will be gravity circulation in that setup (that, after all, is what heated the place before, no?). Then the question is -- why do the boilers run? It may well be a cold start protection, and it may be very hard to get rid of the problem with that piping.

Shahrdad

Jamie, that's what I thought too. I watched the boilers fire one evening, and one boiler came on (it wasn't even that cold) and then the other one came on for a couple of minutes, and then they both turned off. I could feel pipes continuing to get warmer even after the boilers and the pump turned off, so water kept circulating.

Shahrdad

Youngplumber, he wasn't complaining much, but when I went to check out his system (I'm planning on upgrading my 50 year old MW boiler), he said that the only issue he has is what I described above, and that seemed like a strange phenomenon to be having for a new system. I also wanted to see what kind of piping his contractor had done, and now I know I will not be using that company. It seems to be that piping the system correctly with primary and secondary pumping would be easy, would solve his problem, and save him money in the long run. He did seem slightly annoyed by the issue, however.

motoguy128

2 stage boiler control with outdoor reset should help.   

nicholas bonham-carter

Having a total of 300,000 btu of output seems overkill for a 4000 sq. ft house as well.—NBC

Paul Pollets

Using 2 boilers for a 4000sf load is not normally done. There are prescriped piping methods when it is done and it hasn't been followed. A 2 stage control with outdoor reset is used that will rotate each boiler on each firing cycle and bring on both boilers, if the load requires.

Paul Pollets

Where is the home located?

Jamie Hall

That would be the way to do it, @Paul Pollets -- except that the boilers aren't isolated from each other, unless there are some powered valves in there which I can't see.

Paul Pollets

Siegenthaler has written about the proper piping practices for tandem boilers using P/S.
https://www.pmengineer.com/articles/87639-multiple-boiler-systems

I wonder if the DHW uses an indirect tank?

Shahrdad

Paul, it's in St. Louis. I thought the boilers were overkill for a house of that size, and it's just not normal to have warm radiators when no heat is asked for by the thermostat. He does not use an indirect tank.

Jamie, there are no powered valves or any separation between the two boilers. My neighbor was under the impression that the left boiler was primary and the boiler on the right secondary. While I was visiting, the right boiler turned on, and while we were watching, the left one turned on too. I don't know if it was because heat was demanded by the thermostat or the boilers were just keeping themselves warm. I put my hand on the circulator, but couldn't feel any vibrations, but with these new ones, I'm not sure that's a reliable way of telling whether or not it's running.

Jamie Hall

No, vibration can be very hard to feel on the newer pumps -- if they are healthy.

But with two boilers with no separation like that, they both have to be running to get any real heat at all -- the shortest path for the water (remember, the stuff is lazy) otherwise is out of one boiler and into the other, and around and around we go...