Thanks for all the replies....
The pressure relief should not be opening just because the temperature rises to 200 degrees or so. If you have an old style steel expansion tank, it might need to have the water drained out of it.
Temp guage pegs @ 260....does not blow until after the guage has been pegged for a while....the boiler will actually start to boil (you can hear it) and then the pressure builds to 30 PSI and off she goes....
I do indeed have an old style steel expansion tank and it does have water in it....I had drained it a while ago but it seemed to fill up as fast as I drained it!?
Where is the aquastat located in relation to the tankless coil? Ideally it should be located close to the coil so it reacts quickly. The aqaustat itself does seem to be questionable based on your observations. I had one that was doing the same thing on my oil boiler, I replaced it and the new one definitely reacts faster. If you can swing it, a proper length well and a new aquastat would probably be a good investment.
This could be it...The Aquastat is side mounted and with the correct depth well should get real close to the hot water coil, however with this short well it may not be close enough....I will post some pics...
I didn't mention it, but if it heats water ....it is a boiler NOT a furnace
How'd I do in this post?
Domestic hot water demand(how many kids, baths, etc.)
6 showers back to back
Tankless only? Do you have a domestic tank as a buffer between the coil and the demand side of the domestic hot water?
Well length? Is the aquastat bulb touching the bottom of the well?
Don't know....I just know the plumber was grumbling about not have a deep enough well and as a result the Aquastat is going to stand away from tank a bit and the temps could be off...
I will try to get some pics posted of the expansion tank, Aquastat, plumbing Etc.