Boiler failure mystery solved using the pump curve, this weeks video

Was it short cycling with that single small zone running?

@RayWohlfarth , what type of aquastat relay did the old boiler have? If it was a HydroStat or AquaSmart, was it properly configured?

Thermal equilibrium reached at this temperature, with one 60k load on the boiler

(60,000/237,000)(180-70)+70=98ºF.

or is it a gravity conversion so it is a system mass issue?

Actually, @RayWohlfarth , I suspect the problem may have been the very high delta T, at least on occasion. Yes, the boiler was controlled by the aquastat, which would have tried very hard to keep the boiler output temperature where it was set — but with the low flow rate through some of the radiation, that part of the boiler would have been hot, but with low circulation in the boiler the specific area where the return connected would have remained cool, if not cold. Very large differential expansion in the block, and … boing.

What confuses me about this is that usually a ci boiler is very happy to sit full of hot water with no flow and the aquastat keeping it at temp so normally a low flow zone though it might cause short cycling and wear out controls and have terrible efficiency shouldn't damage the boiler.

what does cause damage is a system with a lot of mass with all of the water of the system piped directly in to the boiler with no return water temp protection so the mass of the system keeps the boiler from getting above condensing temps before the t-stat is satisfied.

i can see where the first case could be a problem if the internal water passages of the boiler are convoluted or the burner controls mean that the burner takes a long time to fire again after it has shut off.

A water tube boiler like a Laars or a modcon where the lowest turndown ratio is much larger than the smallest zone is likely to have a problem.

I'm with @mattmia2 . This most likely failed from low RWT not from lack of flow.

Usually in older houses with any basic upgrades such as storm windows, the radiators are 2x to 3x over sized. Add in there an oversized boiler that will deliver very hot water very quickly, the zones tend to have very short cycles. Since you have a lot of water in the system, the top of the rads will be hot but the bottom will near always be near room temp, so return water will be very cold most of the heating season.

Short boiler cycles also means the flue stays cold to make things even worse.

Doesn't matter how you do it, high mass rads with cast boiler needs condensation protection.

The soap box I have been on for years now is the misleading boiler protection options that many boiler manufacturers show. These are not absolute protection methods.

The OP system is a classic example or the reason, room temperature water returning at say 68. A properly sized boiler lifting that maybe 20° so the boiler supplies 88°.

How does 88° SWT blend with 68° RTWT to get you to 130° What pump at what speed could do that?

Sad.

And primary secondary piping doesn't offer absolute boiler protection either, another industry myth.

The setup that you have shown above has bigger problems that the lack of proper protection for the boiler.

Think about what occurs when the radiation has reached close to the SWT.…………….170F SWT and returns 150F. When the RWT gets to the boiler, the bypass sends a certain percentage of the supply into the return which raises the RWT 10F or more on the way to the boiler. This will bring the average boiler temperature up by the amount of the RWT increase. The aquastat will definitely shutdown on HL much earlier than it would otherwise with such a setup.

Without a proper temperature controlled bypass, the setup shown doesn't work at cold startup and doesn't work after the radiation reaches temperature.