New boiler on gravity system

Where does your hot water for kitchen sink and bathroom come from? It sounds like you have a domestic hot water coil installed in the boiler, which would explain the 125 minimum aquastat setting.

But your circulator shouldn't be running all the time. It should only be running when the thermostat calls for heat.

@jakamini

some picture from around your system is really helpful. Make sure to get enough pictures to show all the way around the boiler and the controls and piping, should be about 4-6 photos standing back 4-5 feet from boiler.

Are you sure the "separate" water heater has its own heat source (like gas or electric) and isn't an "indirect" tank that gets heated water from the boiler?

Check if your boiler controls allow for a heat purge after shutdown, if not the electrician can add an off programmable off delay timer. You want to keep the pump on until the boiler cools down after each firing, this gets you a couple % of extra efficiency for free.

Did you call the person that installed it? They might have missed a few things while they were trying to get the heat back on like setting it as cold start. Note that on a converted gravity system, unless it has a flow check at the boiler, if it is set up warm start it is very likely to heat the system through gravity flow even if the circulator isn't running. Set the low setting to not fire the boiler.

That not the way I would have gone about piping that system.

Typically you try to mimic the flow rate. That looks like a 0010 on the piping.

Im not even sure there is any boiler protection there.

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I don’t see any air purger? Also have him support the expansion tank a bit more.

When it fires from a cold start, time how long it takes to get the return to the boiler to 130F. You want that to happen within 10 minutes, ideally

I bet he is using the boiler aquastat as a low limit to help prevent the boiler from condensing. Not ideal but it will help. Thats better than nothing.

That may also explain the ghost flow out to the system and the overheating… especially w/ the cast iron rads.

You don't need anything else. You already have a bypass plumbed in, which is a pipe path that takes hot water from the supply pipe exiting the boiler and returns it to the boiler. The bypass is the copper pipe that tees out to the left just above the top of the boiler. That increases the water temperature returning to the boiler, to help prevent condensation and also to prevent thermal shock to the cast iron heat exchanger.

Apparently your plumber set the boiler up initially with the aquastat having a low limit temperature because as you said, he was just trying to give you temporary heat until the elctrician could come and redo all the wiring correctly. It wasn't for condensation protection. The bypass does that.

I do not see any bypass/boiler protection valve in this system

No air separator, which I guess just means that he would be bleeding the air at the highest points at the radiators.

Also looks like there's at least 2 sets of thermostat wires in the top left of the pictures (old striped cloth wrapped wire) but you only one set of thermostat wires into the control box. Its hard to see how they are all spliced together.

The 120v circuits all hanging in free air doesn't look too safe either.

@jakamini I stand corrected, there is no bypass plumbed in as I erroneously said. One of your photos has two pipes lined up one behind the other, and my eyes fooled me. The horizontal branch of the copper pipe coming vertically out of the top of the boiler is going to another supply main, not the return pipe behind it.

The boiler installation instructions probably do call for a bypass. However, I have two Weil-McLain WGO-5 cast iron boilers that also were plumbed with no bypass, and they have run for 30+ years with no sign of condensation, even with low return water temps, most of the time sub-130 degrees.

So if I were you, I wouldn't sweat it too much. On a call for heat, your boiler should run from a cold start at least 30 minutes at a time, during which time the flue gas will get plenty hot to burn off any initial condensation from startup, regardless of low return water temperatures.

The installation instructions show a bypass and explain why it is needed. However..

That boiler could run hours in condensing mode if the distribution piping and the cast radiators are more emitter than the boiler can keep up with. Thermal equilibrium is at that low RWT.

For example the return water may be 68° when a call for heat A properly sized boiler could maybe "lift" that temperature 5° or so. That indicates the boiler block is running around 70.

Assume 10 gpm is being circulated. There is no way a bypass, even full open can raise to boiler to 130° if the boiler can only make 70° water.

That is why the bypass needs to have a thermostatic function. Could be the aquastat as @kcopp mentioned, or a thermostatic valve.

Two ways to confirm this. Calculate the radiators output, EDR. Or watch the boiler when it fires up, time the cycle, measure the return.

Just because @jesmed1 boilers lasted, doesn't guarantee yours will😉

Observe it and see for yourself

Unfortunately it seems common for installers to ignore the typical bypass requirement for cast iron boilers. I suspect they count on the ignorance of the homeowner (myself included, since I didn't know what a bypass was until I came here). They know the boiler will work whether or not they install a bypass, and if the boiler doesn't last as long as it should due to condensing, they'll be long gone, and the homeowner is none the wiser.

As I've mentioned, our two cast iron boilers were installed in the 1990's without bypasses, and fortunately it seems they haven't suffered from the omission, but now that I know, I will make sure our next boilers do get bypasses.

@EricPeterson why 115F ? Don't the boiler mfr's typically say return temp must be above 130F ?

I'm not saying 130F is right, because as I've said before, our boilers run 90% of the time below 130F return. I'm just trying to understand the logic of 115F.

We can also use those Caleffi 280's when we replace our old boilers. Am just curious about the correct temp.