Minimum temperature of a hydronics boiler

I am thinking that what you are calling a temperature gauge is not a gauge but a Aquastat if there are wires going to it. The tank is actually your boiler.

So you don't need the maintain a minimum temperature in the boiler unless the boiler also makes your domestic hot water by way of a tankless coil. Do you have a separate water heater?

You are describing a cold start boiler and that is a normal way to set up a boiler. there is no reason the burn fuel all year long just on the chance that you might want heat in the radiators on July 5th or September 22 or even February 14. The thermostat in the room will let the burner know if it needs to heat the water so your radiators can heat your home. You can have a cold start boiler.

What you don't want is to operate the boiler so the return water is below 135° for hours of operation. You want that return water to get to 135° within a few minutes of operation. (like maybe 10 minutes)

it would seem it must have had a tankless coil or indirect or hot water zone or something like that at some point if someone went to the trouble of installing a low limit aquastat.

unless some upgrades have been made, a 1940 home may not be well insulated ar sealed

An energy audit that included a blower door and an infrared scan would show all that. In some cases the gas company may have an audit program

Www.dsireusa.org may show programs offered to audit the home

it is always the home that dictates most of the heating costs.

The important number is the amount of gas you get for that monthly bill, how much you pay per therm and if there are additional costs

I would not suspect the boiler is grossly oversized if it runs long cycles to heat up?

Getting rid of the tankless coil will help, contact Peerless or their rep in the area about a damper addition

I have four of these clip-on pipe thermometers mounted on the supply and return pipes of our two boilers, and they are accurate enough for your purposes. I have checked calibration with an infrared "gun" type thermometer.

For best results, get some thermal conductive paste and smear it on the contact surface of the thermometer. That ensures good contact with the pipe and better accuracy.

Tell us how many square feet your house is, and if it has any thermal upgrades like double-pane windows, more insulation, etc. You said it was circa 1940, so if it has original windows and insulation, it's probably fairly leaky. I think I saw a boiler service label that said Boyertown PA, which tells us where your house is and what assumptions to make about the local climate for heating calculations.

Your gas bills do not seem that bad. Where are you located?

What I did with my boiler was to mount a strap on aquastat to bypass the circ relay. When the stat was satisfied the burner shut down and I kept the circ on until the boiler cooled to 125 or so. It would overshoot the stat about 1 degree.

As for post purging the boiler, I'll mention that I've been running a one-year experiment with post purging our two WGO-5 boilers in a gravity conversion heating system from a typical end-of-burn water temp of 140 degrees or so down to 90 degrees, and I haven't been able to measure a statistically meaningful effect on fuel consumption.

I'm doubtful it's helping much since we would normally get continued gravity circulation after boiler shutdown, and the post purge is simply adding some additional force to the gravity flow, thereby speeding up the process somewhat. When I proposed trying the post purge last year, some pros here were skeptical that the juice would be worth the squeeze, and the data so far suggests they were probably right. If there is a fuel savings in our application, it's so small as to be swamped by the noise in the data.

@fentonc That sounds right, and we're 0-for-3 on the conditions you listed, so not a surprise that post purge doesn't appear to be doing anything for us.