Burner Not Shutting Off at 10 Diff From Lo

My understanding is that the lo/diff control the temps when the burner should come on (feed coal-blow air).

I noticed that the temp would often rise after the thermostat cuts the circulator pump, and I know that is normal. However, when the thermostat kicks the circulator pump back on, the temp is sometimes still above the lo/diff setting.

To test, I set the lo at 140 with a diff of 10. Circulator pump should be able to run between 130-150, and the burner should kick on between 130-150.. correct? But the burner is still firing at 160, and I'm guessing that is because the thermostat is calling for heat. Problem is, I'm wasting heat because even if the boiler temp is 160, the burner will still kick on whenever the thermostat is engaged. At least it seems that way (whenever I turn the thermostat down, the burner shuts off, naturally, but if I turn the temp back up on the thermostat... the burner kicks on every time, even if the water temp in the boiler reads over 150).

It's cold. So this pretty much runs non stop over nights these days ... but during times of shut off, when the heat rises (due to lack of circulator activity and the fire still being relatively hot), I'd like the circulator to bring that heat up to the apartments.

Advice much appreciated.

Assuming you are using a standard triple aquastat and the thermostat is connected to the TT terminals, the stoker will run anytime there is a heat call until the high limit is reached.

The low limit only comes into play the boiler temperature drops and there is not a heat call...such as when the tankless coil is producing hot water.

If the unit is running nearly nonstop, the heat that gets stored in the boiler will be quickly used when there is another heat call. I suggest you increase the feed rate a tooth or two so the unit has better recovery, and pay attention to the ash ring. If you have problems with the boiler getting too hot after a hard run, a L4006B aquastat could be used to turn on the circulator(s) at a certain temperature.

Is the boiler insulated?

Keep in mind the aquastat and temperature gauge may be seeing different temps and their accuracy may not be dead on and or relative to each other. Once below the differential, the burner will run on a call for heat regardless of the low limit.

If you change out the triple aquastat to a Hydro level 3250, you will be able to get the results you want. It has a built in purge feature that takes all that heat out of the boiler on a heat call before it turns on the burner.ll

The reason for the differential is to keep the eat up with demand. I don't recommend losing the differential for some assumed savings particularly if it has a DHW application.

Thanks again fellas... couple things:

I'm more interested in getting that heat that accumulates whenever the boiler shuts off.. e.g. at the "end" of a cycle. Basically, I'd like the circ pump to keep running about 5 or 10 minutes after the stoker shuts off because, as we know, the temp of the boiler often rises after the stoker shuts off due to the fact that the circ pump is not running, and there is still a lot of heat in the firebox/stoker.

Rob, When I bought this all refurbed, I did not insulate it, but now it is fully insulated with a nice coating of ash that has built up over the last two years, particularly on the top. Seriously, do you recommend putting some insulation on it? It does sit in the basement... though I could insulate the basement better. On your rec for that Aquastat, Rob, would that replace the triple HW or work in tandem with it?

kinnscience wrote:I'm more interested in getting that heat that accumulates whenever the boiler shuts off.. e.g. at the "end" of a cycle. Basically, I'd like the circ pump to keep running about 5 or 10 minutes after the stoker shuts off because, as we know, the temp of the boiler often rises after the stoker shuts off due to the fact that the circ pump is not running, and there is still a lot of heat in the firebox/stoker.

Did this go anywhere? I was thinking about something similar:

1. Allowing the Tstat to overshoot the setpoint using circulators only until the boiler temp falls to a certain point (not necessarily Aquastat's LO)

2. Eventually allowing the Tstat to "predict" where to stop stoker firing and "coast" up to the setpoint.

We used to have a heat pump Tstat that did the second, no idea if it would work for a coal stoker?