'aquastat' delta-T for modulating boiler...set large or small?

Mike T., Swampeast MO

Personally I'd suggest the setting that:

1) Results in the least amount of gas consumed during a NUMBER of such cycles with nearly steady temps. (Like the lowest possible setting.)

provided:

2) The firing time does not grow unacceptably short in really mid weather. And, no, I can't define "unacceptably short". Perhaps HTP can.

Chuckles_4

I have a Munchkin. The 'delta-T' I'm talking about is the difference between the outlet temp at which the boiler goes off (if it can't hold the outlet temp by modulating) and the outlet temp at which it comes back on again. Assumes continuous heat call during that time.

Of course the Munchkin doesn't actually have an aquastat, it's done with on-board logic. The range of this setting on the Munchkin is something like 5-40F.

1. Large delta-T (greater than 20F): results in long burner cycles. However, on Munchkin it results in burner modulating to full output every time it starts up, which may be less efficient.

2. Small delta-T (less than 20F): burner never ramps up to 100%, which may be good. But it cycles more often.

So which of these is better? I'm asking because, in the current warm (55F-65F) weather, I'm seeing significant differences in gas consumption between a setting of 15F and a setting of 30F. Presumably in colder weather it won't matter because the burner will just run all the time on a modulated setting, but right now the difference is quite noticeable.

Guy_6

Delta

The Munchkin was designed with a 20 degree deltamin mind. That delta is obtained entirely by FLOW, and therefore proper circulator piping and sizing are very important.
As for how the boiler begins its cycle- if it is a newer vintage, you have the option of either a 6 step ramping to full fire, OR jumping immediately to high fire. At ANY point in the start cycle of either method,or any point of operation, the boiler will modulate as the delta dictates.
As a rule, the boiler should have 1GPM flow for every 10,000 BTUs of output, with primary/secondary piping the preferred method.

Chuckles_4

This is not what I am talking about. You are talking about the difference between supply and return temperatures, which is a different thing.

I am talking about the difference between where the boiler goes off and where the boiler goes on again. It's set in the menu that you bring up by holding F3 down for a few seconds.

Also the primary flow rate is fixed, which means even your deltaT can't be always be 20F, it is only 20F if the boiler is running flat out at rated BTU output (no modulation) .