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use cold start aquastat for warm start

zoevic Member Posts: 7
I’m a boiler system newbie but I’ve been having a discussion with someone about their boiler setup and the replacement of their thermostat. One thing they happened to mention in passing is that their Aquastat control was converted from cold start to warm start. The control they are talking about is the Honeywell L4081B Aquastat which has a high-limit and low-limit, but the low-limit is used to control a circulator so that the circulator only runs when the water temp is hot enough. The low-limit relay breaks on water temp fall and makes on water temp rise.

It seems to me this type of Aquastat can’t be used for warm-start. Or is that wrong? I suspect what someone has done from what I can tell is use the high-limit only to control the burner, so that the water temp is kept up at near the high-limit all of the time – with or without a call for heat. So it’s now kind of like a warm-start boiler, but I guess you would say it’s more like a hot-start.

I think what someone did was change the system so that there is continuous power to the Aquastat high-limit relay which in turn is connected directly to the burner - and there is no thermostat connected to the burner.

But there is a line voltage thermostat connected to the Aquastat low-limit relay which in turn is connected to the circulator.

Maybe I’m wrong but it seems to me someone took an Aquastat made for cold start and is using it improperly. If that’s the case I want to tell this person they are probably using much more oil than they should by keeping the water temp too high all of the time and they would be better off buying another Aquastat made for warm start.

But I’m not sure I really understand the Aquastat and these things and may be way out in left field. I certainly don’t want to throw someone off track.

Any thoughts would be greatly appreciated!

Here is the info on the Aquastat



  • zoevic
    zoevic Member Posts: 7
    Thank you very much for your response Hatterasguy and I am certain of the model number (makes me feel good I'm an old senior but I guess I have a few brain cells left)

    Thanks again!!