Aquastat question?

Geurn

Hi, I have a wood boiler in my shop that heats the shop and the house via underground pipes. It's been up and running fine for ten years. One thing that puzzles me is when the house thermostat calls for heat, it turns on the circulator out in the shop next to the boiler, sends the hot water to my basement gas furnace (heat exchanger in the plenum) and a "makes on rise" aquastat waits for the temp then starts the fan (furnace fan). All this time the fan will come on for maybe 5 seconds then stop, then come back on for the duration that it needs to. I can't figure out why it starts, then stops, then goes like it should. I wonder about things like…is the first bunch of water coming underground cooler because its been underground, only to be replaced by fresh hot water, or is my domestic hot water coil calling and a quick spurt of hot water triggers the fan aquastat for a second, then diverts to the domestic water tank? It just seems like the aquastat gets a zap of water hot enough to trigger it, then immediately loses it, only to be fed with hot water 5 or 10 seconds later. Other than this "false start" the system as a whole has been great as I say for the past ten years. I've just lived with it until now I'm questioning why. Thoughts please and thank you?

SuperTech

Is the aquastat on the supply or the return pipe from the boiler? What temperature is it set for?

HVACNUT

What aquastat is on the hydro coil? See if there's a differential setting.

neilc

I'm guessing your drawing a quick shot of colder basement ductwork air, tripping the aqua/air stat back off, then warmer return air from the space follows and let's the heat run as wanted,

Geurn

Its a Honeywell aquastat and its on the supply side. The main dial is on 135 and the little wheel underneath is on 10. I think by the time the water gets to it, it's probably around 160 degrees after it's trip underground for 50 feet or so. There is a loop at the boiler so that it stays as hot as possible until called for. The hope was to keep the temp up so the the return water wasn't going into the boiler too cooled off coming back. I haven't measured recently but the water leaves the boiler at 180 degrees and comes back around 140.

HVACNUT

Move the differential to 15°.

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1832611#Comment_1832611

That is "the little wheel underneath" you referred to.

hot_rod

Often times the aqua stat is mounted right on the coil return pipe, so the coil is hot before the fan starts.

But you are probably correct that it gets a slug of cold water.