Aquastat Settings

Tom Curlee

Hello,

I am a retired IBM design engineer and a trustee at Poughkeepsie (NY) United Methodist Church. With the help of a professional engineer, I have been studying our church's heating system and we are about to replace the circulator pumps on each of our 4 boilers.

Each pair of boilers is controlled by an aquastat - one in the supply side of the secondary loop (for the "lead" group") and one in the return side of this loop (for the "lag" group). The aquastats are mounted about 3 feet from opposite ends of the "common" pipe.

The aquastats have a set-point adjustment and a difference adjustment. I found the difference adjustments to be set, years ago presumably, to 5 degrees. A computer model I developed to study the effects of the new circulator pumps indicates that the system is unstable at this setting as follows:

When the supply-side aquastat is satisfied, it causes the 2 lead boilers to shut down, which causes the GPM in the primary loop to drop by about 45%. This, in turn, causes the water temperature at the supply-side aquastat to drop dramatically (more than 5 degrees) since there is LESS boiler water and MORE return-side water being fed to the supply-side "T." This, of course, causes the lead boilers, which just shut down, to start up again (after a short aquastat thermal delay), beginning a series of on-off oscillations.

So, in summary, I guess I have two questions: 1) What should the difference settings of the aquastats be set to (or how do I figure this out if I've not given you enough information), and 2) What set-point temperature should each aquastat be set to? Is this where the nominal 20 degree temperature drop is involved? Shouldn't the supply-side aquastat be set to a temperature based on the boiler high-limit (fire-down) temperature setting?

Your help with this matter will be greatly appreciated!!!

Tom Curlee
(845) 454-2388

Unknown

Please

please post a drawing showing pump locations and sizes, and tell us what the distribution piping layout is. It would help a lot.

Tom Curlee

Aquastat Settings

Gee, no wonder only one response to my 8/1 forum entry: It takes only about a day to get off the "front page" of The Wall.

ANOTHER SUBJECT (A tip on thermometers)

I found a digital, battery-powered barbequeing meat thermomether at Lowe's for about $21 that I found to be extremely accurate and ideal (for me, at least) for measuring the temperature of heating pipes. The manufacture is ACCU-RITE. It has a 6" probe which has a 90 degree bend in it where it attaches to a 4' cable going to a small display unit. I found it very easy to insert the probe between a pipe and the insulation around it. I bought 3 of the thermometers and found that they give readings within 1 degree of each other at 80 degrees and in boiling water. They respond pretty quickly to temperature changes, as well.

Tom Curlee

Terry

5 degree diff pretty tight! I would set Differential to 15-20Deg. Set Point would depend on terminal units, piping, etc. Typically set around 180F here in the great white north.
I would highly recommend a 4-stage (or 8 Stage if Boilers are HI/LOW fired) Sequencer c/w outdoor reset. Outdoor reset will modify target setpoints according to current outdoor temps (Higher on colder days, Cooler on warmer days, and "warm weather Shutdown")

Hope it helps

Tom Curlee

Aquastat Settings

Thanks, Terry!

15-20 degrees sounds a lot more reasonable to me. In fact, it seems to me that you should must set the DIFFERENTIAL to a value GREATER than the CHANGE IN TEMPERATURE you could see when one or more boilers shut down. Otherwise, the boiler(s) would start right back up. Agree?

Only thing is, I had sort of thought that the DIFFERENCE in SET POINT temperatures between the supply and return aquastats should also be in the 15-20 degree range. Is this reasonable? If so, it's conceivable that the range of the return aquastat OVERLAPS the range of the supply aquastat. Is this acceptable?

Tom Curlee

Unknown

Hey, Tom

Check out this control. I've used it on a pool heater. It's a VERY sensative control.

http://hbctechlit.honeywell.com/techlit/PDF/63-0000s/63-1151.pdf

Noel