Oversized boiler - best aquastat setting?

joebloggz

I have an old 3 story brick building with limited insulation. Approx 3000 sq ft in total.

- The boiler size is 225k. It's about 10-15 years old. Most likely oversized.
- there are 3 total loops. Each loop is a two pipe (approx 2"-3" each) reverse return system
- I measured the supply pipe 131F and the return pipes at 123F. All 3 loops gave about the same readings.
- Rooms have Cast iron radiators of various sizes
- The current aquastat setting (Honeywell high limit L8148E) is 160F.
- The thermostat has 3 remote sensors - it is reading the 3rd floor sensor, which is the coldest room, set at 72F.
- I limited the thermostat to 3 cycles/hour.

I measured the supply pipe temperature by touching a probe onto the pipe.

Should I increase the aquastat to 180/190?
Would a higher aquastat setting lower the heating bill?
Would a higher aquastat setting create a more comfortable temperature?
Also would appreciate your feedback on "short cycling" and "flue condensation", which I don't know too much about.

GBart

You don't know if it's over sized unless you do a load calculation on the building.

You need a pro to come out and check it and install an Outdoor Reset control that will automatically alter the boiler temp based on the outdoor temperature and you'll save tons of energy. They will also be able to ascertain if the unit is oversized.

joebloggz

Its oversized. For sure.

Thanks for the outdoor reset control idea. Currently, I have a high-limit aquastat set at 160. The question is whether it should be increased to 180/190.

John Mills_5

Higher water temp seems to me would raise gas usage. Keeping it as cool as possible without condensing the flue gases will lower it. Just make sure return water doesn't drop below 135. Since it has cast iron radiation, do you know if it has some way to protect itself since on cool & cold starts, it takes quite a while to get the return above 135 and that's hard on the boiler & the chimney.

EBEBRATT-Ed

Water temperature is determined by the heating load and the size of the radiation. If the limit control only shuts off the boiler on high temp and the boiler is actually run by the thermostat as it's operating control then set the high limit at 190. In that case setting the limit too low will cause short cycling and burn more fuel.

joebloggz

@EBEBRATT-Ed
I have programmed my thermostat to allow only 3 cycles per hour. Will this prevent short cycling?

If this protects me from short cycling, can I keep it at the lower aquastat setting (160F)?

My manual:

Cycle rate limits the maximum number of
times the system can cycle in a 1 hour period
measured at a 50% load. For example, when
set to 3 CPH, at a 50% load, the most the
system will cycle is 3 times per hour (10
minutes on, 10 minutes off). The system
cycles less often when load conditions are
less than or greater than a 50% load. The
recommended cycle rate settings are below
for each heating equipment type:

Standard Efficiency Gas Forced Air = 5 CPH
High Efficiency Gas Forced Air = 3 CPH
Oil Forced Air = 5 CPH
Electric Forced Air = 9 CPH
Hot Water Fan Coil = 3 CPH
Hot Water Radiant Heat = 3 CPH
Geothermal Radiant Heat = 3 CPH
Steam = 1 CPH
Gravity = 1 CPH

joebloggz

John Mills_5 said:

Higher water temp seems to me would raise gas usage. Keeping it as cool as possible without condensing the flue gases will lower it. Just make sure return water doesn't drop below 135. Since it has cast iron radiation, do you know if it has some way to protect itself since on cool & cold starts, it takes quite a while to get the return above 135 and that's hard on the boiler & the chimney.

@John Mills_5 What are some signs of flue condensation? There does not appear to be any rust on the metal flue..the flue ducts themselves are much hotter than the hot water supply/return pipes.