Short Cycling - Aquastat problem?

Kevin Walker

I have a 7 year old Weil-McLain WTGO-3 boiler with one baseboard heat loop and 3 in-floor heat loops. The 4 original 3 wire Taco zone valves are in the system. Last fall I installed a Taco SR501 Switching Relay on the baseboard heat loop, a Taco ZVC403 Zone Valve control for in-floor loops, I have replaced the 3 thermostats that I use (the 4th is in the garage, heated by waste heat), and I replaced the Honeywell L8124A Aquastat to try to reduce or eliminate the short cycling problem.

The boiler still short cycles. At the low setting, it will run about 20 seconds every 5 minutes. I can't detect a change in the boiler water temperature. At the high setting, the temperature only changes about 1-3 degrees. Could the aquastat be defective, or installed incorrectly? Is the sensing bulb position critical? Does the boiler have to be drained to change the aquastat? The old aquastat was a L8124A 1155, while the new one is a L8124A 1007.

Thanks for any help, or let me know if you could use more information.

carol_3

I doubt that the L8124 is the reason for your short cycling. That control is designed to control high limit, low limit and circulator. The differential is 10 degrees on the high limit, and adjustable 10-25 degrees on the low limit, so it's not the type of control that should be controlling your everyday boiler operation, and not within a tight temperature range.

The bulb position isn't critical, and certainly wouldn't have the affect you describe.

The boiler doesn't have to be drained if there's a well for the aquastat element to fit into.

Do any of you other folks out there have an idea of what the real cause is?

Rudy

Thanks for the reply, Carol.

I'm not sure I totally understand your response. If the aquastat doesn't control the boiler when the thermostats have called for heat (at the high end), and when the boiler is idle with no calls for heat, but staying warm for domestic water (at the low end) - what is telling the boiler when to start and stop? I'm only getting minimal differential on the high limit, and less on the low limit.

I was hoping that tweaking the sensing bulb would solve the problem.

Rudy

Thanks for the reply, Carol.

I'm not sure I totally understand your response. If the aquastat doesn't control the boiler when the thermostats have called for heat (at the high end), and when the boiler is idle with no calls for heat, but staying warm for domestic water (at the low end) - what is telling the boiler when to start and stop? I'm only getting minimal differential on the high limit, and less on the low limit.

I was hoping that tweaking the sensing bulb would solve the problem.

Unknown

Kevin do you

get domestic hot water from the boiler?

I would also make sure that the low limit is set at least 20 degrees lower than the high limit. A good setting if you have domestic is around 140 to 150 degrees on the low limit. Set the high limit to 180. I assume you have mixing valve for your radiant?

What kind of thermostats do you have? Do they have an anticipator? Are they programmable.

Is the baseboard zone relay a slave to the ZVC Taco panel?

Is there a priority setting in place for your domestic (assuming you get hot water from the boiler or have an indirect)?

Rudy

>Do you get domestic hot water from the boiler?

Yes, it is indirect hot water. There is an electric hot water heater used only for a storage tank, and a separate pump to circulate the domestic hot water through the coils in the boiler. The pump is controlled by the thermostat in the hot water tank.

>
>I would also make sure that the low limit is set at least 20 degrees lower than the high limit. A good setting if you have domestic is around 140 to 150 degrees on the low limit. Set the high limit to 180. I assume you have mixing valve for your radiant?
>
I had the limits set as you suggested until a couple days ago, when I lowered them to about 150 high and 120 low. It seems to work a little better running cooler. Also, I'm going to be gone most of the winter, and plan to keep the house about 55 F while I'm away. I live in Homer, Alaska, where we seldom see temps below zero, and usually in the 20's and 30's. Yes, there is a mixing valve for radiant zones.

>
>What kind of thermostats do you have? Do they have an anticipator? Are they programmable.
>
I have tried several thermostats. A year ago I installed a programmable Honeywell 4730 electronic thermostat controlling the baseboard loop, where we spend 90% of our time. I replaced a standard Honeywell mercury thermostat with an inexpensive White Rogers in the entryway (radiant), and replaced the downstairs zone (radiant) with a standard mercury Homeywell thermostat. They have anticipators, and the anticipators are set at the maximum. I've disconnected the thermostat in the garage, it stays toasty from the waste heat from the boiler and hot water tank.

>
>Is the baseboard zone relay a slave to the ZVC Taco panel?
>
No, they are parallel.

>
>Is there a priority setting in place for your domestic (assuming you get hot water from the boiler or have an indirect)?
>
No, the hot water is the main reason to keep the boiler warm. I'm thinking of disconnecting the pump and letting the water cool off while I'm away this winter.

Unknown

It sounds like

one of the thermostats is causing your problem. Try disconnecting one at a time and see if normal cycle is created with that particular thermostat disconnected. If you get normal cycling on all others then that one needs adjustment or replacement.

Just one more thing you say you are in Alaska, do you have glycol in your system? If so has it been tested recently? Also some glycol mixes can cause bilers to get clogged up and poor temperature sensing to take place.