Honeywell thermostat question

Mike M.

Yesterday I replaced my classic Honeywell T87F with a new Honeywell T8775A (digital round) thermostat in a natural gas, hot water, fin tube baseboard system. It is maintaining set temperature much closer than the T87F ever did but it is running 6-7 short cycles per hour even though the DIP switch is set for the recommended 3 cycles per hour. Does anyone know if this thermostat goes through some sort of "learning" process before it settles down to fewer cycles?

Mike M.

David Sutton_5

Taco Zone heads ?

Mike, are you running zone valves?

Mike M.

Nope...

...just a single zone.

Mike M.

Glenn Harrison_2

Did you check...

to see if it's just the burner that short cycles, but the pump stays running. I'll bet the burner is shutting off due to the boiler getting to max temp but the pump keeps running and the pump only cycles 3 or 4 times an hour.

Just a thought.

Ed T

Honeywell t'stat question

Mike M, I have the same digital round stat on my oil fired boiler with hot water baseboards, and i too am having similar problems. T'stat will call for heat and not even bring loop temp up to satisfy aquastat and system is back off for a little bit. No drafts near stat, false heat, etc.Reminds me of mercury stat with antisipator setting well below amp draw of gas valve or primary control.We may have to install a resistor across w and c terminal to make it compatible with controls ? I too would appreciate any help on situation. Thanks

Ed T

Honeywell t'stat question

> Mike, are you running zone valves?



Mike M, I have the same digital round stat on my oil fired boiler with hot water baseboards, and i too am having similar problems. T'stat will call for heat and not even bring loop temp up to satisfy aquastat and system is back off for a little bit. No drafts near stat, false heat, etc.Reminds me of mercury stat with antisipator setting well below amp draw of gas valve or primary control.We may have to install a resistor across w and c terminal to make it compatible with controls ? I too would appreciate any help on situation. Thanks

Mike M.

No Glenn...

...it's 6-7 calls for heat an hour. The boiler is in my first floor utility room just off the kitchen and I'm very familiar with its cycling characteristics.

Mike M.

Ed_22

T-stat

Heat anticipator on stat and heater calling for the same setting?

tommyoil

did you

have to use the resistor? I know its only for certain applications but could THAT be it?

tommyoil

ALSO

will these new digital t-87 work on an RA117 stack relay? Has anyone heard that they wont???

superMARKet

Power stealer

I would try installing the resistor. Here's why.

The T8775 is a power-stealer. The theory is that whatever control the thermostat is switching doesn't pull enough power through the thermostat for it to steal during the on portion of a cycle. There needs to be at least a half watt or so. Since there's not enough power for the thermostat to steal, it eventually depletes itself and stops working. The heat goes off, and all of the sudden, the thermostat is able to steal power again. The power-stealing circuit is different during the off and on portions of the cycle, so the instant that the thermostat has zonked out and the heat's gone off, it finds that it's able to steal power again and resumes functioning. It has essentially reset itself. It doesn't know that it made an (incomplete) call for heat recently, so it calls again in fairly short order.

If you put the supplied resistor in parallel with the switched load, enough power will flow through the thermostat during the on part of the cycle to keep it running. The resistor is properly sized for the task. Another solution is to put a relay in between the the thermostat and the control it's supposed to switch.

I think this is where David was going when he asked about Taco zone valves. This problem shows up there, but it can also show up in other applicatons.

C-TEC

Short cycling

Did you seal the hole behind the thermostat with silicone? If not, this would allow cool drafts to pass the T-stat wires & enter the stat from inside the wall. Therefore causing the stat to read impropery.

Mike M.

No anticipator...

...on this thermostat. It has a DIP switch to set cycles per hour from 1 (for steam) to 9 (for electric heat). 3 is recommended for hot water and that's where it's set.

Mike M.

Mike M.

Yes...

> Did you seal the hole behind the thermostat with

> silicone? If not, this would allow cool drafts to

> pass the T-stat wires & enter the stat from

> inside the wall. Therefore causing the stat to

> read impropery.

Mike M.

Yes...

...hole is sealed and no other drafts on thermostat.

Mike M.

Mike M.

Didn't use....

...the resistor. It's not called for in my situation. This is on a fairly new Burnham Series 2 boiler. The thermostat is essentially turning on a Honeywell R8285D control center in the boiler.

Mike M.

Mike M.

Okay,,,

...I'll give the resistor a try after work tonight. The display isn't blanking out at any point in the cycle but hey, it's worth a shot.

Mike M.

superMARKet

The display might not go out

I've never touched an 8775, but I have played extensively with 8400s, which are also digital nonprogrammables. When they lose power, the call for heat is cancelled but the display remains on for a little while - maybe 10 minutes or so. During that interval, the buttons don't function, but if you're just looking at the thermostat, it appears to be working.

Even if the display does go out, it would come back on almost instantaneously, as the thermostat is able to steal power during the "off" portion of the cycle.

BillW@honeywell

No, the T8775 does not have...

Adaptive Intelligent Recovery like the better programmables have, so it does not "learn". You don't mention what you are burning, oil or gas. The resistor is used on either the TACO green zone valves or on the oil primary to prevent display blanking due to high current draw. Double check the cycles per hour switch, all stats are factory pre-set a 6 cph. Make sure the switch is in the proper position. You might want to condider the new FocusPro,part number TH5110D1022, it's battery powered, and has virtually none of the compatibility issues that can sometimes arise with power stealing.

Mike M.

And the winner is...

...Mark Mentovai! Installing the resistor did the trick. Even though it wasn't called for, this must have been a borderline case and it made the difference. The boiler's running about 3 cycles per hour now. Thanks to all who responded. This site is great.

Mike M.

superMARKet

Excellent. Have a warm Thanksgiving, Mike!