overshooting thermostat heat set point - Honeywell Pro 4000 & oil furnace

rblakenyc

Background: After many years of fine functioning - this Fall our upstairs heat (on own oil furnace) has been overshooting set point (69 deg) by as much as 4 or 5 deg on an intermittent basis that can last hours (maybe longer) but can also work fine for a day or two (maybe longer).

We set the thermostat on "hold" rather than running a schedule in hopes of isolating the issue - still happens.

There are two zones on this furnace (separate thermostats) with the secondary zone in room over garage set lower (66d) and seeming to work fine even when I swap the thermostats between zones (they snap in and out) - the primary zone overshooting regardless of the thermostat in place.

There is a separate furnace and single thermostat downstairs (big house) that functions normally and still have upstairs issue even when that downstairs unit on hold at 69d.

Based on my inexpert research this seems likely to be a furnace control board issue?

I don't see any obviously loose wires.

Three questions:

• anything else I can try/check? (Assume a new thermostat not worth trying since swapping those on 2 zones didn't fix?)
• will a "standard furnace guy" be able to diagnose
• and is it diagnosable anytime or only when temp is being overshot?

Honeywell PRO 4000 5-2 Day Programmable Thermostat (TH4110D1007)

Oil furnace dates to 1997 and presume control panel does too.

EdTheHeaterMan

EDIT below

Sounds like the zone control has something to do with it. Since you have 2 zones and both thermostats operate properly on the garage zone, and both thermostats have the same issue on the problem zone, there is a strong probability that the zone damper or the zone control panel is the problem.

You will need a higher tier technician that is familiar with ductwork zoning controls to look at the problem

The problem should present at the time when the technician is onsite in order to more easily find the problem.

EDIT:

My first test would be to see if the damper is getting stuck in the open position after the call for heat is completed.   This may leave the end switch from the zone damper to stay engaged for a time period after the thermostat is satisfied.

yellowdog

Certainly sounds like a stuck open damper.

rblakenyc

thank you for the replies/thoughts. had a suspicion that it could have something to do with the zone. will do some research on stick zone damper but appreciate any input on diagnosing a sticking damper... my attic is one of those pretty inaccessible/ hard to navigate ones... no flooring just rafters and insulation so won't be easy to poke around.

rblakenyc

did some preliminary research to better understand dampers prior to climbing up into the inhospitable attic. 2 questions...

. will I need to pull the damper to see if it's stuck or sticking open despite thermostat above setpoint?

. can you clarify earlier reply suggesting if the damper is stuck open "may leave the end switch from the zone damper to stay engaged for a time period after the thermostat is satisfied" - would an engaged end switch keep the furnace running /delivering heat to 70 - 74 degrees (set point at 69) and keep it that high for hours or even days? or does the sustained higher temp suggest it isn't a stuck damper but the control panel?

thanks again!

full earlier comment; "see if the damper is getting stuck in the open position after the call for heat is completed. This may leave the end switch from the zone damper to stay engaged for a time period after the thermostat is satisfied

EdTheHeaterMan

The sequence of operation on a zone damper system.

1. Zone thermostat calls for heat.
2. Zone control sends signal to damper motor to open and or other zone(s) damper to close.
3. Damper opens and the damper end switch makes contact (turns on a switch)
4. End switch (in damper) completes circuit in order to prove damper is open. That signals the panel to operate the heating system.
5. Heating system operates as a result of the end switch signal. (end switch contacts turn on the heater)
6. Heater keeps operating as long as there is a call for heat from the zone thermostat.
7. Call for heat is satisfied. the zone damper motor springs back and the end switch disengages (turn off) the zone panel's signal to the heater
8. Heater turns off

If the end switch does not disengage in step 7 because something is stuck, then the call for heat to the zone panel may continue to operate the heater as long as the damper is stuck open.

There are other zone panels that operate differently. If you can get the part number of the zone panel and the damper part humber, that will help to determine if your panel is the problem or your damper is your problem, or if there is a bare wire that may be the problem. Pictures with the cover off so we can see the wires will also help.