Ask Me Help Desk - A puzzle for the pro's

A puzzle for the pro's

OK here goes. This week I installed a 3 zone system on an existing furnace using a Robertshaw Simline Classic zone control(I've used this dozens of times with great success).
All went well until the customer called me and said that the furnace was overshooting the t-stat by 2 or 3 degrees.This was unusual. I spent a couple of hrs observing and was surprised to see a 4-5 min fire after the t-stat was satisfied.(I jumped R & W, fired the burners, removed the jumper... gasvalve remained open). I called York tech support and was told that it was either a sticky gas valve or faulty board. Tested the gasvalve, OK. Board is doing everything as asked so it seems OK.(I don't always trust Tech support but that's another story)

After further online research I fount this info @ Johnston controls{York}:

"York MODELS: PC9*DH
UNLIKE CONVENTIONAL SYSTEMS, THE
WALL THERMOSTAT DOES NOT SIMPLY TURN THE FURNACE ON
AND OFF. THE FURNACE CONTROL CALCULATES THE DEMAND
AND MAY CONTINUE TO FIRE THE FURNACE DURING PORTIONS
OF THE THERMOSTAT "OFF" CYCLE"

I said to myself "what the heck" now I have a perfectly good zone system and a perfectly good furnace that won't co-operate.

Any suggestions as to how to remedy the problem?

What stat? Post link of manual?

Can you adjust cycles per hour?

There may be other options too.

http://hvactroubleshooters.com/YORK/...20FURNACES.pdf

3 stats... all Honeywell pro 4000
http://www.forwardthinking.honeywell...ll/69_1770.pdf

Robtshaw Zone
http://www.robertshawtstats.com/imag.../150-1575F.pdf

Lower CPH. Can't tell efficiency of furnace. CPH acts like the "anticipator" of yesteryear.

Any idea what it's set at?

Efficiency of furnace?

The York furnace is a 96%. I'm not sure what you mean by "CPH"
The only on board adjustments are for fan speed and such.

The t-stats have no anticipator but they have no effect on the operation. Even if I eliminate the stats, the problem continues.
It seems like a simple problem but because the operation is controlled by the board, I can't see an obvious fix. I told the boss that with a 24v relay I could make the gas valve close when I want it to but that's a last resort.

Have you replaced/tested the thermister/temp sensor probe?

If you haven't seen it already, here's this: http://www.yorkupg.com/PDFFiles/246815-YTG-D-0108.pdf
And this: http://www.yorkupg.com/PDFFiles/292307-UUM-A-0108.pdf
You may have to contact the manufacturer's tech support. It sounds as if this furnace was designed to have a mind of it's own, and ignore the thermostat's demands under certain conditions.

CPH = Cycles per hour. It's adjustable in the install guide.

Page 8 of the install guide option #5 (CPH).

http://www.forwardthinking.honeywell...ll/69_1849.pdf

It should be set to 3 and not 5 which is the default value for a furnace >90% efficiency. That will make it overshoot.

Quote:

Originally Posted by lil joe

With a 24v relay I could make the gas valve close when I want it to but thats a last resort.

Out of the frying pan, into the fire. This could cause the furnace to experience nuisance lock-outs, as flame may not be sensed when the furnace is trying to operate. I hate to be the one to say it, but the zoning system you have installed may not be compatible with this furnace. :(

I would isolate the zone system which signals the furnace to cycle on and off, and see if it operates normally by jumpering r to w at the furnace to determine if it is a furnace issue or a zone system issue..

Re-read 1st post.. sounds like the furnace has a recovery algorythm built into circuit board and or an out door air temp sensor? Thought only t-stat contollers did this..

@ K.I.S.S... I thought you were referring to a furnace setting. It is set @ 3. I tried 3 non digital stats just for kicks and got the same result.

@ KC13... I forgot the smiley face at the end of my sentence. That's the kind of stuff we did waaay back.
I think you may be right about compatibility though. Tech support is off till Tuesday. I'll follow up with them.

Quote:

Originally Posted by johnair77

re-read 1st post..sounds like the furnace has a recovery algorythm built into circuit board and or an out door air temp sensor?? thought only t-stat contollers did this..

Me too. Guess not.

The new breed of York systems is a li'l different than most I've seen. The high-end heat pumps have the hybrid/dual-fuel management controls built-in. The thermostat connects directly to the HP, which in turn manages the gas furnace. I've come across a few that weren't wired correctly, 'cause everybody knows installers can't read! :p The heat pumps sure are noisy, when running at the same time as the gas furnace...

I've come across a few that weren't wired correctly, 'cause everybody knows installers can't read!

I do 90% retrofits and service calls. Quite often the simplest way to fix someone else's mess is to start fresh. The problem starts when I'm the guy who made the mess in the first place.;)