HVAC hacking

Usually an NTC resistor. Check the hysteresis. But if this is a "new" effect it may be time to check other things. Such as air ducts and the hysteresis in the room thermostat. A total runtime of 2min is absolutely not normal. Does something blow air from a register directly onto the room thermostat?

And I assume you did check for a plugged filter :-)

If the problem truly boils down to this electronics board the first thing would be to check the elecrolytics on there. Then maybe resolder if the solder joints look suspicious.

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No, we've been living with it for many years, just need to have it fixed eventually.

Clean and good air flow.

Even when the thermostat is set to max (90C).

No, it's on opposite side of the room.

Yes, brand new filters.

Yes, perhaps with new caps.

The sensor on the heat exchanger is an overtemperature safety cutout to protect against a overheat damage when the blower fails. The other one in the air plenum is usually a bimetal snap action, it closes the circuit to the blower when the plenum ait reaches something like 140oF ( for oil) and opens the circuit when temp falls to something like

Then it looks like a hysteresis problem. Or an inferior furnace design where they figured a brute force on/off pulsing is "ok" to regulate the heat exchanger temperature. The only promising way to figure out if they screwed up the design is to find an honest installer and see if they think this is normal for such a furnace. Or call the folks at the manufacturer.

On our central propane furnace the burner runs all the time until the living room thermostat says that the heat demand is satisfied.

If those check out ok verify the resistor values, voltages and the connections around pins 1 and 7 because I believe that's what sets trip level and hysteresis.

On the photo the component side around the leads looks a bit brownish. Maybe they didn't do a good job cleaing the flux off.

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Makes no sense to fiddle with that board until you isolate the problem to it. He needs to look at the W-input coming from the T-stat. If the furnace shuts off while W is still hot then it is the board, if W drops out then it is the T-stat.

"Furnace" implies oil or gas??

When I last had a "furnace" there was a temperature sensor in the bonnet... adjustable bi-metal operating a relay that turned on the blower after the bonnet was warm, also kept the fan running until the bonnet cooled, thus avoiding wasted heat. ...Jim Thompson

Yes, natural gas furnace.

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I got 13V on the right side of the big resistor, but nothing on any terminal. I assume the blue wire (C) is the common. I guess the control pcb is bad.

Thermostat, probably. Some have hysteresis adjustments.

John

T-stat.http://www.furnacebook.com/understanding-thermostats-for-furnace-inst...

There ain't all that much that could go bad on a simple board like this. Much of it could be checked with a simple DVM.

He's checked that, said the short cycling even happens when setting the thermostat to max.

Looks like either something went kaputt on this controller board or maybe the design had been messed up.

Aha! There's also an over-temperature sensor... safety protection against a failed fan or blocked air-flow... if it's bad that will cause short-cycling.

(I know about these things, I was an expert witness against White Controls _many_ years ago... won the case :-) ...Jim Thompson

The way ours (Lennox High Efficiency) works, the thermostat calls for heat or cooling and the furnace (or AC unit) responds. The purge, ignition, convector and main blower fans are controlled by a module. It's like $70-$120 to replace the module, which I did recently (after

circuitry) and have a pressure sensor, electrode type flame sensor, and overtemperature limit switch input, no sensors that are analog in nature. You can get either spark or HSI ignition types.

So far (knock on wood) just the HSI ignitor has gone a couple of times (I keep one on the shelf- first time it inconveniently failed on New Years eve).

You do know that if the electronics in a gas furnace fails in the wrong way (eg. doesn't purge or opens the gas valve before the ignitor fires up) you can create a rather big bang, right? Really old designs used a thermocouple (thermopile, I believe) in the flame that directly held the gas valve open and a continuous pilot flame, but I doubt anyone has done that for many years.

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I suspect the schmitt is there to do a permanent shutdown when the heat exchanger overtemp cutout trips, this is a standard operating procedure, they don't want the furnace to continue cycling on that cutout. So they have it biased to permanently latch the thing off as long as power is applied. The board is fairly simple, you should be able to produce a schematic rather quickly. Then you can go about debugging it. This is probably one of those scenarios where the board (if it is the board) only acts up once it gets hot, so you'll have to run some clip leads into it while it's in place with the cover on. Also, it is not uncommon for something like the relay coil or transformer to go open once the temperature rises and then re-make contact when it falls, but I doubt it would continue like that for years. Yep- C is the transformer common, W is the T-stat rerouting the transformer hot back onto the control board. It should be AC.

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Can't go by that if the thermostat is screwy, it could be doing anything. There's no substitute for making an actual measurement.

South of the border from you it's more like $350-400. BT, but that was with labor, and it was on a Lennox gas pack which is a large outside unit with A/C plus gas furnace in there.

On our Lennox the service guy was surprised when he saw the (dead) fenwal controller in there. "Wow, it has lasted this long?"

Turns out that only a 24V triac was bad but I didn't want to mess with it, especially since propane is kind of unforgiving upon a major screw-up. The other thing that happened was that the exhaust blower shroud corroded out. Of course, they would not sell that shroud alone. No, you had to buy it with a new fan motor, new capacitor and squirrel cage fan. Plus numerous gaskets that cost tens of bucks each and probably they made >1000% profit on each.

Now, having installed a wood stove, we see all this much more relaxed :-)

The mfg would be in for a major lawsuit if that was possible. Not excluding that some might be that way. Ours has a lot of safety interlocks. For examples, AFAIK the gas valve cannot come on unless the exhaust blower has spooled up and the pressure switch related to it has signaled.

years). The module contains some timers (redundant approved safe

It's a 20 years old Payne. I have not been able to find a replacement module.

The ignition sequence seems OK, except for the short cycles.

I am just going to replace all the parts I can find. I found one quote for the CA3098E for 16 pounds (25 USD). It's still cheaper than the $5000 replacement, include duct works.

Well, a thermostat normally just closes a contact electronically. So Linnix could short the respective wires together if he didn't trust the thermostat. Or just see if the 24V are there all the time (I bet it will be).

These controller boards live in a fairly hostile environment. Electrolytics are always a concern out there and often they don't use top of the line caps.

Ouch! But make sure you get it from a reputable dealer and not some scalper that peddles relabeled stuff.

Make sure to also check the NTC or whatever controls this from outside the board.