Smart valve issues

kja782

Hello,

I have a rebranded ICP gas furnace (NTC7075BFA3) with a SV9501 smart valve and I'm having an issue with the pilot light proving...only when the furnace hasn't been run for a while. If it's been off for ~2-5h it will start up 50% of the time the pilot will always start up fine and stay on but the main burner won't ignite or if it does it will turn off immediately. When it cools down overnight (20C - 17C temp drop) it will fail to turn on the main burner 99% of the time but the pilot will remain lit. If I decrease the temperature and increase it to call for heat again it will work 100% of the time after the failure. This makes it difficult to diagnose any fix as it takes anywhere from 2 to 8 hours to check if it worked. The pilot will always stat on but the main burner won't, the system will think it's heating and just blow cold air until the temperature is lowered and raised

I have cleaned the flame sensor and the pilot assembly with sand paper. Measured 24V across the igniter when on and 4.3ohm resistance when it's cold. When the furnace is running the VAC from the flame sensor wire to ground is 110V to 108V. If I check for resistance between the flame sensor and groud while it's running the system turns of and will turn on when I stop.

I've ordered a new flame sensor/igniter but I'm not sure this is the problem since it works every time except when it's been off for a while.

Any suggestions?

Thank you

ratio

According to the installation manual (https://www.sidharvey.com/documents/SV9501M2528.pdf), if the pilot is on and the ignitor is off (by the end of the 90 sec trial for ignition), the main valve should be open. If this is not the case, replace the valve.

I suppose there's a slight possibility that low 24V power may cause the valve to misbehave, verify that you have a nominal 24 volts on the power connector while the valve is supposed to be open.

kja782

Hi,

Thank you for the reply. I checked and it's sitting around 27V at all the positions expected on the control connector wires while running and the 28VAC on one pin when off.

When the igniter is coming on to heat (at start up) it only measures 12-13VAC across the two terminals...however it glows red and ignites the pilot pretty quickly.

Most likely a smart valve issue and not an flame sensor igniter problem?

Cheers

ratio

That's my take on it. The fact that the pilot stays on means that it has a valid flame signal. If the flame signal is good, it should eventually open the main valve. The fact that it doesn't open means an internal failure of some kind. You should see 24V on the EFT output (see p. 14 of that PDF) some time after the main valve is supposed to be open, too.

kja782

Is it possible that the flame just isn't proving fast enough to light the main burner and it falls out of the retry loop? I looked at page 14 and measured the 24V hot and 24V common at time of start up and it checks out to be ~27VAC so that seems to suggest it's not a transformer or power issue (along with all the rest measuring 27-28V.

If I follow the flowchart on page 14 that would put me at replace igniter/flame rode assembly. Hopefully that works if not I guess it's the smart valve that needs to be replaced

The EFT output is to trigger the main fan correct? That functions fine as the main fan comes on about a minute after that burner ignites (or doesn't ignite in some cases...).

ratio

If it were losing the flame signal, it would attempt to relight. The fact that it doesn't (ignitor turns on again) says it's still got a valid flame signal. As far as I can see, as long as the giddy-up command is present & the flame signal is present (after try-for-pilot) it should have the main valve open (after the timers, etc. run out).

If you've got a steady pilot that isn't trying to relight, it should fire off.

Assuming, of course, that I've properly understood your description of what's going on.

Sandpaper isn't the best for cleaning a flame rod. I might replace it for that reason, but I still think you've got a bad valve (Protip: use a dollar bill to buff a flame rod. It's about the easiest thing to use.)

kja782

You did understand it correctly. Steady pilot that won't light the burner...only when not run for a while (2-5h 50% chance of lighting, 6-7h 1% chance of lighting) although occasionally (1/5when the furnace fails ) it will light the burner and then the burner will turn off after a couple of seconds and the pilot light will remain on.

Canadian here so no dollar bills! but I hear you, I will use some news paper or something on the new one if it fixes the problem, or if I replace the smart valve I'll still have the new one installed.

Thanks again for all your help

kja782

It may have been my thermostat? I'm not sure, I have a Nest (with a common wire so not a battery issue) After testing everything on the furnace that I could (while I wait for parts) I tried switching the wire from Rh to Rc on the nest base...after this the furnace has started up every time (2 days in a row). I'm not `%100 convinced it's completely fixed but could there be any reason for this to work?

JUGHNE

Nest tstat are always suspect, some work for awhile and then maybe not.
If you have a plain jane stat that uses 2 wires try that first.
Smart valves are quite expensive.

Assuming this is the typical HSI/sensor:

Their HSI combo ignitor and flame sensor only have so many years in them.

Usually do not clean HSI ignitor as they are quite fragile and I would replace that before the valve.
You will eventually need a new HSI/sensor in any case.

Also some Smart valves had connector plugs that were just a little short on wire.
The plug would not seat well.
Have seen a temporary "repair" with wooden match wedged along side the plug to make better contact.

kja782

Thanks!

My HSI/sensor just came in the mail. I'm going to leave the system as is for now and if it stays working for a couple of weeks great! I'll replace the sensor after that to be safe (and it's been a while since a new one has been installed, at least 3 years since I bought the place). If it fails again, the new sensor is going in to check...if it fails after that...I think a new furnace will go in, 25ish years is a good run...and when it gets to -40C I don't want to worry about it anymore.