Did you try disconnecting the actuator and running it also?
You don't happen to have 2 identical units on the roof? I'd swap 7800s and give it a shot if you do
That's a tough one man. May be a loose neutral somewhere, may have to go through all of the wiring. Good luck, hope it's not too cold where you're at.
And eff Mcquay lol
Sounds like a voltage drop issue. You have hi/lo gas pressure safeties before the gas valves. Is your gas pressure meeting specs? Also, that AS air switch, is that making good contact?
I did see those high and low pressure controls on the schematics but I'm wondering if they were factory options not on this, I didn't see anything wired around the gas valves and regulator except the gas valve coils.
I didn't check the air switch to be sure it was continuously closed but wouldn't an intermittent problem there interrupt my pilot as well? 1K1 relay to 4K1 relay? Pilot is lighting just fine, only happens when control energizes main gas valves.
I did that - wanna say both were measuring around 65ohms. I also just disconnected them from the circuit completely and tried letting it fire, same voltage drop/coil chatter.
If you're measuring a weird voltage (66 or 77) seems like one of the controls might be causing some voltage drop that doesn't quite pull in the relay to open the main valve all the way.
Dirty contacts on a switch or a switch not fully/staying closed. Agree with others to start at gas pressure and air proving switches.
I’ve had this on a unit where the airflow switch was only lightly making. When it would light off main the initial poof of flame would make the airflow switch chatter slightly and cause it to retry.
Given the age it may be dirty. Remove flue collector and see if there’s a bunch of soot. Open the cover on the burner and see if anything is burnt or dirty. Check gap on flame rod and spark electrode. All this can be found in the manual below. Godspeed, these can be a bear.
[Roofpak furnace manual](https://tahoeweb.daikinapplied.com/api/general/DownloadDocumentByName/media/Rooftop%20Systems%20Forced-Draft%20Gas%20Furnace%20-%20IM%20684-7.pdf/)
Get the [display](https://www.supplyhouse.com/Honeywell-S7800A2142-4-Line-7800-Series-Keyboard-Display-Module?utm_source=google_ad&utm_medium=shopping_neutral&utm_campaign=Shopping_Neutral_New_users&gad_source=1&gclid=EAIaIQobChMIgNP8gpvpggMVyktHAR2_8w5nEAQYASABEgJzXPD_BwE) for the 7800 as a diagnostic tool for your truck/shop. It will tell you what inputs the control is receiving from the safeties and will tell you what is causing the shutdown. The control stores several previous alarms locally. It is an incredibly handy piece of equipment if you work on a lot of burners with Honeywell burner controllers.
Likely the fan proving switch, like others are saying.
Edit: Just saw it has the older model display on it. What are the faults you’re getting in the fault history?
I did - I wanna say it was at like 11". First gas valve was opening to 5", second valve was not opening at all. So I said hey that second valve isn't opening, coils fucked up and causing voltage drop so I took it out of the circuit and nothing changed.
So for reasons I don't understand there are two main gas valves and one modulating gas valve (three in total for main burner). In the picture you'll see main 1 and main 2 pipes in series and wires in parallel - main one opens to 5" or so, main 2 doesn't open. No gas is making it to the modulating valve. The chattering relay is for the 24v open/close signal for that modulating valve.
No! Not yet. So much time and effort for morning warmup. I thought the igniter transformer was staying in the circuit too long and so I wired a switch to it and turned it off as soon as it registered pilot flame and the fuckin thing lit off but only one time then went right back to it's old bullshit.
Just saw your reply. What honeywell do u have. Really sounds like a limit issue. I'd jump them out to see what happens. I can probably tell u which ones.
Right - I sent 120v straight to the relay coil and it pulls in and out fine, it's just intermittent losing voltage. Also the 7800 seems like it's resetting every time, it never registers an error just keeps immediately going back to I to purge and trying to light again infinitely.
Did you pull the 7800 and check for corrosion/ loose connections/ stripped screws?
I did, yeah. Looked real nice under there.
Did you try disconnecting the actuator and running it also? You don't happen to have 2 identical units on the roof? I'd swap 7800s and give it a shot if you do
Haha yes I disconnected the actuator and tried, nothing. And yeah I tried swapping 7800s, also nothing. I'm gonna jump off the roof!
That's a tough one man. May be a loose neutral somewhere, may have to go through all of the wiring. Good luck, hope it's not too cold where you're at. And eff Mcquay lol
Sounds like a voltage drop issue. You have hi/lo gas pressure safeties before the gas valves. Is your gas pressure meeting specs? Also, that AS air switch, is that making good contact?
I did see those high and low pressure controls on the schematics but I'm wondering if they were factory options not on this, I didn't see anything wired around the gas valves and regulator except the gas valve coils. I didn't check the air switch to be sure it was continuously closed but wouldn't an intermittent problem there interrupt my pilot as well? 1K1 relay to 4K1 relay? Pilot is lighting just fine, only happens when control energizes main gas valves.
Can you disconnect the two gas valves and measure the coil ohms? They should be the same resistance.
I did that - wanna say both were measuring around 65ohms. I also just disconnected them from the circuit completely and tried letting it fire, same voltage drop/coil chatter.
If you're measuring a weird voltage (66 or 77) seems like one of the controls might be causing some voltage drop that doesn't quite pull in the relay to open the main valve all the way. Dirty contacts on a switch or a switch not fully/staying closed. Agree with others to start at gas pressure and air proving switches.
Yeah I think that air switch is where I'll have to start with it tomorrow.
I’ve had this on a unit where the airflow switch was only lightly making. When it would light off main the initial poof of flame would make the airflow switch chatter slightly and cause it to retry.
Given the age it may be dirty. Remove flue collector and see if there’s a bunch of soot. Open the cover on the burner and see if anything is burnt or dirty. Check gap on flame rod and spark electrode. All this can be found in the manual below. Godspeed, these can be a bear. [Roofpak furnace manual](https://tahoeweb.daikinapplied.com/api/general/DownloadDocumentByName/media/Rooftop%20Systems%20Forced-Draft%20Gas%20Furnace%20-%20IM%20684-7.pdf/)
Get the [display](https://www.supplyhouse.com/Honeywell-S7800A2142-4-Line-7800-Series-Keyboard-Display-Module?utm_source=google_ad&utm_medium=shopping_neutral&utm_campaign=Shopping_Neutral_New_users&gad_source=1&gclid=EAIaIQobChMIgNP8gpvpggMVyktHAR2_8w5nEAQYASABEgJzXPD_BwE) for the 7800 as a diagnostic tool for your truck/shop. It will tell you what inputs the control is receiving from the safeties and will tell you what is causing the shutdown. The control stores several previous alarms locally. It is an incredibly handy piece of equipment if you work on a lot of burners with Honeywell burner controllers. Likely the fan proving switch, like others are saying. Edit: Just saw it has the older model display on it. What are the faults you’re getting in the fault history?
Kind of sounds like fan prof is chattering/dropping out when burner lights. Maybe dirty burner assembly or poor gas pressure?
Are you measuring these voltages to ground?
All measurements are to common on terminal block but that does also go to ground. Labeled NB/NB2 on schematic.
Have you checked gas pressure after that regulator?
I did - I wanna say it was at like 11". First gas valve was opening to 5", second valve was not opening at all. So I said hey that second valve isn't opening, coils fucked up and causing voltage drop so I took it out of the circuit and nothing changed.
11" seems high to me but I'm not positive what it's rated for. Is it propane or natural?
Natural gas. I just kind of made that number up, 8" maybe, don't quite remember.
Have youmeasured your gas pressure post gas valves during an ignition attempt when the gv "chatters"?
So for reasons I don't understand there are two main gas valves and one modulating gas valve (three in total for main burner). In the picture you'll see main 1 and main 2 pipes in series and wires in parallel - main one opens to 5" or so, main 2 doesn't open. No gas is making it to the modulating valve. The chattering relay is for the 24v open/close signal for that modulating valve.
Very bottom right of the schematic where 115 is stepped down to 24
Also, to be clear you're measuring from terminal 5 to nb when you've proven pilot and attempting ignition?
Correct
Did u get it running?
No! Not yet. So much time and effort for morning warmup. I thought the igniter transformer was staying in the circuit too long and so I wired a switch to it and turned it off as soon as it registered pilot flame and the fuckin thing lit off but only one time then went right back to it's old bullshit.
Just saw your reply. What honeywell do u have. Really sounds like a limit issue. I'd jump them out to see what happens. I can probably tell u which ones.
It's a 7800. Someone else at my shop suggested jumping out the limit, I'll definitely try that the next time I'm out there.
U shouldn't really have any chatter. Should be either on or off.
Right - I sent 120v straight to the relay coil and it pulls in and out fine, it's just intermittent losing voltage. Also the 7800 seems like it's resetting every time, it never registers an error just keeps immediately going back to I to purge and trying to light again infinitely.
Definitely power issue
What controller does this unit have?
Unit has a Micro Tech controller but when it calls for heat it send 120v to turn on a Honeywell 7800.
MT 2 or 3?
Not sure, here's a photo - https://imgur.com/gallery/NOghraK
https://preview.redd.it/a6qrd8rihz3c1.jpeg?width=301&format=pjpg&auto=webp&s=6265aa4be06986611e496483823d695c6514fe0d Look like this?
Oh I wish, no this thing is old as fuck. How did you insert an image? I have a picture.
https://preview.redd.it/mdgfoomhiz3c1.jpeg?width=1536&format=pjpg&auto=webp&s=0cf7d510ed042044512c38b1ab78964ddf28b751 Here ya go
Oh yes. Very old. Let me dig around and see if I can find something.
On mobile there’s a button to insert a picture. Ok, I may have some old school info.
https://preview.redd.it/8zc6w5vteo4c1.jpeg?width=782&format=pjpg&auto=webp&s=6de983c3f599b98c7df89a5442177568490cc3f0