Modine unit heater?

[iceage]

Working on Modine u/h today m/n PA300A, with standing pilot. On a call for heat the unit fires but as soon as the fan kicks on the gas valve looses power and I loose flame. Any after fan shuts down it repeats cycle again, flame on, delay fan on, flame off etc. No schematic so not entirely sure wired correctly.

[monkeyman#1]

How old is this unit. Did you check the heat exchanger? Not familiar with this particular unit, but I wouldn't think there would be that much different about it. Hi temp limit? Stack switch?

[FrostyBeer]

Ok, your losing power to the gas valve. Where are you losing it before the valve?
Does the pilot stay lit?

(rhetorical quesitions)

[Core_d]

Maybe someone come in and wired the fan in series after the transformer and its stealn its juice.

[VTP99]

Check your thermocouple millivolts.

[valdelocc]

Working on Modine u/h today m/n PA300A, with standing pilot. On a call for heat the unit fires but as soon as the fan kicks on the gas valve looses power and I loose flame. Any after fan shuts down it repeats cycle again, flame on, delay fan on, flame off etc. No schematic so not entirely sure wired correctly.

seems like a weak limit, jump the fan control to get the fan to run before the burners fire, see if it runs that way, if it does, fan switch or the limit are the problem.

[Betabass]

I'm thinking the limits are not wired correctly as well.......

[2sac]

There's realy not much to that unit. Should have a maunal reset spill switch, one, possibly 2 limits. One being a 3 wire limit which is where I suspect the problem lies. What was your gas pressure?

[uniservice]

Is this one that has a time delay fan switch wired in parallel with the gas valve? Could be the 24 volt heater on the time delay is dead shorted, and pulling valve off. Does pilot stay lit?

[iceage]

Is this one that has a time delay fan switch wired in parallel with the gas valve? Could be the 24 volt heater on the time delay is dead shorted, and pulling valve off. Does pilot stay lit?

. Yes pilot stays lit

[iceage]

Unit is from 1975, one limit 3 terminals on it, no cracks on exchanger,pilot always stays lit. I have 120v at either side of limit when it opens, longer than usual amount of time before fan comes on after burners, bad delay timer maybe, I was told this particular timer is obsolete and a sequencer would be needed. No schematic but wonder if 120 is normal for these older units at limit. Gas valve 24v

[valdelocc]

Unit is from 1975, one limit 3 terminals on it, no cracks on exchanger,pilot always stays lit. I have 120v at either side of limit when it opens, longer than usual amount of time before fan comes on after burners, bad delay timer maybe, I was told this particular timer is obsolete and a sequencer would be needed. No schematic but wonder if 120 is normal for these older units at limit. Gas valve 24v

Look a little bit closer1975 and no cracks on the HX?, unless they didnt use it for the last 20 years. the limit probably breaks the primary for the 24v transformer.

[Core_d]

Look a little bit closer1975 and no cracks on the HX?, unless they didnt use it for the last 20 years. the limit probably breaks the primary for the 24v transformer.

Prolly cast iron

[MicahWes]

I have a Modine unit that has been heavily used since the late 1970's. The heat exchanger is fine. It is basically just 10-12 oval tubes that the fan blows over.

[valdelocc]

I have a Modine unit that has been heavily used since the late 1970's. The heat exchanger is fine. It is basically just 10-12 oval tubes that the fan blows over.

Well, a furnace in NC runs twice a year

[iceage]

Stupid question but, if limit breaks transformers primary 120, what powers fan?

[Core_d]

Stupid question but, if limit breaks transformers primary 120, what powers fan?

the limit is wired through the tranformers circuit
.

[canusayinsanity]

Sounds a little goofy to me. Usually you see the limits and roll outs wired series in low volt. It may have been Jimmy rigged. Sometimes these modine units,especially older used a gas valve that had thermostat built in with probe and the blower cycled on and off when call for heat without delay off for blower. So basically that switch is probably a limit switch for low volt but somebody tried wiring blower line voltage to it thinking it was auxiliary blower switch.

[Freightshaker]

I had a modine of that vintage and could not get parts from suppliers. I do not know what the issue was but I found some greenhouse websites that had good pictures, schematics and numbers. I pulled the modine part numbers from the site, called my supplier and gave him those numbers and he found the parts. It was like his information did not go back far enough. Those old moodines are tough... I have not seen many cracked compared to other brands of similar vintage.

[Core_d]

Ive seen the limits wired both ways. Either turning off the trans former or just the gas valve, if the limit shuts off the primary side their is usualy a jumper wire from the input of the fan switch to the limit. Shouldnt be to hard to follow the wires. Let us know when u figure it out.