New old modine PA75AB Questions

[burnedzr2]

I have to change the heater in my shop, the reznor developed a crack in the heat exchanger and was very surprised when flame started coming out the front.

I was able to get a Modine PA75AB Old for sure but for a $100. I can give it a go. So I cleaned it up and figured I would get it wired temporally on the bench to check it out. The first thing I found was it does not have a 24v transformer for the thermostat, but I think I can connect a 24v transformer to the 115v thermostat wires then I can connect to my existing thermosat wire. I found a wiring diagram online for it so I checked it out and everything was good. So I tied the 115v thermostat leads together and plugged it in. The fan came right on? That's strange but on further digging I guess this model does not have a fan delay. I did not connect the gas up yet. I will need a new thermocouple I cracked the line near the gas valve.

(EDIT: This unit has a 115v gas valve....and a 115v thermostat wiring.)

So anyone know more about this heater? Think the 115v to 24v transformer will work right for the thermostat? Is there a way to delay the fan until the unit comes up to temperature or is that just asking to much?

 

[gregtwojeeps]

Am confused. Doesn't the unit have a 24 V. gas valve on it ? Or something else control the gas ?

 

[burnedzr2]

No it has a 115v gas valve on it. Robertshaw gas valve.

 

[gregtwojeeps]

O.K. I am still confused here It has a 120 V fan and I see now a 120 V gas valve. What is/was the low voltage transformer needed for can you tell ?

 

[burnedzr2]

I have the 24v thermostat and wiring already mounted in side of the shop from the Old Reznor heater, The Modine I just picked up is I believe is setup for a 115v thermostat. So I am trying to see if by adding a 115v to 24v transformer to the 115v thermostat wires on the Modine.Would It would work right so I can use the wiring and thermostat in the shop already?

 

[gregtwojeeps]

O.K> gottcha now. So you have a wall mounted LV stat on the wall from a old low voltage controlled unit. So you are not wanting to buy a new 120 V line break single pole T-Sat and change the LV conductors to it..... I am just a dim witted retired sparky and not a HVAC tech but, I believe it will go similar to this. A tech will chime in hopefully later and correct me if need be.. so let me be a introduction to what you may be asking , and let a real HVAC tech be your guide to what to do officially...

Yes, you could do the 120v/24v transformer and use your old LV t-stat and wiring. You would have to install a 24 V coil relay in the unit that will have four poles on it. When the 24 V coil pulls in when the T-stat closes and calls for heat, then the two sets of normally open contacts on your LV coil contactor will pull in... One set of contacts will pull in your gas valve solenoid and the other set of contacts putting power to your fan. ( you may find it easier to just redo the old wall stat and wiring to 120 V ratings)

I think you can also pick up a 120 V time delay relay that will also time delay "on" so your fan will stay off while allowing your combustion chamber to heat up. They also make temperature limiting switches that the sensing probe will sense your chamber temp and its contacts will open/ close on temp fall and rise, making a fan time delay for you. JMO

 

[burnedzr2]

So the 24v thermostat would not work without a contactor? The gas valve will not be changed from 115v so that should not be a issue?

I have looked on the net and have found a limiter and fan delay switch combo that is used on the PA75ab but I have to find the schematic on how it is wired. My unit is a little earlier model before they put the combo switch on.

 

[nehog]

So the 24v thermostat would not work without a contactor? The gas valve will not be changed from 115v so that should not be a issue?...

You'll need a contactor to control the gas valve, and you'll need a second contactor (perhaps) to control the fan. You cannot feed 120 volts into that thermostat.

 

[burnedzr2]

You'll need a contactor to control the gas valve, and you'll need a second contactor (perhaps) to control the fan. You cannot feed 120 volts into that thermostat.

The thermostat in the shop is 24v, I was going to try a 115v to 24v transformer to make that work.

 

[burnedzr2]

So I found out that this heater only has the one safety on it and that is the High limiter switch. So it will go for what I have in it and look for a newer model. In the long run it will be cheaper than trying to add systems to this one.

 

[nehog]

The thermostat in the shop is 24v, I was going to try a 115v to 24v transformer to make that work.

That won't work without at least one contactor. You need a transformer and one or two contactors (relays) to do this. I can do a schematic if you want.

Do you have a schematic of the original heater?

 

[burnedzr2]

Here is a wiring schematic, Upper left hand one is the correct one.

 

[nehog]

Here is a wiring schematic, Upper left hand one is the correct one.

Well, it is simple isn't it?

A transformer, thermostat (your existing one) and a relay/contactor are what's needed. If you want a schematic try this:

 

[gregtwojeeps]

A 24 volt coil relay/contactor I mentioned above that Nehog has now drawn in his schematic.

 

[burnedzr2]

Really simple conversion for the thermostat. Thank you very much I will get on that tomorrow.

Now if I could put a delay on the fan so the fan comes on after the chamber heats up. A fan delay was added a year later model. But I looked all over the net and could not find a wiring diagram.

(thanks nehog!)

 

[burnedzr2]

So that will work in place of 2 separate pieces, nice! I will have to look for that.

(thanks gregtwojeeps)

 

[burnedzr2]

Today I had time to get the unit up and suspended temporally so I could attempt to get it lit. Well after setting it up and putting a new thermocouple in it and propane connected I tried to light it....... and tried, Well I took all the panels off and pulled the pilot out. It is clogged and to top it off the pilot orifice is **** (rusted) in the pilot housing. So I got it out in pieces and of course not supply house in my area carries anything like this. So I looked around on the net and found one close on QC supply. It will bolt on the same way and it is for propane so I will try it unless I come up something else.

 

[burnedzr2]

I picked up a replacement pilot assembly for the heater. I installed it with the new thermocouple, after bleeding the line the pilot lit up and then the main burner, after adjusting the shutter on the burner it burns very nice. Once I get thermostat wired I should be good to go.

 

[gregtwojeeps]

I picked up a replacement pilot assembly for the heater. I installed it with the new thermocouple, after bleeding the line the pilot lit up and then the main burner, after adjusting the shutter on the burner it burns very nice. Once I get thermostat wired I should be good to go.

Good job Just be sure your high temp limit switch is in your wiring circuit so that it will shut the gas valve off, in the event of a overheating situation. JMO

 

[burnedzr2]

I was using the pa75 the other night just to take the chill off and the burner shut off! I quickly shut off the gas line and power. (I worry) I checked out a few things once it was cooled down. I checked the high limit switch first. seemed ok. Next the gas valve, well there is no click when powered up. So I called a guy I know who works at the gas company if he could stop by and take a look. Anyway long story short the gas valve is No good and will need a new one. So I will fully convert it now to 24v setup with the help of Tim. Here is a schematic of what is planned. I cleaned and repainted the heater and it is hanging in the shop so I will just fix it and get some years out of it since it will be almost new..

 

[burnedzr2]

I changed the heater over to 24v system, I was going to buy all the parts but I got lucky and was told about a Pa unit that was taken out of service that was Natural Gas in back of the Gas Company. I asked about it and was told I had to strip it there in the yard and could not remove it. It was a Pa 105 and the controls were what I needed. I took all the wiring, junction box, transformer, high limit and Fan and fan delay switch and the gas valve out.
Funny all the holes were in my unit to fit all the newer parts? Go figure that out. Everything went right in pretty easily. I took the gas valve apart and just cleaned the inside and put it back together, Nice silicone gaskets in it.
Converted the valve to Propane with the help of a gas tech I know, It was easy 6 complete turns in and then he put a gauge on and adjusted it a little more.Now it runs great off the old thermostat. I put a CO detector in the shop just in case.