Need diagnosis help with Unit Heater

[Ken Smith]

HI all,
I have rebuilt a nat. gas unit heater for my shop. There are no identifying labels so I don't know the brand. It could be 20+ years old, so is very basic.
I have replaced all gas components; main valve, pilot & thermocouple. I have completely rewired the unit and replaced the only 2 sensors; high limit and fan switch. With final installation done, I fired up the unit - pilot is perfect, thermostat calls for heat and the burner lights perfectly. When the unit comes to temperature, the fan comes on as it should.
THE PROBLEM - the fan runs for 10 seconds and shuts off. After 25 seconds, the fan comes on again. It continues to cycle like this but the burner never shuts off. I replaced the fan switch again - same result. I can't find another component that controls the fan except this fan sensor (F140 to 120).
I would appreciate any suggestion that will resolve this problem. I don't expect the unit to provide adequate heat if it only runs 20 seconds out of each minute - 10 seconds at a time.
Keep in mind that all parts are new and everything works perfectly except the cycling of the fan.
It is supposed to snow again tonight - I'd really like to have some heat!
Thanks for your help,
Smitty

 

[ctfjr]

It sounds like you have isolated the problem (the fan switch). Does it mount against the heat exchanger? Did you replace it with a higher temp unit? Is it adjustable? Does it have an adjustable 'deadband' setting (differential, where it might turn on at "X" degrees and has to drop through the differential before it turns off)?

 

[Ken Smith]

To ctfjr,
The "Therm-O-Disc" sensor was so old that we had trouble cross referencing a replacement. My local HVAC parts house contact was able to find a new sensor, which I installed. It mounts in the housing above the heat exchanger compartment and protrudes through the normal round hole. It is rated as an F-140 like the original but indicates that it opens at 120 degrees. It is not adjustable. When the fan started cycling as it did with the old sensor, I replaced it with the original sensor and got the same result. Since I had the unit apart, I thought it a good time to replace all the parts with new ones. In this case, both the old and new sensors give the same results.
Smitty

 

[Rookie2]

Did you try directly powering the fan bypassing the fan sensor ? Did you adjust the gas pressure to the burners after replacing the gas valve ?

 

[RECox286]

Sounds to me as if the clicks-on is activating in too high

a range, I would try again with a sensor that activates

at a lower setting. Ask the parts counter guy what ranges

of sensors the store stocks...I should think one in the 120*

range would be better suited, or IF an adjustable sensor is

available, that would be my choice.

Uncle Bob

 

[ctfjr]

One other thought. It sounds as though the temp of the heat exchanger is cooling down too fast. Did you by any chance increase the speed of the blower motor?

 

[LS6 Tommy]

One other thought. It sounds as though the temp of the heat exchanger is cooling down too fast. Did you by any chance increase the speed of the blower motor?

X2. Wrong fan motor or low gas pressure.

Tommy

 

[Ken Smith]

Hi Rookie2,
I'm making a list of things to try - clipping around the fan switch will be first.
I can't compare to the past since I never saw the unit in operation before I became the "proud owner" and have had to replace all the parts. Gas pressure is to spec but I am curious why you asked.
Thanks,
Smitty

 

[Ken Smith]

Hi ctfjr,
The fan rotated freely so the only thing I did not change is the motor. It is quiet and smooth. There is heat coming out but it is not excessively hot. If it would run to satisfy the thermostat and not cut off after 10 seconds, I think it would do the job.
Smitty

 

[acmikee]

check your blower rotation. also what is your temp differential inlet and outlet.
what is your gas pressure

 

[Rookie2]

Hi Rookie2,
I'm making a list of things to try - clipping around the fan switch will be first.
I can't compare to the past since I never saw the unit in operation before I became the "proud owner" and have had to replace all the parts. Gas pressure is to spec but I am curious why you asked.
Thanks,
Smitty

Inlet gas pressure is not the same as burner pressure. If the burner pressure is low then the flame is low and the fan switch may be cycling. I'm not an expert so I won't tell you how to adjust it so you don't burn the place down.

procedure may be in the second page below. (not knowing what unit you have)

www.modine.com/download/m3k97mur1ojs6b8mgghatfhjq2/75-551.pdf

www.h-mac.com/product-catalogs/modine/Modine-PDP-Catalog.pdf

 

[brewchief]

What's the temp in the shop? If it's cold enough the fan will keep shutting off from the cold air cooling the fan switch right back down.

How long have you let it cycle on and off?

 

[LS6 Tommy]

What's the temp in the shop? If it's cold enough the fan will keep shutting off from the cold air cooling the fan switch right back down.

How long have you let it cycle on and off?

Low ambient temp does not make the fan switch short cycle. It would just delay the initial on time.

Tommy

 

[Ken Smith]

Temp has varied from 35 to 55. I've let it run, cycling, for about 5 minutes.

 

[zmaxmotorsports]

The switch you wired it through is the wrong one. Can yost a picture of what youve got?

 

[zmaxmotorsports]

If you dont have a board and it has a standing pilot and thermal couple Id say you need an old style limit switch like this,But I need a picture of your furnace wiring/burners to be sure.
http://www.diychatroom.com/attachments/f17/14475d1256774931-fan-limit-switch-help-006.jpg

 

[ambenz]

I was actually thinking the thermo protect inside the motor was tripping because the motor was never changed. But the OP claims it runs quiet and smooth....
I would put a amp meter between the power to the motor to verify the power is cutting out to the motor and eliminate the possibility that the motors thermo protect isn't tripping out internally inside the motor.

 

[Ken Smith]

I will get some pics and post them tonight or in the morning.
This is an old unit,, very basic. No boards, relays, etc. There are 2 sensors, a fan switch (F-140) and a high limit. There is a common line voltage feed to each with the fan switch feeding the fan motor and the high limit feeding the transformer to the gas valve. I have maintained wiring so the fan continues to run after the thermostat is satisfied and shuts off the gas feed. It has a standing pilot and every aspect of the set up works perfectly except for the cycling of the fan. Someone suggested that I wire around the fan switch to test the motor, which I haven't had time to do yet. I can't figure out what else would control the motor other than the fan switch. It doesn't run long enough to cool the unit below 120, which is the "open" temp for the fan switch or satisfy the thermostat. The only part I did not replace is the fan motor, so that needs to be tested. I traced the wiring for the third time to make sure the fan switch feeds the motor and the high limit feeds the transformer. I need to adjust the pilot down a bit but all the gas components work like they should and it produces good heat. The fan just doesn't run long enough to get the heat out into the shop. I'll get back to you with the wiring diagram I made and some pics.
I appreciate your interest.
Smitty

 

[Ken Smith]

I believe you are on to something. The fan is the only thing I didn't replace so it needs to be tested.

 

[ctfjr]

Low ambient temp does not make the fan switch short cycle. It would just delay the initial on time.

Tommy

What's the temp in the shop? If it's cold enough the fan will keep shutting off from the cold air cooling the fan switch right back down.

How long have you let it cycle on and off?

I think you are onto something there brewchief. Very cold air returning to the UH will drag down the exchanger temperature, right through the deadband and turn off the fan control. Its an easy test to see. Simply jump the fan control so the fan runs continuously. Set your thermostat for whatever temp you are looking for & let it rip. After the ambient room temp comes up to a 'normal' level (say 50 deg or so) you can shut it off, disconnect the jumper and try operating it again.

 

[csp]

Low ambient temp does not make the fan switch short cycle. It would just delay the initial on time.

Tommy

If the ambient is so cold that the heat exchanger cools off faster than it makes heat it could definitely short cycle the fan for a few cycles. My dad's 100btu Reznor has done this a couple of times.

Being in the same area that the OP is in I can say that's not the case with the ambient temps being what they are currently, but if it were 10 below it very well could.

 

[James-W]

My suspicion is that the burner is not giving off enough heat, that is to say, it's getting hot enough. This could be caused by low gas pressure, a partially clogged orifice, a kinked gas line, etc.

 

[LS6 Tommy]

If the ambient is so cold that the heat exchanger cools off faster than it makes heat it could definitely short cycle the fan for a few cycles. My dad's 100btu Reznor has done this a couple of times.

Being in the same area that the OP is in I can say that's not the case with the ambient temps being what they are currently, but if it were 10 below it very well could.

I guess if your climate is THAT cold it could happen. I've never seen it here in NJ...



Tommy

 

[zmaxmotorsports]

Still waiting on a picture here.

 

[Ken Smith]

XMax,
Sorry for the delay. I'll attempt to attach 2 pics. I wired around the fan switch. You can see the "clippy" in the pic. Fan runs fine, no cycling. Both fan and limit switches are correct and connected to the fan and transformer, respectively. Fan and motor were untouched, rotate appropriately. I don't see a thermal switch on the fan. I'm at a lost.
Smitty

 

[zmaxmotorsports]

XMax,
Sorry for the delay. I'll attempt to attach 2 pics. I wired around the fan switch. You can see the "clippy" in the pic. Fan runs fine, no cycling. Both fan and limit switches are correct and connected to the fan and transformer, respectively. Fan and motor were untouched, rotate appropriately. I don't see a thermal switch on the fan. I'm at a lost.
Smitty

Thats not the kind of heater I was exprcting,Disregard the picture of old style limit switch I posted.
This is exactly why pictures are so important.

 

[Rookie2]

Be nice to see the gas valve you changed . With the fan switch bypassed ,does the unit keep the room warm ?

 

[BD1]

We all try to safe money but a buying a NEW one is what I would have done. You gotta have lots of $$ in that already for the parts. I wonder what shape the heat exchanger is in ? I would also have a carbon monoxide detector with that heater.

 

[Ken Smith]

"Worth a thousand words"! Given the info you have - if this was your problem, what would you do next?

 

[Ken Smith]

I've had too busy a week. I'll get more pics tonight and post them.
I only bypassed the fan to check function - not with the burner on.
It would warm the shop fine if it stayed on. I'll have more time tonight and will start the burner and keep the fan bypassed to see what happens.
Let you know later.