I've had good luck with this version of Modine heater for 20 years. Went to fire it up today and no dice. Only thing that came alive after a while was the blower that moves the hot air. Thermostat and 24VAC transformer OK. It does not start the exhaust blower first like it always did before thus it doesn't even try to ignite. I'm leaning toward the gas control valve board is out but if there is anything else I should check before I look for a replacement let me know. This is an older model with a simple terminal board (no troubleshooting LEDs on board) but the LED on the gas valve is a solid green which indicates normal behavior. I did have a nearby lightning strike in summer that took out my router and a couple of TVs so I'm leery that this heater has also been affected (even though it was not powered up). Started a search for a new gas valve with prices nearly as much as me buying a new Modine ($1000+). Any good places to look for the HoneyWell control valve that are reasonable in cost? Thanks.
Modine HD45 Hot Dawg troubleshooting
- Thread starter PeterN
- Start date
fitter30
Well-known member
Take a pic of the wiring diagram and do u have a volt meter? Take a good look at the board with a good light look for burn marks or hot spots.
I don't see any circuit board overheating on the Honeywell gas valve circuit board. I've checked the 120VAC in and 24VAC transformer output (including to the gas valve. All is OK. The blower fan works as expected (comes on 30sec after thermostat requests heat and shuts off when thermostat is at requested temp) but its circuit is outside the circuit that controls the exhaust fan and gas valve. I've contacted Modine support but they have yet to get back to me. I'm thinking the gas valve circuit board is bad. Wonder if you can replace that without removing the entire valve assembly.
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JeepJohn62
Well-known member
I replaced the board on mine a few years back. Mine is the HD75 model from 2002. I don't think it was that expensive, The old board had the time delay relays that failed over time. My other issue was that the flame sensor is located in the bottom of the unit. This also lines up with the lowest gas jet which clogs at the bottom of the feed tube. I occasionally have to tap the tube to dislodge debris and allow the bottom jet to fire. The symptom here is it fires briefly and then shuts down.
Just to clarify, did you replace the circuit board on the Honeywell gas valve? If so, where did you find a replacement? The newer models have taken the electronics off the valve and put them on their own circuit board. I have seen those for sale. Mine has a simple terminal connection board where the 120VAC and 24VAC are split up to different locations. My heater is a 1998 model HD45.
fitter30
Well-known member
Replace the hot surface igniter. Most hot igniters look for a set minimum amp draw. Can remove it see if a appliance parts house can match it up. Theres only a few different ones.
Thanks for the suggestion on the surface igniter but the heater doesn't get to that stage on a troubleshooting chart. The "smart" Honeywell gas valve does most of the control and the first thing it should do is start up the inducer (exhaust) fan. That is not happening. Without that occurring the chain of command no longer functions. The valve does have an LED to identify issues but it is telling me (solid green, no flashing) that everything is ok. I'm assuming that something has failed on the circuit board that is bolted to the gas valve. The newer than mine Modine heaters removed the circuit board from the gas valve and created its own control board. I have found a possible replacement Honeywell gas valve but it costs more than if I bought a new complete 50,000 BTU Mr. Heater garage heater.
I'm no HVAC expert but I have checked the 120VAC in to the unit and transformer and gas valve. The transformer is working and outputting 24VAC to the gas valve. There is 120VAC to the gas valve but no output to the draft inducer fan. The only thing that works properly is the circulating fan, which comes on about 30sec after the thermostat calls for heat. It also shuts down properly when the thermostat value is achieved. This fan has no connection to the gas valve circuit and is operated by a simple terminal board and relay. I have attached a wiring diagram to this post earlier in the discussion. I am open to other suggestions to troubleshoot this before I make any changes since I may have missed something. I need to study the wiring diagram and understand how the thermostat call for heat is signaling the gas valve circuit board to power up the inducer draft fan. The thermostat is working properly since the circulating fan is seeing it operating. Any suggestions are welcome.
Wrench97
Well-known member
If the inducer motor does not run it won't open the gas valve.
Make sure the motor can spin it's not unheard of for rodents/birds/insects to find their way in and jam the fan.
Make sure the motor can spin it's not unheard of for rodents/birds/insects to find their way in and jam the fan.
I take it this heater has one of the gas valves with the flame control built into the valve?
On the valve is there a couple connectors with 4-6 wires in each?
**Looked at the diagram and it sure does. I hate those things.
One of the wires in that 8 wire connector should get 24v when you have a call for heat. Definitely need to check that. I can't tell you which wire, could always check each one while someone turns the thermostat on/off
On the valve is there a couple connectors with 4-6 wires in each?
**Looked at the diagram and it sure does. I hate those things.
One of the wires in that 8 wire connector should get 24v when you have a call for heat. Definitely need to check that. I can't tell you which wire, could always check each one while someone turns the thermostat on/off
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Update. I applied external 120VAC to inducer draft fan and it works. The connector on the gas valve does have a constant 24vac on the red wires and 24vac on the orange wire when the thermostat is calling for heat. The blue wire is common. It seems all the inputs to the circuit board on the Honeywell "smart" gas valve are OK, just not getting any output from it. I also noticed the time delay relay on the circulating fan is acting flaky. That part is NLA but there is an updated part that combines the terminal board and the time delay relay. The original gas valve is NLA but there is a replacement that should work for >$500. Terminal board replacement is close to $150. I can get a new Modine for around $1000 or a Mr Heater for $400. Just seems to be more logical to replace the entire unit.
You got it diagnosed. I can't really argue to much with your logic. Hanging heaters are easy to change. No duct to mess with.
I usually fix until there's nothing left to be fixed but I've got a pretty good supply of used HVAC and electrical parts. It's easier to fix when the parts don't cost anything.
I usually fix until there's nothing left to be fixed but I've got a pretty good supply of used HVAC and electrical parts. It's easier to fix when the parts don't cost anything.
I installed a replacement Honeywell smart gas valve and that fixed the problem. Nice to have some wiring diagrams to troubleshoot with. I still need to replace the flaky time delay relay that runs the circulating air fan. Bought a new one off eBay. It is taking a long time to get to me and it's original location was only 50 miles from where I live. I seem to be having trouble with sellers getting the packages out. They create the shipper to make it look like they are sending it but don't promptly take it to the carrier. This relay seller stated same day shipping in their listing but didn't get it to the carrier until 7 days later. Next time I'll buy from a dealer that is local and pay up for that availability. Around here some local dealers won't make retail sails unless they install it.