Hello TFP friends. I have a Hayward H400 Millivolt heater which is about 20 years old. This thing is a dinosaur but when it works properly it’s amazing. I’ve had some issues over the last couple of years where it would take a little bit for the heater to turn on or wouldn’t at all. It’s very finicky. This year, it wouldn’t turn on. My buddy thought it was a bad igniter so I swapped that out. Pilot is lit and I get about 430 Millivolt reading but still won’t fire. I jumped the gas valve and it opens and fires. That’s good news cause that’s probably the most costly part. I tried following the troubleshoot guide but I’m new to jumping and multimeters so I’m not even sure I’m doing anything right. I was getting it to fire jumping the pressure switch so I replaced that. Still nothing. I know the pressure switch works cause when I jump the gas valve and have the heater running, I turned off the pump causing the pressure switch to disengage and it turned off the heater. I tried jumping the thermostat and again not sure I’m doing that right but it wouldn’t fire up. But I do here a click when I turn the thermostat knob so I think that’s working properly. Kind of at a loss now. So here I am. Any help is appreciated.
Hayward H400 Millivolt heater problem
- Thread starter Scottr81
- Start date
It is not likely that it will ever function with only 430mv. A standard pilot generator is rated at 750mv. On a really good day, after the pilot has been lit for a day or two, you might see 700mv. Saw 720 once. Usually they are in the range of 650mv when in use for a while. That's plenty. When you first light the pilot, you have to hold the pilot valve button/plunger down until the output is 100mv, then it will stay open.
Each item in the circuit will cause about a 100mv drop in current and with everything running you should see a reading of 100mv. At 430mv, by the time the system lights and the valve opens the "battery" (pilot generator) is essentially dead, has nothing left to give. A millivolt gas valve of the size a 400K BTU unit has requires about 2-300mv to open the main portion itself, plus the 100mv to keep the pilot lit. Doesn't leave a lot extra and at 430 the heaer won't work, as you have found out.
A generic Robertshaw that matches the Hayward (which is a Robertshaw in a Hayward package) will work fine. Prices for that little item are usually around $40.00. About half that on eBay and Amazon.
CHXGEN1930
Appreciate the tip, but if I swapped out the pilot generator with the part number in the operators manual, why wouldn’t it be producing the proper millivolts? That’s why I’m confused. I didn’t buy an aftermarket part or one that isn’t specific for my heater.
Said you swapped an igniter, not pilot generator. The generator may not be fully in the flame. The pilot-burner orifice may be clogged a bit and not allow a large enough flame to heat the generator enough. With the heater in the on position, even though it may not light, check the mv across the terminals of each device, switch, hi-limits, pressure switch, thermostat. If any read higher than about 100mv it is using too much energy (likely shorted to ground) and can cause the problem.
This is what I replaced. It’s the pilot light plus the thermopile all in one. Sorry if I’m not using the proper terminology. I bought it brand new so shouldn’t it be in the proper position to be fully in the flame? I will check the mv for the other parts tomorrow. I checked each of them for continuity and they all had continuity 