Hayward H200 Miillivolt Not Starting

[Floater22]

I have a Hayward H200 millivolt natural gas heater that isn’t starting. Just replaced the pilot assembly and the pilot lights and stays lit, but the unit won’t fire. Can’t detect any loose connections, and have good water flow. Planning to test switches and sensors with a multimeter tonight.

Any other suggestions?

I have seen a few troubleshooting guides mention when lighting the pilot to hold the button down for up to 2 minutes. Does holding the button a longer amount of time mean anything, other than to be sure it doesn’t go out? Like, as long as it stays lit after being let go and turned to “on”, would it matter if it was held down for 30 seconds vs 1+ minutes prior?

 

[1poolman1]

I have a Hayward H200 millivolt natural gas heater that isn’t starting. Just replaced the pilot assembly and the pilot lights and stays lit, but the unit won’t fire. Can’t detect any loose connections, and have good water flow. Planning to test switches and sensors with a multimeter tonight.

Any other suggestions?

I have seen a few troubleshooting guides mention when lighting the pilot to hold the button down for up to 2 minutes. Does holding the button a longer amount of time mean anything, other than to be sure it doesn’t go out? Like, as long as it stays lit after being let go and turned to “on”, would it matter if it was held down for 30 seconds vs 1+ minutes prior?

First, be sure that the pressure switch is closing when the pump is running. That's the first place to look. If so, proceed to the following.

A millivolt heater operates on just that, thousandths of a volt. The pilot generator is labeled as a 750mV generator (3/4V DC). A really good one, new out of the package will usually, after being heated for a while, get you up to 700mV, usually around 650mV.

It takes at least 100mV to keep the pilot portion of the gas valve open, that's why you have to hold the button, so the generator get hot enough to put out that voltage. Holding the button after that makes no difference.

It then takes about 400-450mV to open the main portion of the gas valve and keep it open. Your generator may just be worn out. You need a millivolt-meter to test the output, but may not have enough voltage.

Replacing the pilot assembly should include replacing the pilot generator as well if it didn't come with the part. If it did, be sure there are no shorted wires, even a tiny one. Pull the terminals off the gas valve and be sure that there is zero corrosion. With such low voltage it doesn't take much to stop voltage from flowing. Do the same with each item in the control circuit, the pressure switch, limit switches, the thermostat.

 

[Floater22]

First, be sure that the pressure switch is closing when the pump is running. That's the first place to look. If so, proceed to the following.

A millivolt heater operates on just that, thousandths of a volt. The pilot generator is labeled as a 750mV generator (3/4V DC). A really good one, new out of the package will usually, after being heated for a while, get you up to 700mV, usually around 650mV.

It takes at least 100mV to keep the pilot portion of the gas valve open, that's why you have to hold the button, so the generator get hot enough to put out that voltage. Holding the button after that makes no difference.

It then takes about 400-450mV to open the main portion of the gas valve and keep it open. Your generator may just be worn out. You need a millivolt-meter to test the output, but may not have enough voltage.

Replacing the pilot assembly should include replacing the pilot generator as well if it didn't come with the part. If it did, be sure there are no shorted wires, even a tiny one. Pull the terminals off the gas valve and be sure that there is zero corrosion. With such low voltage it doesn't take much to stop voltage from flowing. Do the same with each item in the control circuit, the pressure switch, limit switches, the thermostat.

Thanks for your reply. So I did replace the pilot generator assembly. After lighting the pilot, it is putting out approximately 430 and rose to 450MV after a few minutes. I have not checked after a longer amount of time to see if it still rises.

For the pressure switch, I did replace it in 2021, but started checking sensors for continuity. Would it make sense to check the pressure switch for continuity, or no because it is a switch? How do you check to see “if the pressure switch is closing”, as you described?

 

[ajw22]

Would it make sense to check the pressure switch for continuity,

Yes. check it.

How do you check to see “if the pressure switch is closing”, as you described?

Pump on - pressure switch in heater should test closed.

Pump off - pressure switch in heater should test open.

 

[1poolman1]

Thanks for your reply. So I did replace the pilot generator assembly. After lighting the pilot, it is putting out approximately 430 and rose to 450MV after a few minutes. I have not checked after a longer amount of time to see if it still rises.

For the pressure switch, I did replace it in 2021, but started checking sensors for continuity. Would it make sense to check the pressure switch for continuity, or no because it is a switch? How do you check to see “if the pressure switch is closing”, as you described?

450 will not run a millivolt heater. You need at least 600 at a minimum to be safe. It takes 100 to hold the pilot open, 400 to open a gas valve, then there must be at least another 100 "just hanging around" as my first instructor said. Without that 100 reserve, not being used, a millivolt heater will not operate properly.

 

[Floater22]

450 will not run a millivolt heater. You need at least 600 at a minimum to be safe. It takes 100 to hold the pilot open, 400 to open a gas valve, then there must be at least another 100 "just hanging around" as my first instructor said. Without that 100 reserve, not being used, a millivolt heater will not operate properly.

Interesting. I will check it when I get home. Not sure if it would be expected to rise after the initial reading. But the 430 and 450 readings were immediately after lighting the pilot.

If that isn’t normal, any idea what would cause a low power reading off of a brand new pilot and thermopile?

 

[1poolman1]

Interesting. I will check it when I get home. Not sure if it would be expected to rise after the initial reading. But the 430 and 450 readings were immediately after lighting the pilot.

If that isn’t normal, any idea what would cause a low power reading off of a brand new pilot and thermopile?

Sometimes the pilot generator is not completely in the pilot flame. That will not allow it to get hot enough to generate its full potential. Sometimes the part is just bad, but that is rare.
Are you taking the reading with the terminals off the gas valve? It takes someone holding the pilot button down wile the wires are disconnected. Test the voltage right on those wires. By now, if the pilot has been continuously lit, if there isn't at least 600mV on the wires, it won't run the heater. Those numbers I gave are pretty much absolutes. It takes at least 500mV to start and run a heater like yours and there needs to be 100 left over. With the pilot lit and the heater turned "on," test voltage across the mv/pv and mv terminals at the gas valve. If not 400 or so it won't open. Be sure the t'stat is set to its highest temp, the pump running, the pressure switch closed.

 

[Floater22]

Did some more testing…pressure switch, thermostat, and on/off switch all checked out ok.

Still only getting about 400MV to the terminals, and the highest I was able to get directly to the lines was 540, and it still wouldn’t fire. When I turn it to on, and turn the thermostat, it’s drawing some power.

So, I guess the big question is, why is a brand new pilot assembly (sparker, pilot orifice, and thermopile) not generating more than 540MV?