old National boiler

Flashv208 · January 25

I have an old boiler. National US Radiator 0-4GW. Looking for manual or drawing to show location of flame sensor. I see one for the pilot flame, but can't locate sensor for main burner flame.

The pilot stays lit, circulating pump is running, main flame shuts off before reaching set temp.

109A_5 · January 25

How old is it ? Pretty old I bet. It may not have a flame sensor like a newer boiler would.

Something in the electrical control may be shutting the burner off.

mattmia2 · January 25

typical gas trains just prove the pilot flame and if the pilot flame is proven turn on the main burner. if it is a total shutoff system it will turn off the gas to the pilot if it doesn't prove the pilot. if it is an older wild pilot system the pilot burner is controlled with a manual valve and it only proves pilot to control the main valve.

can you post some pictures of the gas train?

it could be shutting off on high limit on the aquastat or it could be a problem with proving the flame. if the main valve is opening for part of the cycle it is more likely that some other safety is shutting off the main burner valve although it is possible that whatever proves flame(thermocouple, thermopile, mercury switch, thermostatic switch) is intermittent.

HVACNUT · January 25

How long does the main flame stay on?

Whats the limit setting, and at what temperature is it shutting off?

Are you sure the temperature gauge is correct?

Are there 2 aquastats (high and low)?

Is there a tankless coil?

Captain Who · January 25

So, do older boilers such as this have no blocked vent switch in addition to no spill switch?

Steamhead · January 25

https://forum.heatinghelp.com/discussion/comment/1884022#Comment_1884022

In most cases, yes. I don't think these became standard on residential boilers until at least the late 1980s.

4GenPlumber · January 25

Nationals used to have Baso Switches. The pilot stayed lit, but the baso wouldnt allow the main burner to fire. If it has a Baso switch, reset it. If that doesnt work, replace the thermocouple and try again.

Flashv208 · January 27

so is it a thermocouple or a flame sensor. it is mounted right in front of the pilot flame. sometimes if it didn't turn on main burner, it would if i held a flame to the other side of it. I also removed and tried to clean it. I read they would get a build up of carbon or soot. It does not have a wire attached to it but rather a thin metal flexible tubing

109A_5 · 1:29AM

Sounds like Baso switch scenario. If the thermocouple goes bad or it not correctly orientated into the pilot flame to heat it sufficiently the gas valve will not open.

Pictures would help.

Your system may be like this. Where a bad thermocouple will not shut the pilot off but will lock out the main burner.

4GenPlumber · 2:29AM

It could have a powerpile(wire with 2 leads), a thermoplastic (head of power pile but one bi-metal lead with a nut) or a thermocouple (smaller head bi-metal lead, same as most water heaters). Usually a baso set up has a thermocouple, but it depends on the base. Send pics of what the lead from your "sensor" goes into, and the "sensor" at the pilot assembly.

mattmia2 · 2:52AM

if you take a picture of the gas train we can tell you how it works although it sounds more like it is cycling on the aquastat

109A_5 · 3:59AM

So does the burner cycle as normal just shuts off before reaching the thermostat's set temperature ?

old National boiler

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