Erratic gas pilot

I have an old gas burner and a digital thermostat. On some mornings it does not come on when heat is called for. I manipulate the thermostat, thinking it may be faulting. I have it set to come on at 4:30am with the temp set for 72. When Its not coming on as it should, the temp of the house says 68. I lower the set temp just below 68 and i hear the click on the thermostat, then i raise it again to 72 and i hear the click again and the heat comes on. I cannot afford to replace this boiler just for that reason. Does anyone know what causes this and how to rectify it, other than buying a new boiler? and possibly the cost. Maybe it needs a new pilot switch? BTW, when its not running with this problem, I do hear a constant running in the background...sounds like a burner running, but there is no flames .
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Boothbay wrote:

When the heat is called for little relay inside the thermostat energize to supply the voltage(usually 24V AC) to the main gas valve at the burner. If you have a multi meter, I'd check the voltage at the valve terminals to see if it is getting the proper voltage.
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It is a bit hard to tell from your post, but does sound like problem is with thermostat. This is not expensive to replace, if necessary. Needless to say, be very careful if you are really talking about about problems with gas burner, and you're not experienced fooling with these. Might be good idea to spend a few hundred on a plumber/ heating contractor, at least call a _knowledgeable_ friend.
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Well I got a plumber and heating guy over and after some testing, he came to the conclusion...it was not the pilot switch or thermostat at fault..it was my low water cutoff gizmo. He banged against it a couple of times and the boiler came on. He suggested 2 remedies. Replace the water cutoff valve..$250 for that and $250 for labor and/or do a chemical cleaning of the boiler for $120. He said he would have to drill a hole in one of the pipes above the boiler and insert some chemical cleaning fluid to scrape out the crud that is at the bottom of the boiler which in turn is making my cutoff valve getting stuck. I never heard of drilling a hole for that purpose. I seem to remember years ago in another house there was a valve nearby on the boiler where you can open it ..aka the radiator valve on a car, and just pour the liquid in.Is there a plumber here that can give me an explanation of the chemical cleaning?
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Don't call him back. Ask around at the local wholesale supply houses for the names of a few of the best boiler service companies. The low water cutoff should never have gotten that bad, and that's not how to service a boiler.
Any boiler needs a yearly service by someone who is experienced and good. A well maintained boiler could last 25+ years without major problems.
(no boiler maintenance is kinda like buying a car and never taking it to a mechanic, just keeping it going with a little of this/that till it dies and then back to the dealer for a new one. I'm NOT saying bad on you! This is just how boilers are generally looked at and have been looked at for years by most people.)
-zero
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