Thermostat is programed correctly for weekdays and weekends. Sometimes therms says heat but damper does not open and boiler does not come on. If I try to turn it on manually same thing happens nothing. At some point the boiler may come on when it feel like it. What could be the problem? National Grid was called in 5 times last yr could not pinpoint a problem nor could any plumbing professionals??

It is possible your boiler is being controlled by the boiler temperature control device. If that is the case the boiler may not come on when the thermostat calls for heat but the boiler will come on when the temperature of the boiler goes down enough to cause the boiler temperature control to fire the boiler over to replenish the heat that has been removed from the system. Usually under this kind of control the thermostat will bring on the water circulating pump.

Boilers can be wired to run in many different ways and that can create problems for service people especially if they have not done a lot of boiler service.

Running a constant temperature boiler can cost a bit more on energy usage but it is necessary especially if the boiler has a domestic hot water supply coil that heats the hot water for showers etc in the home.

If you could provide an actual wiring diagram with all the controls I can look it over and see if that is exactly what you have. It will take you some time to do the diagram but at least you will have it for future reference. Another thought is to post pictures here of the controls and the brand and part number. It is possible to tell from that also.

Just my opinion, but I don't feel that programmable thermostats work well with hot water heat. I suggest going to a non programmable stat. Hot water heat takes a lot longer to bring up the temp of a house(compaired to forced air). IF you are let the temp of the house drop say five to ten degrees, its going to take awhile to get it back up. I agree with hvac1000, many different settups on boilers may cause confusion for service techs that don't deal with these things on a regular basis. Could they not locate the problem because the boiler was hot, and all lines were circulating as they should? Hot water heat is usually very efficient, but it is important to set the thermosat to desires temp and just leave it there. Other wise it can take hours to raise the temp of a room. Hope this all makes sense. Temp of boiler water, pressure of boiler system, and circulation of boiler water are key in any boiler.

What does work well is a Tekmar control system. The little 256 unit I use here on my hot water system. It modulates the water temp according to the outside temp and is extremely accurate. Tekmark supplied one for a class I gave about 12 years ago and I have used it every year in class to show just what can be done with hot water heat. My system is instantanious as far as the boiler is concerned so I have no standby heat loss and the hot water fan coils act just like a gas fired furnace with the exception that hot water is used instead of a direct fired gas furnace but that little Tekmark is a sweet unit. For many years we used Tekmark equipment to control some huge steam boilers/generators and they never let us down except for the utility bill which always went down after we installed a system. Here is a link to the small unit.

tekmar Boiler Control 256 (http://www.tekmarcontrols.com/prod/256.shtml)