What can happen if I use a heat cycle of 5 versus 3. I bought two honeywell thermostats. RTH6300b. There is some confusion as to what heat setting to use. I have a new oil furnace, a system 2000 furnace with forced hot water. The thermostat says to use a setting of 5 if I have a gas or oil furnace that is less than 90% efficient, which it is. Setting 3 is used if you have a hot water system or a gas furnace that is greater than 90% efficient. When they say hot water system I am not sure if they mean the baseboard. If that is what they mean then settings 5 and 3 apply to me. What will happen if I use 5 vs 3? What does it mean?:)

By default, the thermostat instructions refer to forced-air heating systems, which are most prevalent nowadays. You say you have an oil furnace with forced hot water - does this mean that the heat in your rooms comes from radiators (either cast iron or at the baseboard) with water circulating through them? If that is the case, you do have a hot water system like they're talking about, and should use setting 3.

Also - the heating device on a hot water system is usually referred to as a boiler instead of a furnace :)

Does heat cycle mean that the furnace will only start that many times in an hour? Would if it is very cold and the furnace calls for heat and it has already started 3 times? Sorry about the terminology, everybody calls our heating devices furnaces!! I do have water circulating through baseboards.

Our thermostat has some dip switches on the back that you can use to select different programs. I'm not sure about yours, but I can tell you what it does on our 'stat.

We have hot water radiators. With these, the radiators continue to throw off heat even after the boiler has stopped (because the water is still warm). To compensate for this, the thermostat shuts the boiler off a little bit early, and the temperature will continue to rise until the water cools down. This requires a different set of programming than using a high-efficiency forced-air furnace, where the temp goes up while the furnace is running, then effectively stops going up when the furnace stops.

You'd have to look at the documentation for your thermostat to determine what the differences between the different programs are.

As far as instructions it only states what setting to put it at depending on the system. I have it set to 3 which runs longer, I guess. The other option is 5 which will keep a steadier temp. but will run more often. I do not know which one will use more oil. On the system that I have the circ pump keeps running even though there is no call for heat. It runs until the water temp comes down to a specifice temp. This way no heat is lost.