Programmable Thermostat - any tips for gas fired boiler/radiator heating?

I recently installed a programmable thermostat in a house with natural gas fired boiler/radiator heating.

I noticed that the house takes more than an hour to heat up from 17 degrees to 20 degrees. And once the temperature is reached, even though the thermostat shuts off the furnace, the temperature keeps rising to 21+ degrees and takes about 2.5 hours before it starts to cool down. This appears similar to lake effect heating where land close to large bodies of water tend to stay warmer at nights.

Question is, if water takes longer than air to cool down (as it looses BTUs slowly), would it also take more BTUs to heat up? If that's the case, would it be more efficient to simply keep the house at a constant 18.5 degrees 24 hours a day (and using less natural gas to just keep the water warm) instead of changing the temperature from 17 degrees to 20 degrees twice a day where the gas burner runs at 100% for a couple of hours?

Very good question. I've been wondering the same thing with radiant in-floor heating (water-based). I find that it takes even longer, maybe as much as a day to get to the right heat level. But I'm not sure as this is my first winter with it. I don't know the answer to your question - I would suggest doing a little experiment of a week one way and a week the other way, and compare meter reading results.

There is a delay between the thermostat instruct your boiler to start/stop to the actually temperature of your house. You just need to factor in the delay when you program your thermostat.

The delay is due to the fact hydronic heating heats the house then the air not the air which in turn heats the house. your heating the concrete of your home which then radiates it back into the room., very similar to your lake effect heating.

If your house is properly insulated it wont take any more energy and turning down the thermostat will save energy. But it requires some trial and error into getting the timing right as there is over shoot and lag.

The best systems are those with independent zones with independent set points. So you can program the system to heat the kitchen and bath rooms in the morning and leave the den/family room at a lower point.