The general feeling I have seen about radiant set-back is not to do it. One of the reasons is the delay when using high thermal mass emitters (e.g., concrete floor). Does your room actually get much cooler before the system needs to start heating again? It can also lead to overshoot when it is reheating the thermal mass. The degree (sorry) of success is also Dependant on the intelligence of the thermostat.
The physics is that if the room is cooler, less heat will be lost to the outside across the insulation layer. With forced air, the responsiveness makes the effort profitable. For a conventional heat source not using power vented combustion air, a longer cycle on return from reset will probably make that period of operation a bit more efficient.
My guess is that if you were using a modcon, you may cause the boiler to run at higher temps and may actually reduce burn efficiency a bit while bringing the mass back up to temp.
It is another one of these hard to quantify numbers. It involves interaction of complex factors. Mass, thermal loss characteristics, comfort, boiler performance characteristics. There are complex modeling systems that can evaluate these things and calculate the expected results. You are more likely to use a thermometer, wet finger, walking barefoot in your skivvies, and reading your fuel bills. Less scientific, but the results should be good. But not as quick as a simulation. Your situation for tracking heating cost is difficult because you are using the gas for DHW and maybe cooking.
If you are comfortable in the house with the existing thermostats and setback scheme, it probably is not making a big practical difference one way or the other (with respect to the cost of replacing lots of equipment). As I think about it, there is one issue you may have with the water heater and heating setback. Because of the shared heat source, you can not identify when DHW load is occurring. A smart boiler will stop heating the floor when DHW demand occurs. By demand I mean when it would have to recharge an indirect hot water heater. Morning showers and setback recovery may be an issue unless the boiler can supply BTUs for all the concurrent loads. You can figure this out by finding the inlet temp on the water heater (e.g., your water supply temp) and desired output temp. Figure out the flow rates of your showers and fixtures, decide how long they are going to be used, if they are going to have simultaneous use, anything else taking hot water concurrently, and add them all up. This gives you the worst case flow. Calculate the BTUs needed to raise that many gallons to the required temp. That is how many BTUs you need to have to keep the showers warm if you are not heating the house at the same time. You could mitigate the heating load by using your thermostat setbacks to turn heating off when you have hordes of guests showering. Remember that we have been talking about output BTUs; not the input to the boiler, which is what the ratings you see represent.
Something I was going to mention earlier. Some areas will not allow a hot water heater to be used as a heating system source. Obviously you have on already, but I thought I would mention it. Note that the Phoenix is designed to do this.
Perhaps you heating guy can provide you some input on these issues.
I had a better idea about DHW vs heating demand. Make sure the piping at the output of the tank for DHW has a accesable place on it near the system. You may need a loop to stop convection. Attach a sensor to the pipe and when I gets hot from running DHW, use the sensor to disable the heating system.
A possible sensor would be a high temp limit switch. It just clamps on and has a switch and adjustable temp.
Last edited by alternety; 11-16-2007 at 11:49 AM.