gaw wrote:Just a little food for thought about an indirect water heater and an internal coil/instant heater. The question worth knowing is just how cold the boiler will get while maintaining a fire. The fire can only idle along so low and it is always giving off some heat. Unlike an oil burner there is always some heat output from a coal stoker. I am not using a boiler that is built with this sort of sophistication so my experience may be totally irrelevant but I have noticed during warm weather with the low limit set at 140 I always have at least a 150 reading on the boiler gauge and this is just from the fire required to maintain itself (enough fire to stay lit) as far as I know. Obviously I am not standing there 24/7 to see if it has to run to satisfy the aquastat when there is no demand.
A good experiment would be to turn the aquastat down to about 100 and then see just how low the boiler temp will go when lit. If you are going to maintain at least 120 to 130 degrees no matter what the aquastat setting then an instant coil type heater may be the most economical choice.
Hope this makes “cents”
I don't have the data to make a knowledgeable recommendation. I believe the answer will be unique to the AHS and/or A-A boiler design. What we are trying to answer is "What is the most economical operation mode for summer production of domestic how water?" The boiler produces very little heat when the combustion blower is not running. So with the proper control system it could approach the ideal of a truly on-off boiler like oil or gas units. The data point that's missing is how small the coal fire is possible when there is no need for heat. Only those with the optional Fuji thermocouple ash probe can answer this. I don't let have one. Let's assume with no heat demand the long term boiler water temperature is really low, say 75 degrees. This might be a reasonable possibility, it's only an 11 inch round fire box surrounded by 27 gallons of water. A barely burning coal fire would not make much heat. Now the Fuji controller is a fancy unit adapted by AHS to measure ash temperature with a thermocouple probe in the ash. The controller can predict future temperatures, it's called fuzzy logic. The idea is this based on past measurement of temperature overshoot. Basically you turn off the combustion blower before the desired set point boiler temperature is reached. The coal fire dies down but the continued heat input allows the desired set point to be reached. There is no temperature overshoot. I don't think the factory programs the Fuji controller in this way but it is capable of such operation.
Now lets assume the purpose of the Fuji controller in summer mode is only to keep the fire from going out. Let's further assume there is an indirect hot water heater and the thermostat in the hot water heater calls for heating. The combustion blower comes on heating the boiler water. But the circulating pump doesn't come on until the boiler water comes up to aquastat set point. The domestic hot water tank get reheated and everything shuts off. Boiler water cools and the Fuji controller continues it's job of keeping the fire from going out.
Now the next step... All hot water heaters have snap action thermostats, on or off. What if the indirect hot water heater thermostat is replaced with a Fuji like fuzzy logic controller. Now you can turn off the boiler's combustion blower early and let the circulation pump transferring the boiler water heat to the hot water heater tank and run greatly reducing the boiler water losses.
Hopefully E-bay will have Fuji controllers cheap enough for me to install several and I'll live long enough using coal heat to answer my initial question.