Boiler Throughput

Stokehold

I am installing a small solid fuel boiler rated at 90,000 BTU.
This boiler will be supplying a single zone which is a fan coil installed in the furnace plenum. There will be a dump zone to discharge excess heat if the boiler gets too hot and there is no call for heat. Also, a thermic valve will regulate return temperatures.
Under ideal circumstances, I am presuming you would not want to flow more than 9 GPM through a boiler this small.
If my presumptions are correct, then I should be able to get away with 1" piping from the boiler to the emitter and back. The 1" thermic valve has a Cv of 10.6.
I would be somewhat hesitant to increase this piping to 1¼" because of velocity issues. Again, if I am presuming correctly, the ideal flow through 1¼" copper is about 14 GPM which I am thinking is too much flow through the boiler. It looks as if 1" pipe is too small and 1¼" pipe is too large.
Again, as always, comments are appreciated!

hot_rod

A couple thoughts, is the 90,000 actual rated output? If so 9 gpm is plenty, 1" is perfect for that flow rate. Can the fan coil transfer that 9K output?

Why an additional dump zone? The fan coil could be brought back on for a short, over- temperature condition. A small UPS could power the circulator in the event of a power outage.

A bit of buffer tank can help smooth out cycling and inefficient burns as you may not be at design conditions much of the season.

Put your DHW on the wood also if possible, a reverse indirect tank could buffer and provide DHW for you.

Stokehold

Excellent points, Hot Rod!
I should have mentioned that I will have DHW on that boiler since it has it's own coil. Basically,I was just going to plumb that coil through the existing water heater with a small recirculator and high and low limit aquastats to control temperature. One to stop circulation if boiler temps fall too much and one to stop circulation if tank temps get too high.
Yes, your suggestion to restart the main circulator and dissipate excess heat into the fan coil rather than a separate dump zone is a good idea. That also could be controlled easily with an aquastat and simplifies the job.
Fan coils should be sized (I presume) so to expose as much finned area as possible to air flow. You do not want to restrict CFM because that may cause issues with the normal operation of the gas furnace. In my case, the coil has a much higher BTU rating than the boiler. That is a coil 20x22x3.5 rated at 176,000 BTU at 180° with 20° delta or 17.5 GPM. Of course, at 9 GPM things change. I am still concerned it may be TOO big causing short cycling of the circulator. Since the limits here will probably be 160-180, I am presuming that boiler is going to lose heat very quickly running through that size emitter.
Am I correct?

EBEBRATT-Ed

if you pump 9 gpm through the fan coil the heat output will probably not change a lot. You just will get a larger td across the fan coil. Flow will be slower so more heat will be removed from the water. Flow will be less turbulent which would tend to reduce heat transfer lots of variables.

Does the fan coil have multiple fan speeds that you can use??

You could also pipe a bypass around the fan coil or primary secondary piping would be best if you don't have variable fan speed

Stokehold

The coil fan will actually be the blower on the LP furnace. It is currently set at 1805 CFM via dip switch on the electronic control.
I was going to control the call for heat to the boiler circulator and the furnace blower with a separate wall thermostat set at approx. five degrees higher than the LP unit. I am presuming that removing the jumper between RC and RH can handle that. RC can control the fan relay on the LP furnace while RH can be wired to TT on the boiler aquastat.
This boiler is not a primary heating source, only supplemental. There is some room for the variables.

hot_rod

The coil manufacturer should have performance data available. It would indicate performance a various conditions, sometimes a graph. Flow rate, temperature drop, and air flow are all part of the variables.