fin tube, modcon boiler and indirect WH

Scott B.

so i did a heat loss and i need about 65000 net btus to heat the house. they have fintube heat and want to add an indirect heater for the domestic hot water. the gas company supplying burnham freedom boilers. two baths with 3 people in the house. the 70,00 freedom would be perfect for heating but small for an SSU. figured upsizing to the 93,000 freedom would help out the SSU. I called burnham and they suggested a 30 SSU,because recovery would be fast but this is way to small for 2 baths and 3 occupants. I called SSU and they didnt want to make any recommendation but i did get the tech to say that he suggests a 60 to add capacity. the problem here is the recovery, it will take to long to get off priority and back to heating the house. anyone have any suggestions . This seems like a pretty typical situation and I am wondering what the rest of you are doing in cases like this. size the boiler simply based on DHW or compromise and go in the middle? maybe a 45 gal SSU? What would you do?

Uni R_2

Maximize the condensing...

Get the smaller boiler, a larger indirect and widen the differential setting on the indirect (making sure it has a safely set mixing valve on it). With a small tank, a larger boiler, a tighter differential and any water temperature near 140°F, you'll seldom condense making DHW.

In addition, during the off season, you strand BTUs in the boiler's HX and the piping between the DHW and the boiler. With a larger tank, you'll do fewer firings (stranding fewer BTUs but slightly higher tank losses) and with the wider hysteresis more of the DHW recovery firing will be in the condensing range.