Tankless hot water/boiler question

MikeAR

After searching on-line for options to replace my old boiler, and probably the hot water heater at the same time, I found some tankless hot water heaters that said they could also be used as boilers. With my very limited background in this area, I can't see any reason why it wouldn't work as long as the output was high enough. Any suggestions? If it helps, I have a 100 yr old 2 story home with hot water radiators in all the rooms (10 total).

Thanks

Jack_23

GPM X Delta T X 500 = BTU

I will speak of a Rinnai as I know it well. On a model 2532FFU-C (Commercial unit req'd for heat)you can use them for space heat...but! Due to pressure drop across the unit you are going to get no more than 5 gpm thru the unit with any reasonably sized circ (009 or equiv). With a 20 F temp drop you have made (5x20x500=50,000btu). The flow rate is constant as is the 500 number and with your old system you cannot push the delta T number around so I think you should go with a bit more conventional system. Certainly put one in for your hot water but in this case not for heat. By using the on-demand for hot water you can properly size your boiler for the true space heating load. I like these for low btu heat apps on radiant and hydro-air. Less so for baseboard and I like to size the baseboard for 160F operation. Again low btu systems

Firedragon_4

Just make sure that it's legal

in your state. Many states prohibit the use of water heaters for space heating. Quick way to start is to find out if it's ASME listed as a 'heating source'.