I am considering adding baseboard heating to my bedrooms (there are four) because we don't like the forced air heat from the furnace. The main living area has floor heat using a tankless water heater which also provides our domestic hot water and it has worked great for over 10 years but this is not an option for the bedrooms due to the slab foundation. The problem is, based on what I have read, baseboard heat requires 180 degree water which I know our Takagi won't handle. We live in a moderate climate where, in winter, it rarely gets below 30 degrees and the house is well insulated. This second water heater would be solely used for the baseboard heat in these bedrooms. Any suggestions.
The
real problem is the misconception about what baseboard heat actually requires. A average water temp temp (AWT) of 180F is just one temperature on a continuum, often selected as the point for specifying it's capacity.
It usually takes 190-195F water to even hit an AWT of 180, a temperature almost nobody is using in real systems today. At an AWT of 180F
typical fin-tube baseboard delivers about 600 BTU/hr per running foot of baseboard. At a SUPPLY temperature of 180F the AWT is usually about 170F (
180F supply, 160F return), delivering about 500 BTU/ft.
At an AWT of 120F (125F supply, 115F return) it'll be delivering about 200 BTU/hr per foot, which works just fine for condensing efficiency, and works just fine for water heaters.
The problem of heating with the Takagi would be
the same as with a condensing boiler- if there isn't enough baseboard to emit the full minimum-fire output at whatever temperature you're running it's going to short-cycle itself into low efficiency and an early grave. If the min-fire output is 10,000 BTU/hr and it's set up for 130F output the baseboard be putting out about 230-240 BTU/hr per foot, and it'll take at least 40' of baseboard to suppress short-cycling, 50' would be better. That's probably do-able if the bedrooms are all operated as a single zone, but damned-near impossible if trying to micro-zone it.
Start with running
a room by room I=B=R type heat load calculation for each room on the zone to get a handle on the magnitude of the load and the relative amount of baseboard needed for each room. Most likely the total load number for the zone will be less than the minimum firing rate of the Takagi. Then using an output spec for the baseboard decide how much baseboard is needed to avoid short cycling at the temperatures you're running. In most cases that will be WAY more baseboard than is actually need to heat four bedrooms with low-temp water, but baseboard is cheap- boilers & tankless water heater are expensive, so don't try to cheap out by going short on the size of the radiation on the zone.
