Replies

1. Expert Member
   Dana Dorsett|May 05, 2014 11:17am|#1

   There are multiple combi-systems available based on tankless HW heater technology- Navien and Rinnai both have reasonable offerings. While a DIY combi system based on a tankless HW heater is possible, there are several ways to screw it up (and badly), requiring at least some amount of hydronic heating design skills.

   Getting to the right space heating solution starts with a room-by-room heat load calculation. If the baseboard in question is already installed, the amount of baseboard in each room needs to be noted too, since that's what determines the water temperature requirements. The output of fin-tube baseboards at typical domestic hot water temperatures is between ~200 BTU per linear foot of baseboard at 120F average water temp, to about 250 BTU/foot at 130F average water temp. If your temperature requirements are higher than that you need turn up the temperature on the tankless (outside of the condensing zone), and temper the hot water distributed on the potable side to a non-scalding value.

   If you don't have sufficient baseboard to deliver the tankless output BTUs at it's minimum-firing rate at the temperature you're running it'll short-cycle, taking a hit in efficiency and a big toll in wear & tear on the tankless. This is true for combi boilers too, some of which which have a fairly high minimum firing rate compared the actual heat loads of many smaller or higher-R houses. (The Rinnai E50C has a min-fire input of 13,000BTU/hr though, which isn't bad, but with only 85,000 BTU/hr at max imput you'd have to schedule your other hot water uses to avoid cold showers.)

   Without knowing your actual heat load, there's no saying whether any of the above would be an appropriate solution for you.

2. 
   user-2890856|May 05, 2014 12:40pm|#2

   There are other types of equipment to combine the 2 functions . What are your space limitations ? What do you presently have ?