I'm looking to heat a small house with a gas tankless hot water system. Is this a good idea?
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I have a new 180k btu tankless hot water unit and want to know if I can also heat my house with the hot water system. The baseboard heat is the second source of heat. Heat 80% w/ pellets. I know I would need a heat exchanger of some sort to prevent mixing domestic hot water w/ heat water.
This question is not terribly clear, but I would always recommend starting your home energy design process with an energy audit.
For example, do you have access to natural gas? If so, pellets are a more expensive form of heat. The energy auditor's cost-benefit analysis of improvement options should help quantify options that are and aren't cost-effective.
Tankless hot water units are designed to heat water from 35/55F to 115/130F. Hot water boilers are designed to heat water to 170/190F. You really want mechanical equipment that matches your system/strategy.
There are combination (combi) type units that do both hot water and heat. Navien, a very popular manufacturer of super high efficiency on-demand water heaters, has a unit that you might want to look into.