You can buy a gas-fired tankless water heater for $600 — or even $300, if you want a bare-bones model. These appliances are remarkable: they are compact enough to hang on a wall and can begin producing an “endless” flow of hot water almost instantly. Many people look at these small appliances and think, “Why can’t I use one to heat my house?”
The answer is, you can. However, just because you can, doesn’t mean you should.
It’s true that a tankless water heater can easily heat most single-family homes. It’s also true that a tankless water heater is smaller and less expensive than a boiler or a furnace. So what’s the catch?
There are several catches, and the issue is fairly complex. The basic problem is that tankless water heaters need a high firing rate — as high as 180,000 BTU/h — to quickly bring cold tap water up to 120°F or 140°F. These appliances work best when the incoming water temperature is relatively low (below 85°F). On the other hand, a space heating appliance for a single-family home or apartment needs a much lower firing rate — usually between 10,000 BTU/h and 20,000 BTU/h — and must be able to tolerate incoming water temperatures that are higher than the temperature of cold tap water.
Trying to use an appliance designed for the instantaneous production of domestic hot water for space heating is possible, but doing so requires the system designer to accept certain compromises and to engage in several contortions.
Before we can analyze the advantages and disadvantages of using a tankless water for space heating, we need to discuss three questions:
Systems that use a single heating appliance for both space heating and domestic hot water are called combi-systems (or combo-systems).
In theory, a tankless water heater can be…