Great question, Annie.
I believe what you are describing is called a 'combined hydronic' system.'Combined' because is uses the water heater as a heat source for the furnace and 'hydronic' because water is the medium used to transfer the heat.
Combined hydronic systems can be the most efficient method for heating your home with gas for the following reasons:
- there's only one combustion appliance instead of two and that one, the water heater, will likely run at a higher efficiency when used a lot.
- the furnace itself is a bit smaller, is now called an air handler, and needs no gas supply.
- the flow of heated water supplied to the air handler is adjustable and can be fine tuned to match the air handler's needs.
Also, combined hydronic doesn't run over 125 or 130 degrees F, while a gas furnace can have a hot surface that can run in the hundreds of degrees. This is an important distinction because organic dusts start to crisp and char when near a very hot metal surface; and I've read research that suggests that organic dusts are more of an irritant once they've been overheated and crisped.
You may have guessed by now that I love combined hydronic systems. There are, however, a few things to understand.
- They are most efficient when the air handler and water heater are near each other.
- And it takes a large and powerful water heater to supply enough hot water for the air handler and still give you enough hot water for all your domestic needs.
- Tankless water heaters don't work well when asked to do both.If you get a tankless that is large enough to do so, 199,000 BTUs or higher, it likely won't throttle down low enough to stay on and give you hot water at a small fixture such as your bathroom sink.
The solution I use is a high efficiency tank type water heater such as a State Premier GP6 50.
- AO Smith and others also make similar models.
- These not only store 50 gallons of hot water but are rated as high as 96% efficiency, higher than almost all on-demand water heaters, and are capable of supplying an air handler and still have enough for your showers.
Your unique home and circumstances are best addressed by a Home Performance Contractor in your area.
- Use the 'Find a Pro' button on this site, or others such as RESNET or BPI, to find one near you.
- Ask him to drop by and discuss your needs. Most are happy to do so.
I hope this gave you enough information to get started.