and what has to be done to change out the current HVAC system? Please keep in mind building science, being green and sustainablity.
She builds green
Boy Judi, where to start?
First, a hydronic system can be used with any kind of heat delivery package you want in a structure -- hot water baseboard, panel radiators, radiant floor/wall/ceiling heat and hydro-air using an air handler and duct work (it's basically a forced air system using a boiler and air handler instead of a furnace). In addition, you can also use the boiler to make domestic hot water very efficiently using either an indirect hot water tank or an "instantaneous" option using a flat plate heat exchanger.
Take my home as an example -- I have a high efficiency boiler supplying panel radiators and some radiant floor heating for my basement. It also supplies an air handler with a variable speed ECM blower motor for forced air heat for the main level (there's an AC coil as well). And I have a flat plate heat exchanger that makes all the hot water we'll ever need without having to "pay" to keep 40 gallons of water hot just in case I might use it. The panel rads and radiant floor keep my finished basement (family room, two bedrooms, bath and office) WAY more comfortable than the old forced air system ever could (the basement was, in fact, almost unusable during the winter before we installed the panel rads. We had to use portable electric heaters to actually use that space), and the variable speed fan keeps the upstairs very comfortable - moreso than the fixed speed fan in the old furnace the I took out. I also have an air-to-air HRV for indoor air quality.
In addition, last year my gas bill for the winter -- mid-October to mid-April here in Minnesota, totaled just over $506.00 for heat and hot water.
Other things to consider -- you would need an 8 x 14 duct to move the same amount of BTU's as a 3/4" copper pipe - you can easily drill out floor joists to run the 3/4" pipe -- you would have to hang the 8x14 duct below the joists - which takes up usable space and makes finishing the space a pain.
In addition, with the hydronic system you could take care of all your heating and hot water needs with just 1 burner. That means only one appliance to gas-pipe, one appliance to vent and one appliance to service. There's more room in my mechanical room/laundry room/home office, too.
In addition, hydronic-based heating systems lend themselves very nicely to alternative fuel sources - whether it's solar, geothermal, wood/pellet boilers and another other fuel type that transfers its heat to water.
This is a start -- I can post some pictures of my system if you like, as well as other examples. Let me know...
Here's a link to my blog that might be useful to you....
Hi Judi. John interpreted your question as pertaining to hydro-air, a hybrid system.
I'll let others chime in, and simply say one very good thing about hydronic systems is that at their best they're much quieter than air-moving systems, which frequently remind us they're working for us.