Using an air handler with a boiler is most often done to add suplemental heat, addressing a small zone or allowing constant warm air circulation for filtration purposes. Other then that, they are usually used in conjunction to radiant. If your desire is efficiency, you would be better served with a high efficiency gas furnace and an on-demand gas hot water heater.

Using a boiler with the piping required and circulator, is not an efficient way to transfer heat to warm air unless there is a need for hot water besides domestic.

With regards to the Phoenix, I would look at other boilers. (search for HTP/Munchkin failures, circulator requirements and annual service required to understand what I mean)

Look at the Triangle Tube Excellence, If thats what you want that is the way to go.

zl700 - Thanks for the response. Can I pick your brain for more details. I've read elsewhere that it isn't a big deal to install a hydronic heat exchanger in existing furnance duct work. Are you saying that the efficiency loss in the exchange is so great that it's not an economic way to provide heated air to my home?

If I can get a solar DHW combi system that backs up a gas boiler to heat the forced air for nearly the same price as the installation of a new condensing furnace with upgraded duct work and an on-demand DHW system, wouldn't the first option be more cost efficient over time?

What do you think of Viessmann products?

While it is not hard to install a external hot water coil to existing ductwork, many times you are restricted in physical size, number of rows & fins per inch because you cannot exceed the static pressure that the blower can handle with all things measured. All these plus CFM, GPM & water temp through coil dictates BTU output.

Word of caution with an AC coil, filter, ductwork and adding a hot water coil, the airflow is many times restricted to a point of excessive loading and noise. Manufactures many times donÂt expect that much to be added and donÂt oversize their blowers and motors in an attempt to keep efficiency up and costs down. These issues usually restrict the size and type of coil that can be added. If you take a look at hi-velocity systems, hot water coil options tend to get bigger because the blower can accommodate it.

ThatÂs why you will see low standard hydronic air handler performances as compared to their gas or oil brothers or AC capacities. Given a low heat loss requirement, you probably will be able to match one up.

Hey solar is free, providing the sun shines, the expense is capturing it, sometimes storing until needed, and getting the heat dispersed to the space. That is a financial and part green choice you will have to make, most often dictated by your return on investment.

Viessmann, is top of the line with all the bells and whistles available. Their Vacuum tube solar collectors that convert solar to steam, and heat the solution circulated to it are slick.

Okay. So if my idea is to buy a new blower (air handler) to push the air past the hydronic coil (no AC here) that receives heat from a high efficiency condensing boiler/DHW appliance (supplemented by a solar hot water system) with new duct work added (I think there are 7 new duct runs to connect to the old system above the basement.), then I can hopefully size the air handler and ducts to produce enough BTUs. The question is whether this is cost effective versus installing a new gas furnace and duct runs and a new hot water heater. Obviously the initial cost is much higher but possibly operational savings would make it viable. What do you think? ALso, assuming proper blower and duct work, do you think the operational costs of a boiler with heat exchanger to move the same number of BTUs through my house would be the same as a furnace? These boilers are more efficient in producing heat NG but the heat exchanger must decrease the efficiency. I wonder if the operational costs would be a push.

Thanks for all your information.

Before making any statements that involve efficieny and return on investment, I would need to have a good idea about how much solar capability you expect to have. Any idea how many gallons at what temp and the recovery based on medium sun day with 8 hours exposure?
This factors into the equation whether or not it is worth your while of all the added expenses, versus a 90%+ furnace and perhaps a on-demand gas heater.

Personally, I dont have a big heating load but I am considering solar for preheating my domestic before my on-demand via storage and rejecting extra unneeded to my pool/tub.

The solar people's program estimated SF to be 20% of of my DHW and heating. I don't use much DHW so the only economic reason to go solar would be savings on the heat side.

But leaving aside the solar input and installation costs, would a boiler airhandler system, properly designed, use about same amount of NG to heat my home as a furnace would?

Quick answer-

Boilers perform better, especially condensing boilers, when cooler water is returned to it from a large mass system.

Being that your heat terminal off the boiler would be a fan convector coil, this is a low mass heating device. That meaning the temperature difference between supply and return would be minimal not allowing the boiler to perform as it desires, thus short cycling, among other things. Because of this the efficiency drops also.

Without additional zones off the boiler, a hydronic air system of its small size would not be your best choice in both material/install costs, versus a gas furnace.

Your money would be better spent on a high-efficiency multi-stage burner with a variable speed blower.

For comparison, the furnace and boiler cost about the same, and for the boiler you would still need the air handler, coil, pumps, relays, piping, etc.

In addition-
You could always use the solar to produce domestic hot water and if you find you have excess, add a coil in the ductwork on the gas furnace later and treat it as a 1st stage of 2 or 3 stages.

With this option, get a 3 stage stat such as the H/W Vison pro with apropriate number of wires run so that you are ready and not have to buy a new stat again later.

I live in Louisiana were we now have a 50% tax credit for solar systems, plus the 30 percent federal credit.

We don't heating a lot of heating demand (rarely gets below freezing), so my thought is to use solar to heat water for domestic HW as well as to heat the house through a water coil in the air handler. If necessary, I could supplement the solar preheat with a demand heater before it goes to the air handler.

If I spend $30,000 on a new HVAC system that uses solar to provide home heating, my out-of-pocket cost should be only $6,000. That's less that putting in a new system with gas heating.

If I have a hot water coil in the air handler, can I also have a gas burner for the really cold days?

Also, what's the best equipment?

jim