Viessmann boiler instead of two furnaces

[omeriah]

I am building a 3,500sf house including full basement in SE Michigan. We're doing all the standard things to maximize our efficiency, but unfortunately, we do need two furnaces. We are also doing radiant heat in the basement (only). We had originally planned to do a 75gal powervent water heater for radiant heat and domestic water, a gas furnace to heat 1st floor, and a gas furnace on the 2nd floor.

Our plumber suggested that we would save $$ in the long term if we switched to a single Viessman boiler in the basement. This would heat water for the radiant and domestic water, send hot water to an air-handler for the 1st floor, and send hot water to an air-handler on the 2nd floor. This would cost about $3,000 more upfront.

Does anyone have positive/negative thoughts on this? And does anyone have an estimate of savings? Plumber suggests 20% savings, but that seems awfully high.

Thanks very much for your feedback.
Andrew B.

[aemeeich]

Depending on the Viessmann boiler selected, it could be way more efficient than using a water heater. If it is the Vitodens 200, that is a modulating condensing boiler. This boiler (along with similar boilers from many different manufacturers) has the ability to vary its input rate so the gas burned exactly matches the heat needed in the house. There is no short cycling of the unit. When running a condensing boiler at radiant temps (120ish degrees) it is capable of 98% efficiency. A water heater is only around 55-65% efficient.

Also using a water heater for a combined use - radiant floors and domestic water is a very bad and dangerous idea. In the summer time, the water in the floors will stagnate and can start to grow bacteria. This will then come back into the domestic piping and make you sick. The floors need to be in a closed system - isolated from domestic water.

My suggestion to you is to go with a single modulating/condensing boiler. Viessmann Vitodens 200, Munchkin, Lochinvar Knight, Buderus GB142, Weil-Mclain Ultra, as well as other choices. Go with whatever your installer is familar with and recommends. Get an indirect water heater for your domestic water needs. Size the water coils in the air handlers for 140 (or preferrably even lower) degree water. Think about also heating the floors in bathrooms and kitchen. All this will add up to a low temperature system that will run at very high efficiencies.

Michael

[omeriah]

Aemeeich--

Thank you so much for the information. That's very helpful. I probably misstated one fact, which is that we would still have a separate water tank for domestic water. It wouldn't be a gas-fired heater though, the boiler would actually heat it. But I will confirm this and I get your point about the separate systems for radiant and domestic.

On the subject of the air-handlers, we are considering exchanging two 80-90% efficiency furnaces with two air-handlers combined with the boiler. So is our efficiency really going to increase that much?

Thanks again,
Andrew

[key]

I agree with aemeeich...

[slimwoodie]

i wouldn't buy Any kinda Viesmann ....

[johnsp]

Just be aware of hydronic pipes and tubing in a sub-freezing attic. If you're prone to power outages, there's a chance of freezing/bursting pipes. Yo may need a glycol mix.

Other great mod/con boilers out there now from Burnham, Triangle Tube, WeilMclain, that will bring that cost difference down even more, if you don't want to go for the top-end of boilers.

If your contractor uses small hydro coils, you might need to run the boiler water temp over 140° and you'll loose some efficency.

[aemeeich]

Originally posted by omeriah
Aemeeich--

On the subject of the air-handlers, we are considering exchanging two 80-90% efficiency furnaces with two air-handlers combined with the boiler. So is our efficiency really going to increase that much?

Thanks again,
Andrew

If you use a condensing boiler coupled to "oversized" water coils, the heat supplied by the two air handlers won't be any more efficient than a 93% efficient furnace, however where you will pick up more efficiency is heating your domestic water, and the radiant zones that you do install.

You also will only have 1 appliance to service, whereas with separate units you would have 3. (However there is no backup heat if something malfunctions on the boiler.) You also don't have to worry about cracked heat exchangers and the CO risks that they pose.

The main thing above all things, is to get a proper load calculation done. This will tell you the size of the unit or units that need to be installed. If too large of ones are installed, you will loose comfort and efficiency.

Michael