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For Heat and Hot Water: Combi Boiler or Boiler with Indirect Tank?

I recently bought a house just outside of Boston with radiant floor heating that used an electric micro boiler just for the heat. The house is 1200 sq ft with one bathroom, though we hope to add a second in the next year or two. The hot water for the house (showers, sinks, laundry, etc) is done by the traditonal electric hot water tank. The micro boiler for the radiant floor heat has to be replaced now.

I have gotten several estimates on replacing the boiler for the heat and they have recommended a combi boiler (such as the Rinnai EC 110) for the radiant floor heat and to also produce on demand hot water for the house. The last estimate I received said we would be unhappy with just a combi boiler and recommended putting in a high efficiency boiler (Bosch Greenstar 100) for the heat in combination with an indirect tank for the hot water. The last plumber's reasoning was that as we shower, did laundry, or used the dishwasher the combi boiler by itself would divert water from the heating side and the house would cool off until we were finished with the hot water.

Does anyone have recomendations on using a boiler and indirect hot water tank vs. just using the combi boiler for both hot water and heat?

Re: For Heat and Hot Water: Combi Boiler or Boiler with Indirect Tank?

I should have also added we want to convert everything from electric to natural gas at the same time and take advantage of the conversion and high efficency rebates right now. The gas is already piped into the house and the gas company has confirmed the line is more than sufficient.

Re: For Heat and Hot Water: Combi Boiler or Boiler with Indirect Tank?

Converting from elec to gas is a good move, RC---it should save you big $$$$ on your fuel bills.

To choose between a high eff. boiler & indirect vs a combi, you have to first calculate the peak hot water usage (called domestic hot water, DHW) that is used by the family members, usually during the morning rush hour; you can then figure out which system will be best for your family----the heating unit installers should have run this by you when they were over there to size up the installation!

In other words, if your family uses a certain # of gallons of HW during the rush hour for simultaneous showers, shaving, dishwashing, etc., the combi or indirect hwh unit would have to be able to deliver that many gallons without running out of HW.

I like the Triangle Tube Solo Hi Eff boiler with their separate companion Triangle Tube Indirect HWH, but there are many other mfgrs like Slant Fin , Peerless Pinnacle, NTI, Viessmann (German, expensive), Lochinvar---I would recommend a stainless steel combustion chamber instead of an aluminum cc, especially if you have hard water in your area.

Many people don't like the hassle of the extra service calls that can happen with Hi Eff. boilers, that often need periodic adjustment during the 1st year of ownership & opt instead for a more traditional gas-fired Buderus, Burnham PVG or Revolution, Crown Cabo, Dunkirk EV; these units won't give you the same fuel savings as the Hi Effs but close to it (88% vs 95%), & the technology is tried & true over the years & they usually require very little servicing.

Use the site below to calculate how many gallons of hw your family uses during the morning rush hour for showers, shaving, washing, etc., then check to see if the heating/hw system you intend to buy can produce what's known as its "peak HW recovery rate"; a stand-alone boiler with an indirect hwh that has the standard holding capacity of 40 gal is more expensive than a combi, (approx $2k-$3k), plus the cost of the boiler itself, but you will never run out of DHW.


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