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Oil-fired water heater vs. oil-fired boiler for radiant heat

DaveGateway Member Posts: 568
A diy radiant project for a new construction home. I've been getting conflicting info on what is best. If the maximum supply water temperature the system requires is 131 deg. F and the minimum is is 94 deg. and a maximum heating load at design temp of 38954.78 Btu/hr for a total square footage of heated space of 3791.6(1185.6 sqft for concrete basement, remaining in 1st and 2nd floor joist installation), what is the best heat source for this application and why? My radiant consultant recommends a water heater with a heat exchanger with a 65000 minimum btu input. My plumber is recommending a boiler with an indirect water heater.

My domestic hot water needs are pretty high. We'll have 2 1/2 baths, with 1 oversized bathtub in the master bath. We are a family of 6, 2 adults and 4 kids, so you can imagine that in a peak hour we can really stress the water heater. We quite often empty a 66 gal. electric heater, it just can't keep up. It happens often that we're trying to fit baths, clothes washing and automatic dishwashing in at the same time. We're still in the design phase, and trying to figure which will work best for our needs, oil-fired boiler or oil-fired water heater?

I've been told "Boilers are, primarily designed to provide high temperature water or steam to the distribution system. Some boilers, and most fuel oil boilers, are very sensitive to low return water temperatures. That is to say that, if the return water on fuel oil boiler is lower than 130 degrees F, a condition can be caused on the surface of the internal heat exchanger where the fuel gases condensate on the burner and become very corrosive. That condition shortens the life of the internal "fire side" heat exchanger. This problem arises from the fact that the boiler has a relatively small volume of water that can easily be influenced by the temperature of the return water. There are very complex controls that can eliminate this problem to some extent, but not entirely. So, if the boiler is run at a very low temperature, or if a mixing valve permits very low return water to the boiler, either scenario can cause problems in short life and in poor efficiency.

I don't know what size boiler your plumber recommends, but a substantially oversized boiler may not be efficient for the heating season and would definitely not be efficient during the summer when it is used only for potable hot water. As it is now I show a maximum heating load of around 40,000 Btu per hour at design. Another 20,000 Btu's would be good for acceleration loads. At 80 percent combustion efficiency, you would need a heat plant with around 75,000 Btu per hour input. That falls well within the range of most fuel oil fired potable water heaters.

Now fuel oil fired storage type potable water heater is designed to supply a wide range of output temperatures within the range of those that are used in radiant panel heating systems in general and in your project in particular. Since they incorporate a large mass of water in the storage tank, they are not adversely affected by the very small flow rates that occur when just a small number of zones are calling for heat. Boilers are adversely affected by small flow rates. This is why radiator and convector systems are often grouped into large uncontrollable loops with many rooms on one thermostat.

The fuel oil fired storage type water heater is well insulated and very well equipped to handle the potable water demands in both the summer and the winter. I think it is a better choice."

I've been going back and forth for weeks now and am still no closer to finding an answer.


  • Tony_8
    Tony_8 Member Posts: 608
    My opinion

    use a boiler. Lots of oil boilers have high volume water content and high mass cast iron. Injection pumping cures low return temps to the boiler. Water heaters have higher standby losses as well. Another site to ask is Oiltechtalk.com
  • DaveGateway
    DaveGateway Member Posts: 568
    What is an injection pump?

    What's an injection pump and where does it fit into the schematic? Is it an injection pump instead of mixing valves or in addition to? Like is it mixing valves for water to the radiant floors and an injection pump on the return to boiler?
  • Robert O'Connor_3
    Robert O'Connor_3 Member Posts: 272
    Always a boiler

    It might be a cheap radiant source solution with a gas WH, but an oil-fired water heater with a heat exchanger is very pricey and will probably need replacement in ten years. Just get a 60 gal indirect on priority. The radiant can wait 15 to 20 minutes while the tank is getting all the heat.
  • Tony_8
    Tony_8 Member Posts: 608

    pump takes the place of the mixing valves and works better to boot. Again, my opinion.
This discussion has been closed.