Reduce boiler temperature

[hvacbs1]

What is the reduction in BTU delivered to radiators attached to a hot water boiler as you reduce the temperature of a boiler from 180 degrees to 170, 160, 150, 140, 130, or 120 degrees. Is it linear? For example, if a boiler delivers 500 BTU per foot of radiator at 180 degrees today and you reduced the temperature to 120 degrees, would I reduce the BTU per foot of radiator to 500 x 120 / 180 or 333 BTU? Are boilers designed to run efficiently at one temperature (this is not a mod-con boiler) and then lose efficiency if you turn down the temperature?

The scenario is an old house that is not well insulated. I am planning to seal the house much tighter, add a lot of insulation, and replace many windows/doors. I am trying to anticipate what potential adjustments might be necessary on the boiler to accommodate these changes.

[snupytcb]

WHAT KIND OF RADS?

[hvacbs1]

Classic hot water radiators of various sizes. There are at least 15 of them in the building...

[snupytcb]

are they cast iron?

[hvacbs1]

Yes

[snupytcb]

i dont think they will put out much with 120 deg water. 160 deg is about the lowest i would go.

[beenthere]

They don't publish extended data for cast iron rads.
But, they do publish it for cast iron baseboard.

1GPM 180° water=590BTU per3.4 sq ft
170=520
160=450
150=390

If you do a little extrapolation, it gives you about 218 BTUs per 3.4 sq ft of radiation at a water temp of 110°F

Cast iron rads give off good heat at lower water temps. Thats one of the reasons that they provide a nice even heat in a house.

[Rhinohead]

I wouldn't turn down below 140 unless its designed for it

[hvacbs1]

beenthere,

Thank you very much for the reply. This was exactly the type of information I was seeking. This helps me better understand the implications of turning the temperature down. One other question, will the boiler lose efficiency as I turn down the temperature? The model is a Weil-McLain EG-75 SPD (300k BTU with advertised 81% efficiency).

[beenthere]

It won't lose efficiency at lower temps.

Care must be taken not to have too low of a return temp. Or you could get condensation in the boiler combustion passage ways.
Your boiler should be rated for 130° return water temp.
This is the prolonged return water temp.(a cold fired boiler would of course have lower at initial start up)

[Kevin O'Neill]

We always figured 20 degrees drop from supply to return. 20 degrees drop gives you 10,000 BTUs/Hr per gpm. You can find flow using a pump curve measuring pressure drop across the pump.

[harrytbs]

i would be very careful with cast rads .you can thermal shock a boiler if the return temps are to low for long periods of time. i would go with a taco reset control. It has setpoints for cast iron radiators in the program . also depends on the boiler .If you have a buderus boiler can run 120 return temp for long periods of time. where a weil mclain can not .Also if it is a oil boiler i have seen sooting problems with low return water temps.

[beenthere]

The Weil McLain EG, is a gas fired boiler.
Its also rated for 130°F return water temp.

It would probably take 100 years to get a ROI to get a Buderus to get that 10° cooler return water temp allowance.

[harrytbs]

i missed the post stating it was a gas boiler. I wouldn't bother either for the price difference between the buderus and a weil mclain for a gas boiler. I would spend the money on a outside resent control all day long. I would also make sure the boiler has a bypass or a primary secondary loop on it,if there is a lot of water volume above it .

[harrytbs]

"outside reset control" fat fingers