A boiler add-on system that uses outside air temp in determining water temp

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Old 03-23-10, 03:19 PM
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A boiler add-on system that uses outside air temp in determining water temp

Watched This Old House last night and the HVAC guy installed a new bells and whistles device which was comprised of an outdoor temp sensor, wire, and an inside computer fastened to the outside of the boiler. The HVAC guy (I forget his name), said boilers are usually run at about 180 degrees so they can keep a house cool say if 0 or less outside. But he said there are many, if not most days, that are above 0, and you do not need the boiler water temp as hot as 180.

And he said that you should have the boiler trying to run longer to maintain the temp.

Nothing was said about any risks regarding condensation at lower temps. Seems to me that topic has been brought up here.

Any opinions on this?
 
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Old 03-23-10, 04:14 PM
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Of course... it's called OUTDOOR RESET. And as explained the idea is that you may only need 180 degree water on the coldest days of the year. The other times, such as now, you might be able to heat the home with 140 or 120 degree water... so the control monitors the outdoor temp and adjusts the water temperature to compensate.

About condensation... there is a setting on the OD Reset controls that takes this into consideration... Tekmar calls it BOIL MIN, and that would be the lowest temp that the control would 'target'. It would be set above the condensing temperature to avoid that problem.

There are of course boilers that are DESIGNED to condense, and these can be run all the way down to room temperature water if necessary.

The part about the burner running longer... this could become a problem with an oversized boiler during the spring and fall... since not as much temp is needed to heat the home, the burner would fire up for short cycles... not good for efficiency, or excess wear on the components...

Widening the 'differential' helps with this... and the ODR controls have a feature called 'AUTO DIFF' that will automagically widen the differential at these times to try and eliminate the short cycles.

A better approach might be to use a 'buffer tank'... have the boiler heat the mass of water in the tank, and have the space heating pull water from the tank instead of directly from the boiler. You will get longer, more efficient burn times. See: http://www.boilerbuddy.com for details. (there are other manufacturers also)

Personally, I think INDOOR FEEDBACK is a much better approach. Just using outdoor temps is rather limited feedback. It can't compensate for sunny days and solar gain... or windy days... etc... if INDOOR temp is used, it would take all these things into account.
 
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