Temperature rise calculations?

dasniderSeptember 13, 2009

I'm trying to figure out if the hot water heating system installed in a house is adequate to heat the house in the winter. I tested the heat production by turning the furnace on for an hour and seeing how much the temperature rose. (Outside temperature was about 70.)

It went from 71 degress to 78 degress in one hour, which seems to me woefully inadequate for a 2,500 square foot house just outside of New York.

Does anyone know what a decent heating system should produce, not in BTUs but in temperature rise, for an older house in NY (built in 1940), with mediocre insulation, single-pane windows, etc.? Not looking for pinpoint accuracy here, just a rough estimate.



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I don't think you can do it that way.

You are better off doing a manual J and then look at your btu input as well as the efficiency of your boiler.

If it's undersized, the right answer is to add insulation.

    Bookmark   September 13, 2009 at 4:48PM
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'Temperature rise' is dependent on many physical factors, such as how well your house is sealed (tightness) and insulation, etc.

Bear in mind your temperature rise will be much less when it's 30 degrees outside.

Unless you're planning on using drastic setbacks, you would probably be maintaining temperature within a small number of degrees.

You don't want an oversized boiler as it will be inefficient and provide less comfort. Have a heat calculation done to see what your home's heat loss is and compare that to the (output) capacity of your unit.

You could also be having problems with the radiators (type?). Does the system need to be bled?


    Bookmark   September 13, 2009 at 4:53PM
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Thanks for the advice, guys. The boiler is a brand-new 225,000 BTUs, and is more than adequate for the house. (The old boiler, which dated from about 1950, was 195,000 BTUs.)

The problem is in the hot water loop: the installer is feeding the original 1 1/4" main venturi pipe with a 3/4" copper pipe off the header, i.e. throttling down the flow so not enough water gets routed up to the registers. I've told him to change it, he says it's no problem, he does this all the time and all his customers are happy.

I don't want to wait until the dead of winter to be able to prove to him that it's not throwing enough heat. If I can show him that rasing the room temperature 7 degrees per hour when there's NO HEAT LOSS (i.e. durng the summer), will not be enough to heat the house when the outside temp is -20, then I may be able to force him to fix it. But I need some ammunition here!

Does a 7-degree rise under these circumstance seem reasonable to guys that have a lot of experience in this area?? Seems to me an old house in NY would probably lose a LOT more than 7 degress, but I don't really know.

    Bookmark   September 13, 2009 at 8:37PM
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It hard to tell you about you system with out having more info.Instead of trying to check temp rise, check the temp of the water leaving the boiler and then check the temp of the return water coming back to the boiler If you are dropping
more than 15 degrees then you are not getting the flow you
need. feeding a 1-1/4 with a 3/4is just plan stupid, you need a boiler man that knows what he is doing. Is this a monaflow
system where you have 3/4" loops going off the 1-1/4 main and then coming back into the same main? If so it should be 1-1/4 all the way to the boiler, later paulbm

    Bookmark   September 13, 2009 at 9:33PM
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It's not quite this easy, but not seeing the install or knowing the size of the circ, the 3/4" pipes, restrict the flow to 4-5 GPM, 7-8 if high velocity, which means you get 40,000 to 50,000 BTU's low end and 70,000 to 80,000 BTU's high end actually being delivered to your radiators or convector's. Of course that is way under the potential of the boiler and existing piping.

Where do these installers come from, Sheesh!

    Bookmark   September 13, 2009 at 10:02PM
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Yeah, it's a monoflow system, just as paulbm describes. But it seems to me that if the water temp. is dropping >15 degrees between supply and return then that's a good sign - it means the registers are delivering heat to the rooms. If it's not dropping much then it means it's just circulating through the 1 1/4 pipe and not reaching the registers, right?

zl700, this is good info - if the max. heat that can be delivered through a 3/4" pipe is 80,000 BTUs, then the 225,000 BTU's the boiler is delivering are wasted.

Anyone know of a good boiler/heating consultant in the NY (Scarsdale/N. Yonkers) area who might be able to take a look at it - and has the credibility to tell the installer he's done it wrong?

Really appreciate all the great support I'm getting here!


    Bookmark   September 14, 2009 at 1:30AM
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