System Efficiency

[Philip Lewis]

So i was down in the basement looking at the boiler since the thermostat was calling for heat and the pipes were cold. I noticed that the pump was hooked into a thermostat that was clamped to the feed pipe. It was set to turn on when the line temp reached

125 deg F.

Is there a good way of calculating the best temperature for the feed?

The boiler is old and inefficient (I estimate the boiler is at least

50 years old), so i would think that getting the heat to the house quicker would result in a shorter boiler run time.

We have several variables here.

1: cost of running the inefficient furnace 2: energy usage of the pump 3: efficiency of transfer of heat. (the 125F water in the pipes would trasnfer to the 60F room more efficiently than lower temp water since the diff in temperature is greater, yes? ) 4: thermal mass of the system 5: thermal mass of the house 6: air exchange rate for the house (I would say this is high) 7: insualtion in the house (extremely low) etc etc etc

i have no idea where to start on the heat calculation to find out if a lower pipe temp would save money.

My gut feeling is that running the lower temp water through the pipes longer will be more efficient since the boiler is *so* inefficient that anything to get the heat out faster would be better.

Other datapoints: Gas is about $12.71/MCF here electric is $0.0877 /kWh

[Bubba]

Phillip, Spend your time worrying about something much more worth your time. Your old inefficient boiler is going to cost you a lot to operate.....Period! If you want to save money, Turn it off or replace it. A lower pipe temp wont change your fuel bill enough for you to notice. HOWEVER, I would set the pump control from 125 degrees to about 100 degrees. It will all depend on how accurate the control is. Too low and the pump will run constantly and just circulate cold water. Too high and you have warm water left in the boiler that will just be cooled off by the natural induced draft of basement air going up through your furnace and out the chimney. Keep in mind though that a lower pipe temp and longer running pump may cause the temp on your thermostat to overshoot its actual setting. Bubba

[William W. Plummer]

There are several problems with this answer. Heat flow is proportional to temperature difference. And, ineffecient burners waste fuel in proportion to the length of time they are on. Thus in a well insulated house, the furnace will come on just long enough to pump a pulse of heat into the house which will raise its temperature to the desired level. It will then stop. Because of the good insulation, it will be a long time before it has to come on again.

Note that the circulator pump uses quite a bit of electricity, another reason you want to limit the time it runs. Again, a pulse of very hot water is what is best. I don't know how the idea of a continuously running system arose, but it cannot be more efficient.

If your themostat is allowing the temperature to overshoot, it is adjusted wrong. Fix it!

[Bubba]

Bill, I have never seen such utter nonsense posted from someone (you) about a hot water system. Especially in your other post about "bleeding the radiators". Have you even seen one? Im beginning to think you have a tin foil hat. Where did you learn this info? A relief valve is not in a system to purge air from it. A circulator pump does NOT use a lot of electricty. Especially a properly sized one in a residential home. There is NOT cardboard in a vent valve. Pump a pulse of heat???? WTF? You are on your own with that one. An expansion tank is not in a system for pulses or glugs or whatever you were rambling on about. An expansion tank is in a system for EXPANSION of water. Get it? EXPANSION tank? EXPANSION. As cold water heats, it takes up more space. Thus the expansion tank. A place for water to expand to insteading of it increasing the pressure in the system and relieving out the relief valve. You really need to bone up on hot water heating before you spout your gibberish. There are a few very good books out there. Of course, 25 years of on the job training helps a bit too. Bubba Bubba Bubba