This page will be a broad discussion of boiler temperatures and there implication for chimney life and boiler life, as well as the transmission of BTU's most comfortably to the building at the least expense. We would hope that folks will send us their comments for inclusion and discussion.
Boiler operation. We all know the boiler installers installed much larger boilers than were needed over the years because the manufacturers did not instruct them as to what good sizing practices were. Nor did the instructions for installation include sizing practices. Installers failed to read the instructions provided and the piping disasters in the field waste gallon after gallon of fuel oil by not properly balancing the output of boilers with the BTU needs of the building.
One additional poor practice was that of down sizing the gallons per hour being burned to operate the boiler. Some genius felt if the nozzle size is reduced the customer would burn less gallons per hour. The logic was terrific and the customers thought it was brilliant. Problem was that the boiler manufacturers were not asked if there was any sense to the method. When they were asked, manufacturers told folks that if you down size the nozzle, you will under fire the boiler and the fire will instead of heating the water, shoot by and go right up the chimney. Time and again I find boilers in the field that take forever to heat up and find the cause was a less than specified gallons per hour going into the boiler fire box. I asked a mechanic why this was. He asked, "It heats the building, doesn't it. I have not had any heat complaints." When I mentioned that it takes forever to heat the building, and it is wasting fuel. He asked "How and why?" I asked him to call the manufacturer with the model, size and BTU requirement, to find out the gallons per hour required, and be able to run the boiler as designed, more efficiently. Or you could look it up on the manufacturer's website. Answer. " I'll look at it when I do the next annual service." --Right.
Thermal shock: Thermal Shock is the rapid changes in temperature in a boiler when the return water from the heating pipes is cold and the boiler temps at the top of the boiler are hot. The difference in the two temperatures is called the difference or change in temperature or the "Delta T". The problem occurs as the difference in temperature causes contraction and expansion of the metal sections of the boiler which can cause flexing and cracking of boiler sections. It can cause the boiler to leak as sealers used to join boiler sections come apart. In extreme cases it can cause noise, and immediate boiler failure from leaks through cracks in the metal sections. This will cause water to leak into the fire box and steam to come from the chimney. Extreme cracks will cause a loss of fire and flooding. To reduce thermal shock the difference in entry and exit water temperature should be kept to a minimum (20-40 degrees F).
The best way to reduce this delta T, is to have the feed water from the top of the boiler as low as possible and the return water temperature above 140 degrees. The best way to do this is to lower the boiler temperature of water going out to the radiation. This will cause longer circulation cycles and the water returning will be warmer. Delta T (difference in temperature) should be kept to as little as 20-40 degrees F. Additional boiler protection should be provided with a feed pipe pumping water from the top of the boiler to the return pipes at the bottom of the boiler, to preheat incoming return water. This will further protect the boiler from exposure to temperature difference from return ports at the bottom of the boiler and the heated water going out the top of the boiler. Boiler manufacturers instruction for installation will provide for proper boiler protection piping. You should always follow the boiler makers instructions to preserve your warranty
Chimney Condensation: of exhaust gases from boilers can occur when the hot gases cool off on their way up the chimney. Condensation of flue gases can occur within the boiler chambers themselves if the gases leaving the boiler chambers are cooler then 350 degrees. If the stack temperatures are above the 350 degree mark the condensation is limited to the chimney area. Low stack temperatures should be serviced by an approved stainless steel chimney lining that will not corrode or disintegrate from the effect of condensate which can cause the rotting of chimney materials ( brick & mortar) . These relining materials are usually a special stainless steel liner placed in the chimney from top to bottom.
Boiler condensation should not occur if the temperature of the return water is maintained above 130 degrees. For condensing boilers above 80 AFUE separate instructions are provided for proper venting and condensate treatment. The only way to maintain higher boiler temperatures on the return side is to have a bypass pipe running from the top of the boiler to the intake at the bottom. This y pass pre heats the return water with the hot water off the top. This pipe is usually one third to one half the size of the other near boiler piping and may require its own pump when the boiler is activated. This pumping activity adds efficiency to the boiler heat exchange as the water is moving and absorbing the energy from the fire box at all times.
Domestic Hot water from heating boilers must be maintained year around. These temperatures should not go below 130 degrees F as low domestic hot water temperatures can be an environment for the development of Legionaries disease. Legionaries can be transmitted from the less than hot water by the mist in a shower. Be sure you know all you can about this disease before you set domestic hot water temps too low.
End-Use Efficiency: in boiler heating systems supplying circulating hot water are most efficient when the supply temperature is just hot enough to replace the heat loss of the building or home.. Circulating hot water with a 70% thermostat demand ratio should bring the lowest hot water temperature needed to the whole heated space evenly and efficiently. This is an efficient adaptive control technology that applies to all temperature exchange systems. For best results in the design phase the system requirements should be for heating the building on a design degree day( 15 degrees F outside), for radiation to meet these requirements at 140 degrees F. This requirement would need a smaller boiler at less temperature with increased radiation in the house.
For geothermal and solar systems integrated design would allow the excess thermal units to be injected through a heat exchanger into the return water side of the heating system. By increasing the temperature of the return water temperature with excess thermal solar energy the need for the boiler to raise the temperature to meet thermostat demand would be reduced.
5/18/09
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