Will Mod/Cons work efficiently on Copper Tube Baseboard?
The answer to this question, which has been asked many times, is yes, it can work on copper tube baseboard systems. The question I ask is "how efficiently?" This is not a cut-and-dry answer. I have seen systems that operate OK with the boiler and systems where the boiler can work fairly efficiently. Remember, there is a difference between working or working OK compared to working well or efficiently.
Let's first understand how and when these boilers condense. The boiler fires at a given rate and modulates its gas input up and down to maintain a setpoint water temperature, adjusting according to the outside air temperature. The control uses an OD reset sensor and operates on an OD reset curve to determine water temperature. Hopefully, your OD sensor was installed when the high-efficiency boiler was installed, or it was not turned off after installation. The boiler will modulate to that water temperature. The lower the temperature, the more condensate will be produced. Conversely, the higher the water temperature, the less condensate will be made, and the boiler will cease to condense, resulting in decreased efficiency. The boilers are most efficient when condensing. How, you ask? Good question. The boiler flue gases (exhaust) will be cool enough to run at the dew point by sending cool water through a running boiler; the flue gases are at a certain temperature. When the cool flue gases give off enough heat, the boiler starts sweating on the heat exchanger, just as the grass does in the cool summer mornings when the ground is still warm, or a cold beer on a hot summer day. In the boiler, we have given up enough heat (BTUs) from the flue gases that the boiler starts to sweat. While this is happening, we can recover about 9% of the heat from flue gases into the home heating water. The magic temperature at which this all happens is usually around 132F +/-.
Looking at these two charts, we can learn two things. The higher the water temperature gets above the dew point, the boiler stops condensing, and the efficiency is reduced. So any modulating/condensing high-efficiency boiler will react the same way. This is physics, and we cannot change physics
Now let's look at our baseboard heating systems. Let me first say that any style of boiler will work in any system. I know this is a very general statement, but within reason, high-efficiency boilers, cast-iron boilers, those that are too large or too small, and those that are micro-zoned, will all work and heat OK. The difference between working OK and working well (efficiently) is two completely different things. Working well means the boiler is properly sized, properly piped, vented correctly, and not too many zones (thermostats). The more zones we have, the less efficient the boiler will be unless the smallest zone's BTU output is less than the minimum firing rate of the boiler.
All high-efficiency boilers should run for at least 15 minutes at a minimum time. The longer the boiler runs, the more efficient it is.So, back to the question asked in the header. The answer is "it depends," or should I say yes or even no? There is no clear-cut answer because not all the facts are available. Since the question asks "Efficiently," the question must be more specific. It depends on two basic things. Proper boiler sizing and the amount of lineal feet of element in your baseboard per zone that would exceed the actual heat loss. As the ratio of boiler heat output (BTUs) and the amount of baseboard element increases, the water temperature decreases. As the water temperature decreases, the boiler efficiency increases. I know systems with copper tube baseboard that operate at 127f water temperature at 32f OD air temperature. This guy is squeezing every drop of heat and efficiency out of his boiler. This cannot be done without fine-tuning the boiler OD rest control and proper boiler sizing. We cannot ignore the fact that if the boiler is designed to input 150,000 btu/h, but the burner is running at a reduced input of 15,000 btu/h. All that fireside heat surface and a slower fan speed will keep the efficiency a little higher.
Maximizing your efficiency starts with an accurate heat loss, which we can reasonably price for you. Next, determine your maximum water temperature setting, adjust the low OD air temp for your area, and dial down the fan RPMs to meet the heat loss, which limits the maximum input. Most units will allow you to do this for heat and hot water needs.
The more baseboard element you have above the heat loss of the zone the lower the operating water temperature can be. The closer the heating element is to the heat loss, the higher the water temperature must be. There are charts to help you figure this out. Once the heat loss calculations are complete, add the rooms together per zone to determine the maximum water temperature needed in that zone, and use this value as your maximum boiler BTU output.
The question was, can a mod/con high-efficiency boiler operate efficiently on a copper tube baseboard system? Yes, if everything is done correctly, from boiler sizing to fine-tuning the control. While you may not continuously operate at maximum efficiency, the boiler still reduces fuel input, which in itself saves fuel.
You might want to check your boiler's run times. Many boiler controls on the market will offer you run time hours and boiler cycles. Figuring out the average runtime is easy if your boiler model shows boiler cycles and run time hours on the display. Check your boiler's installation manual to find out if it does. If the cycle time is too short, less than 15 - 20 minutes, it is time to make your boiler more efficient.
Where;
RTh = Run time minutes
RTm = Run time hours
BC = Boiler cycles
RTm = (RTH*60)/BC
For example RTH = 758
BC = 37098
RTm = (758*60)/170985
2.66 = 45,480/17,0985
The boiler is short-cycling
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