Maximizing Mod/Con Efficiency

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 boilers operate OK and systems where the boiler can work fairly efficient. Remember there is a difference between working or working OK compared to working well or efficient.

Let's first understand how and when these boilers condense. The boiler fires to a given rate and will modulate its gas input up and down to try to maintain a setpoint water temperature according to 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 not turned off after installation. The boiler will modulate to that water temperature. The lower the temperature the more condensate will be produced and of course the higher the water temperature the boiler will stop condensing and the efficiency will decrease. The boilers are most efficient when condensing. How you ask, good question. The boiler flue gasses (exhaust) will be cool enough to run at dew point by sending cool water through a running boiler the flue gasses are at a certain temperature. When the cool flue gasses 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 (BTU's) from the flue gasses that the boiler starts to sweat. While this is happening, we can recover about 9% of the heat from flue gasses into the home heating water. The magic temperature that this all happens is a boiler is usually around 132f +/-.

Looking at these two charts we can learn two things. The higher the water temperature gets above 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, too large, too small, micro-zoned they will all work and heat OK. The difference between working OK and working well (efficiently) are 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 is not less BTU output than the minimum firing rate of the boiler.

All high efficiency boilers should run at least 15 minutes at a minimum time period. 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 due to not having all the facts. 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 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 actually 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 start with an accurate heat loss which we can do reasonably priced 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.
Maximizing boiler efficiency. When you take the time to do this you will be operating as efficiently as possible.

The more baseboard element you have above the heat loss of the zone the lower the operating water temperature can be. The closer the amount of heating element you have to the heat loss the higher the water temperature must be. There are charts to help you figure this out. When the heat loss is done you can add the rooms together per zone to determine the maximum water temperature needed in that zone and use that 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 properly from boiler sizing to fine tuning the control. While you may not always operate at the 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 hour 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.

To calculate the runtime in minutes the formula would be;
RTm = (RTh * 60)/BC

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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