Condensing Boilers, Don't Force it!!
Before we get started, let me first say that I like condensing boiler technology. The title and summary may suggest otherwise but I am a huge fan of the technology. I have designed many buildings with condensing boilers at the heart of the heating system. Unfortunately, I see far too many projects with condensing boilers that probably shouldn't have them. They may actually be costing the client more money in the long run.
Condensing Boiler Basics
So why are condensing boilers so efficient? When natural gas is burned, there are three major products of combustion; heat, water vapor and carbon dioxide (other byproducts are present, mainly pollutants in trace amounts, but for this discussion only heat, water vapor and carbon dioxide will be considered). In a non-condensing boiler, a significant amount of heat is lost thru the flue. Typically 12-25% depending on the type and age of the boiler; and the conditions that it's operating at. A condensing boiler, operating at ideal conditions, can limit those losses to as little as 2-3%. This is accomplished by "condensing" the water vapor in the flue gasses and reclaiming the latent heat of vaporization. Without getting too detailed about how this is done, the hot flue gas is used to pre-heat the cool return water. The cooler the return water, the more condensation is formed on the heat exchanger and the more latent heat is recovered. As you can see in the chart to the right, the efficiency varies quite a bit with the return water temperature. In fact, the warmest the return water can be and still condense the flue gas is about 125 degrees F. Above that, the boiler operates as a traditional non-condensing boiler with correspondingly traditional efficiency.
A "Typical" Fan Coil Building
The key to achieving condensing operation in a boiler is maintaining low return water temperature; so logic would dictate that the entire system should be designed with a maximum supply water temperature of about 140 degrees to ensure the return is lower than 125 degrees. This is exactly what designers are doing; and while the logic is sound, the practice can sometimes be flawed. One example of this that I see far too often is a fan coil system. These systems are very common in Boston, especially at the many college campuses in the region. A simple system but very reliable and flexible.
Essentially, we have a central cooling and heating plant and a piping distribution system. The "typical" design includes cooling capacities rated with 45 degree entering water and a 10 degree temperature rise. This results in about a 400 CFM/ton cooling rate. In heating mode, the entering water temperature is about 180 degrees with a 20 degree temperature drop in the water. In the Boston area, these conditions result in far more heating capacity than is needed so the sizing for the fan coil units is determined by the cooling capacity. The rest of the system design, such as piping sizes, pump sizing, expansion tanks, etc., follows.
What happens when these conditions are designed around condensing boilers? According to "Condensing Boiler Basics" above, the entering water temperature should remain 140 degrees or lower and the water temperature returning to the boiler should be below 125 degrees. Rating the fan coil at these conditions effectively cuts the heating output in half. A second look is needed to determine if the fan coils can still be selected based on cooling capacity.
- 500 SF/ton (1.25 CFM/SF)
- 20 BTUH/SF (Envelope & Ventilation)
- Heating Degree Days - 5,641
- Cooling Degree Days - 678
- Gas Cost - $1.25/therm
- Electric Cost - $0.16/kWh
100,000 Square Foot Office Building
What happens when fan coils are designed with condensing boilers in a simple office building in Boston? See the sidebar to the right for the building design conditions and assumptions. This building has a cooling load of about 200 tons and a heating load of about 2,000,000 BTUH (the focus will be on the heating system so the cooling system will only discussed for how it is impacted by the changes to the heating system). Good design practice for a boiler plant would be two boilers, each sized at 1,500,000 BTUH each. This allows for some redundancy while lowering the first cost of the system (the sequence of operation will allow the boilers to operate together to keep the firing rate lower and improve efficiency).
Designing the building based on cooling (and keeping this analysis simple), 200 fan coils, each sized for 1 nominal ton is used. This provides 200 nominal tons of cooling, which meets our peak cooling design day. In heating, the 1-ton fan coil provides about 16,000 BTUH of heat which translates to over 3,000,000 BTUH for the building. More capacity than is needed. When the condensing boiler conditions are used, the 1-ton nominal unit only provides 8,400 BTUH each or 1,680,000 BTUH for the building. This is less than our design day heat loss. When using these conditions, the system cannot be designed on cooling capacity anymore, it must be designed for heating capacity. Each fan coil now needs to be a 1-1/2 nominal ton unit. This provides 11,500 BTUH per fan coil and 2,300,000 BTUH for the building. The cooling system is now affected because each fan coil is over-sized for the cooling load.
Building Design Impact
So what affect does changing the fan coils from 1 nominal ton to 1-1/2 nominal tons have on the building? The first thing is that the piping to each unit changes. The cooling flow rate changes from about 2.3 GPM per unit to 3.1 GPM. Seems like a small amount but across the building, it is a rise from 460 GPM to 610 GPM.On the heating side, there is actually a decrease in heating flow rate overall even with the increase in fan coil unit size. The flow changes from 360 GPM to 200 GPM. On the pumping end, the chilled water pumps change from about 10 horsepower to 15 horsepower. The heating pumps change from 3 horsepower down to 2 horsepower. Overall there is an energy penalty. The distribution system is affected as well. Although there is a credit for reducing the pipe sizing on the heating side, there is a penalty to increase the piping size on the cooling system. This piping is larger and more expensive so there is a net increase in pipe size.
Piping & Insulation
Starting with the energy savings, comparing a natural gas condensing boiler with a high efficiency non-condensing boiler, the improved efficiency saves about 11,900 therms. This is based on 97% efficiency condensing vs. 88% efficiency non-condensing. The cost of natural gas for commercial customers in Massachusetts is about $1.25 per therm for a total annual savings of about $14,875. There is an energy penalty for the pumping system that needs to be subtracted from these savings. At $0.16/kWh (average MA cost), the penalty is about $1,000. The real energy cost savings is only about $13,800 per year.
The energy savings has to stand against the increased cost to install this system. The boilers themselves are about $16,000 more than comparable non-condensing models. Some designers are just using this cost and comparing it to the savings to justify their 1.1 year simple payback. The piping changes, pumping changes, variable speed drives and other components all have a cost associated with them. These are spelled out to the right. All in all, the condensing boiler system costs the owner a staggering $110,000 more than the non-condensing system. All for a savings of about $13,800. The client better be happy with an 8 year payback!
Heating Poll Question!
Which factor should be most important when selecting heating systems?
I took some liberties with this analysis but it's not too far off from a potentially real building. I didn't even talk about the effects on the cooling system with over sized fan coils. The energy penalty at the chiller and the effects of occupant comfort due to the short cycling and inability to properly remove the humidity in the building.
There are some perfect applications for condensing boilers. The whole point of this article isn't to dissuade the use of them but to properly apply the technology. If condensing boilers must be used, pair it with a system that inherently benefits from the lower water temperatures. Radiant heating systems and water source heat pumps are great examples of systems that should be designed with condensing boilers.
When faced with this challenge on future projects, think of the big picture. If the system dictates fan coils (or other high temperature terminal systems, ie. finned tube radiation), don't use condensing boilers. If the building or design dictates the use of condensing boilers (EnergyStar or LEED buildings), use radiant heating, water source heat pumps or a combination of these or other naturally low temperature technologies.