August 2, 2014 by MARK NORRIS
How do you determine the efficiency of the heating system that you are recommending to a customer? It is an important question that comes up regularly, but there is no simple answer. The simple answer would be the published boiler efficiency, right? Simple, maybe, but probably not correct because the published efficiencies will rarely be an accurate representation of real system efficiency. The testing that residential boilers undergo for annual fuel utilization efficiency (AFUE), or the BTS-2000 tests that commercial boilers are subject to, are controlled environment tests, intended to provide a base line for all appliances in that category. These appliances are tested to the same conditions, under the same standard, so you can compare them all with equal data. Unfortunately, it is not possible to represent all of the variables that our heating systems are exposed to in what is essentially a laboratory test. There are also several things we can do to make our system efficiencies better that the tests do not consider.
So where do we get the information required to determine our system’s efficiency? Go to the source for that installation. Look at the design, the equipment and how you intend on using them. Under the right conditions you can potentially exceed the AFUE or other standard tests in the field. Other times you may not even get close to the rated efficiencies. Mostly it comes down to the designer or contractor installing the system. In the case of a retrofit, the equipment that will remain will also influence the efficiencies.
When designing or assessing a new or retrofit system the place to start is the heat emitters. Higher temperature emitters will be less efficient than lower temperature types. Fin tube heat emitters, for example, require hotter water and more energy to supply the same amount of heat to the space in comparison to lower temperature radiant floor designs. Older fan coils will require higher temperature water than newer fan coil designs, so the same logic applies (see Figure 1). It starts with how hot you need to get the water to provide enough heat on a design day (coldest day of the year). Do not make the mistake of thinking that you will need to supply 82-85C water to fin tube heat emitters for them to function.
You can get a lot of heat out of a higher temperature emitter at lower temperatures if you have enough emitters. The same applies to most other types of heat emitters as well. The heat emitter manufacturers will have a chart or table that will give you a method to determine how much heat their product will deliver at lower than the published design temperatures and flow rates.
With any hydronic heating system, outdoor reset temperature controls can adjust the system water temperature required to heat the space on the design day to the lower temperatures needed for a warmer winter day. Remember, heat moves from a warm place to a colder place faster with a greater temperature difference, so the building will lose heat faster on a colder day. Building envelope efficiency is also a factor in the rate of heat loss. A building with good insulation and tight windows will lose heat more slowly than one with poor insulation and leaky windows on a day of the same outdoor temperature. Because the heat emitters do not change from warmer to colder days, we compensate for that greater heat loss by increasing the water temperature, or reducing it on warmer winter days when the heat loss is less. That is what outdoor reset does automatically (see Figures 2,3).
The three to one rule states that for every three degrees C that the average heating system temperature is reduced, one per cent less fuel will be consumed. This works with all hydronic heating systems. In reality we typically only need our design day temperatures for about two to four per cent of the heating season almost everywhere in Canada. How many contractors change the factory reset heating curve settings? I find that nearly half of the students in my classes do not change the heating curve values because they do not understand what it does, or how to adjust them correctly.
If I apply that same outdoor reset control to a high efficiency condensing boiler, I also get the added value of the condensing process returning energy that would normally be lost to outside through the venting system back into the heating system. When the flue gas changes from a vapour to a liquid (condensate) it has to give up a bunch of energy. When that happens at the boiler’s heat exchanger, that energy can be transferred to the heating system (see Figure 4). How much energy is that? It is approximately 8095 Btus per USG of condensate.
The deeper the reset, the more condensate can be produced and the more energy is recovered (see Figure 5a). Measure or calculate the amount of condensate coming from the boiler to determine how much energy is recovered for determining efficiency. The efficiency numbers from your combustion analyzer only tell part of the story. Your combustion analyzer only sees the sensible heat (non-condensing) portion of the process. However, your combustion analyzer does provide more information about the boiler’s efficiency than simple efficiency percentage.
If you are using a burner that requires combustion setup, how much excess air that is introduced will change the CO2 percentage and consequently the dew point. Newer premix design burners require little or no setup because they can be factory calibrated. Even if the boiler’s burner is preset, you still need a combustion analyzer to verify the machine is working as designed and is operating safely. Without combustion analyzing you are driving a car with no front window.
Dew point temperature will affect when and how much condensate we get from a condensing boiler and therefore effect efficiency (see Figure 6). The higher the C02 in the flue gas, the higher the dew point, so you can start condensing higher return temperatures. If we lived in a perfect world, we could set our burners for 0 per cent excess air (Lambda 1.0, or Stoichiometric) and get the higher efficiency at slightly higher flue gas temperatures. Altitude, humidity, dirt and so on, make it necessary to add excess air so we do not potentially produce CO, which is toxic. As shown earlier, adding too much excess air will lower the efficiency.
Another way to control efficiency in a condensing boiler is to increase the system delta T (the difference between the supply water and the return water to the boiler). Because the rate of condensate is based on the boiler return water temperature, not the supply water temperature, a larger delta T between supply and return can increase the boiler’s efficiency by returning the water to the boiler below the dew point. You can increase the delta T simply by reducing the pumping speed.
If we build a system around a standard 15C delta T with a design day supply temperature of 71C the water will return at 56C, higher than the dew point of approximately 54C at 10 per cent CO2 in the flue gas. If we increase the delta T to 20C or 25C we can get the return temperature down to 51C, or 46C respectively, below the dew point. This will potentially get the boiler condensing even for those few design days (see Figure 7).
Oversizing the boiler will cause boiler short cycling on milder days and that is a big efficiency robber. Undersizing the boiler can be just as bad because boilers with modulating burners will spend more time at a higher burner output. We can typically add between 1C (at low fire) and 12C (at high fire) to the return water temperature value when calculating the flue gas temperature for wall hung condensing b
oilers, depending on the burner’s actual modulation output. Once you know the estimated dew point from the excess air calculation you can determine the amount of condensate based on the return water and boiler modulation calculation and therefore the energy being recovered. From this we can see that a condensing boiler with a lower firing rate that is not short cycling will provide higher efficiencies.
When I talk about oversizing, I am talking about the minimum output not really the maximum output. Boilers with modulating burners and efficient burner algorithms will modulate to find the lowest firing rate needed to do the job. But the minimum output is the lowest that burner can go and if the load is smaller than that, we get short cycling under partial load conditions, which occur most of the heating season. Increasing the system mass with a buffer tank or more heat emitters can give us extra stored load capacity for systems using low mass boilers. This adds to system installation costs but is worth doing under some conditions.
So, are bigger turndowns better? Not necessarily. More excess air is typically required for lower firing rates to maintain flame stability. The larger the turndown, the more excess air is required. We have already seen that higher excess air makes it harder for the boiler to produce the condensate that is the result of the higher energy recovery. This results in a strange little relationship that ends up with less fuel burned but what is burned is burned less efficiently.
How do we increase system turndown without adding too much excess air? Let’s look at this scenario. Our building has a turndown between the design day load and the base load (smallest load we still need heat for) of eight to one. The boiler design has a functional turndown of five to one. To install a boiler that can supply enough heat for the coldest day, it will be oversized on the warmest day that heat is required by roughly eight per cent. That may lead to boiler short cycling. However, the total boiler turndown can be increased to 10 to one if two smaller boilers are connected with a cascade control that can efficiently operate them as a team. This requires communication between the boilers and the cascade so the cascade can know how hard each boiler is working.
Depending on the cascade control’s functionality, on mid level loads you could operate two boilers in low fire instead of one boiler at a higher firing rate and reduce the net flue gas temperature. This keeps the flue gas temperatures further below the dew point, which will produce more condensate. Because the lowest firing rate is 1/10 of the maximum, less heat is provided without short cycling on partial load days.
Multiple boilers will cost more to operate and install than a single boiler because of the extra pumps, controls, piping and so on, but this can usually be offset by the higher condensate volumes and reduced short cycling that results when multi boiler designs are applied to buildings with larger turndown loads. It also provides a level of redundancy in case of equipment failures.
Looking at the maximum boiler output compared to load, a boiler system that has a larger output than design will provide extra capacity for morning boost to recover from the setback faster (see Figure 8). This also requires enough emitter capacity to take that capacity and a control that can provide a boost function. (This is not the optimization algorithm that some setback controls have; this is the ability to increase the outdoor reset set point by a value for a short time during morning warm up. Most optimization logic starts the system earlier or later depending on previous history).
Up until now I have focused on the combustion and thermal heat transfer efficiencies. Electrical efficiencies are also part of the equation. It is estimated that pumping accounts for up to one fifth of the electrical energy used in a hydronic heating system. One pump manufacturer estimates that 99 per cent of pumps in commercial buildings are oversized. When you consider that most of the year we are under partial load conditions this makes the oversizing worse.
As we include more zoning in our systems the volume of water circulated changes. The use of high efficiency variable speed pumps can reduce the electrical consumption for pumping by up to 75 per cent.
While I believe that variable speed pumps are a wise consideration for a lot of pumping needs, they are not a complete solution. Changing the flow through the heat exchanger of a modulating boiler will tend to lead to burners hunting to find the stable output. This is because a boiler sees varying flow as a varying load. This is another good reason to pipe systems in a primary secondary configuration.
A fixed speed circulator through the boiler stabilizes the flow so the burner can be more efficient in its modulation, and selected variable speed pumps in zones where zoning requires flow changes to maintain delta T through the heat emitters.
In summary, remember the follwing points and you will be well on your way to delivering truly efficient systems:
• Lower temperature heat emitters set up to operate with a larger delta T and flow rates that can adapt to zoning flow changes will get us lower return temperatures.
• Higher CO2 levels in our flue gas will get us higher dew points.
• Lower return water temperatures and a higher dew point will lead to higher efficiencies for the hydronic system.
• Higher mass systems with more heat emitter capacity and burners sized for minimum loads with extra capacity and control for morning boost will reduce short cycling.
In addition to the topics covered here, efficient piping layouts, pipe insulation, and water treatment, will also affect the overall fuel, thermal and electrical efficiencies of the system, but these are topics on their own. <>Mark Norris is a technical instructor with Viessmann Manufacturing Company Inc. www.viessmann.ca