Want to get the most from your condensing boiler? Lower the temperature.
by Roy Collver
One of the most effective energy saving technologies we have available to us right now, is the condensing boiler. Coupled with a well-designed low temperature hydronic heating system, it is easy to produce a 30% to 50% or more reduction in fuel consumed over more conventional space and domestic hot water heating systems. As much as I am a big promoter of renewable energy technologies, the cost vs. savings profile is still waiting to resolve in favour of renewables – reliable and proven condensing boilers are available today. Many manufacturers have done a good job of developing the equipment; it is up to the hydronic system designer to use good practices to make sure these boilers work at the top of their efficiency range.
Therein lies the pivot point of this article. All condensing boilers work within an efficiency range. Just because a boiler claims to be “up to” 98% efficient doesn’t make it necessarily so (like hardly ever – I challenge you to find just one). Until you condense the water vapour out of the flue products, releasing the latent heat of condensation, these boilers give you a much less remarkable efficiency advantage over a good quality conventional one. A respectable manufacturer will be able to give you data to show you what their efficiency range is and under what conditions – in other words, ask them how they tested it, then ask them to prove it with the proper documentation. I know of no condensing boiler that will operate above 89% efficiency while delivering 180°F water out to the system with a 160°F return (the design parameters of many traditional hydronic heating systems). Most condensing boilers will operate in the 93% plus range if the system is running at 120°F supply 100°F return (the design parameters for many radiant floor heating systems). In between these two, the efficiency will be, well – in between. Variables include: the above-mentioned supply and return temperatures – cooler is better, fuel type – natural gas is better, altitude – sea level is better, how thoroughly the burner burns the fuel – high CO2 = better condensation, what the combustion air temperature is – warmer is better, the ratio of heat exchanger surface to firing rate – more heat exchanger surface is better.
- Altitude you’re stuck with.
- Until we clean the sulphur out of our fuel oil, you will have to use natural gas or propane.
- Quality of equipment (good combustion, good heat exchanger) you can influence through choice (and a fatter wallet).
The biggest other single factor you have influence over in getting the most condensation, is system design for lowest supply and return water temperature – the point of this article.
High mass radiant floor heating systems are a no-brainer. Any time you can use this type of heat emitter, go for it. Tighter tube spacing is the simplest and best design practice for a radiant panel to achieve lower water temperatures. Staple up radiant systems have to be carefully designed to maintain low enough water temperatures and heat transfer plate are a must. The very best candidate for low temperature radiant systems to show the maximum energy savings? Industrial/warehouse/light commercial spaces. Think about it. Concrete floors, high ceilings, poor insulation, high energy bills for heating – perfect.
But if radiant floors are not possible, what other good options are there?
It seems counter-intuitive, but fan coils and air handlers are right up there. Many people believe that a fan coil has to run with 180°F supply water temperature – who wrote that rule? The chart below shows the specs on four different sizes of air handler. Look at the how water temperature effects the output of each unit. The model 40 gives you as much heat at around135°F as the model 23 does at 180°F. The model 99 delivers the same heat at 130°F as the 55 does at 180°F. Also keep in mind that it is really easy to operate an air handler with a high temperature drop on the water side, creating nice cool return water. Just what a condensing boiler thrives on. Oversize your fan coils so you don’t need more than 140°F at design and you’re good to go. Of course you will operate your boiler on outdoor reset so that it doesn’t even get close to the 140°F supply temperature until the coldest day of the year and most of the heating season it will be purring away at below 120°F. There is a little more to it of course. You will probably have to reduce your fan speed a bit but any competent Manufacturer should be able to advise you on the finer points.
European style panel radiators are another excellent choice for heat emitters. You can easily size them to run on as low as 140°F supply water at design, although 160°F is more common. Keep in mind that they are also typically designed for a 40°F water side temperature drop which brings your return water back at 120°F during design conditions, low enough to allow condensation in most boilers. Again, using outdoor reset most of the year, will put you well below that. The chart below shows the effect that water temp has on sizing panel radiators. It can be pretty dramatic as you can see, with the 20” model delivering just slightly less heat at 180°F than the 64” model at 120°F. Most designers will not drop below 140°F in order to keep the radiators a reasonable size and cost, but this is less an issue in warmer coastal climates. Available wall real estate is usually the ruling factor here.
What about conventional baseboard convectors and wall fin? Although not the ideal choice, they can still work well if you have enough wall space available. As you have probably guessed when it comes to baseboards, longer is better when you want to lower water temperatures. The chart below illustrates this. At 140°F supply, the output per lineal foot in all models is approximately one half of the output at 180°F. In other words, double the length of element and you can operate baseboard and wall fin convector in the boiler’s condensing range even during design conditions.
So, work with the numbers, consult with your heat emitter manufacturer, and see how you can best design for condensing operation. It may not be practical in the case of baseboards and panel radiators to lower your supply water temperature all the way down to 140°F, but any reduction will help your boiler work better. Design conditions only occur during a small fraction of the heating season, so don’t run your system hot all the time. This is worth repeating: the design conditions only occur during a small fraction of the heating season. If your boiler rises above condensing mode occasionally, no big deal, given the fuel you will be saving 90% of the year. Crunch the numbers – you will be sold!
Enroll in Condensing Boilers in Hydronic Systems and spend 6 weeks optimizing your system designs and control strategies to take full advantage of the energy savings and comfort benefits boilers have to offer.
About Instructor Roy Collver – President, OTBC Inc
Hydronics Specialist / Educator, Class 1 Gas-Fitter and Illustrator, Roy has applied his creative force and problem solving abilities toward the improvement of hydronics in North America for over 35 years. His varied experience at the very cutting edge of new development in hydronics has allowed him to apply his unique knowledge and skill set toward becoming a premier “translator” of new technology to a traditionally conservative and slow-to-change Industry. His training is up-to-date, comprehensive, and entertaining. After ten years at the top level of his trade “on the tools,” Roy worked closely with many eminent design and application engineers on new and innovative products—from initial concept to final production and marketing. Working in the very “inner sanctums” of these most progressive companies; he was a key partner in helping them design and bring to market, many game-changing hydronics products. He produced many outstanding training programs along with the technical and promotional materials needed to explain the effective application of these new products in the real world.