Ask the Plumber: What's the Most Efficent Boiler System?

Question: I have an ancient boiler and heating system in my two-family brownstone. What can I do when I upgrade to create the most energy-efficient system?

Good question given the time of year. First, some tough love: Replacing an old boiler is a 20-year-or-more investment. While today’s boiler technology provides greater efficiency than in the past, the piping from the boiler also plays an important role in how efficiently the boiler and system work together and how long the equipment, new or existing, lasts. The distribution piping immediately exiting the boiler (commonly called the near-boiler piping), if not done correctly, can have the effect of a blocked artery, forcing the heart of your system to overwork in its attempt to get the fluid to where it’s needed, at the radiators. The thermostat in your living space will be telling your boiler, “Come on, more heat”, your boiler will obediently respond, but the main beneficiary of the flaring burner and the accelerating rotations of your meter will be the toasty warm rodent in your utility room.

Compounding this inefficiency is the fact that conventional thermostats aren’t as clever in telling the boiler where the fluid is needed as the brain is in telling the heart. Most brownstone homes have a single thermostat placed in some random location. If the thermostat is located on the top floor, because of the phenomenon of heat rising, the folks right there are comfortable but may be disturbed by the chattering teeth of those on the ground floor. Conversely, having the thermostat on the ground floor causes the system to run a longer cycle, replacing the warm air that’s rising from there to the upper floors where the occupants are tossing, turning and coughing in the sauna-like heat. It’s not uncommon to register a temperature difference of 15 degrees or more from the bottom to top level. Small wonder that a wintertime walk down a brownstone street reveals upper floor windows wide open, spilling the expensively overheated air into the frigid outside atmosphere.

Even the pro-active energy saving measures of properly insulating walls and installing thermally efficient windows don’t alleviate the lack of balance and efficiency of a typical brownstone heating system. Indeed, such actions, if effective, inevitably mean that the existing radiators are bigger than needed and need more of that expensively heated steam or water to fill them than is necessary. What is needed is greater control of our boilers in order to make the difference between an efficient and comfortable system and living with wild, costly temperature fluctuations.

Control systems are available as add-on devices in a retro fit application but work best when the system is set up for the type of control they can provide. The picture at the top of this post shows a new boiler and distribution manifold we designed and installed in an Adelphi Street home this week. In this case, we replaced two small boilers with one capable of heating the entire home as well as warming a tank holding the family’s domestic hot water.

The trick here is to take advantage of the fact that the occasions when there’s a demand for both heat in the home and hot water out of the faucet at the same time amounts to a small percentage of the day’s total heating cycle. After all, why spend the money on a big boiler and its energy consumption to keep your domestic water hot all the time for those few minutes a day that you’re showering? In this set-up, we used a smaller boiler with a control that detects when you turn on a hot faucet and immediately directs all of the boiler’s energy to heating the hot water, which it does very efficiently using what is called an indirect water heater (pictured on the right). Turn the faucet off and the boiler automatically returns to its home-heating mode.

The indirect water heater is often preferred to instantaneous or tankless models, especially where venting is an issue. The indirect water heater requires no vent. In this example, there are two main control strategies at work: Primary/secondary piping and priority control. Priority control is the system of enabling the boiler to most efficiently direct its energy to the specific task that is demanded of it. In this case: space heating and hot water heating.

With primary/secondary piping, on a call for heat, a pump moves the water through the radiators independent of flow through the boiler. As the temperature of the water cools down (because its heat is released by the radiators) the boiler is then fired up and another small pump moves water through the boiler (where it is reheated) and injects it into the loop of moving “system” water.

One major advantage: On a 15 degree day, the system may require water temperature of 180 degrees so the boiler will run long enough to meet that demand. However, on a 40 degree day, that same system may only require a water temperature of 120 degrees. For that, the boiler will only run (and burn fuel) a fraction of the time even though the thermostat is still calling for heat and the radiators still receive adequate flow of heated water.

This is called “Outdoor Reset”, which is probably worth a mention, but may become a discussion on its own. This is a simple drawing of that system. The circled arrows are the pumps:

There are many positive results achieved here. The end user will experience lower fuel bills and significantly narrowed room temperature swings by accurately targeting comfort settings. But the smart system designer and future maintainers will enjoy much greater control without over-firing the boiler, more effective air elimination from the piping, less wear on the boiler components as flow through them is at a fraction as it is through the rest of the system, the very efficient selection of smaller pumps that consume less wattage, and more.

As the continuously improved efficiency controls from the market leaders in Germany, Italy, Switzerland and Canada become available in America (oh, for the day when we, as a nation, care as much about energy consumption!) the system is ready for any upgrade without having to incur re-pipe costs. As you know by now, Green building options tend to cost more initially than conventional methods. This is no exception, but there is a big question that should be answered: Shall we finally commit to long-term efficiency and comfort, or the short-term enjoyment of a low-cost installation?

John Cataneo is the chief excutive plumber at Gateway Plumbing & Heating; he’s also been a long-time advertiser and participant on the the Forum.