Cruise control for your hot-water heating system

Last week I talked about how sustained flue-gas condensation can shorten the useful life of a cast-iron hot water boiler. Today I’d like to share with you my favorite method for preventing flue-gas condensation while making your home more efficient and comfortable.

The goal of any control and/or piping strategy designed to eliminate sustained flue-gas condensation in a cast-iron boiler is to maintain the return water temperature above the dew point of the flue gasses. With natural gas this dew point is typically 130˚ - 140˚F. There are several variables that contribute to the returning water temperature and to the flue-gas temperatures. These include the type of room heat emitter, the volume of water in the system, the size of the boiler (as compared to the heat load) and the ambient combustion air temperature, to name just a few. All of these variables are unique from system to system. They can also change from cycle to cycle in the same system.

Here’s an example: Let’s say you have a system with cast-iron radiators and large distribution piping. This may originally have been a gravity hot water system from the early 20th century. There’s lots of water and metal to heat up in a system like this. Let’s also imagine it's a been a sunny but cool day so your home has benefited from solar gain during the day and the boiler hasn't fired for hours because the thermostat is in that south- or west-facing dining room. The sun goes down, the dining room cools, the thermostat calls for heat and your boiler fires up.

Now think about the water inside your heating system. It’s been sitting there all day in the cold pipes and radiators. It can’t be any warmer than the air in your house or basement — probably 60˚ - 70˚F. It starts to circulate through the boiler, the piping and the radiators. Maybe it runs for a half hour or so, and the boiler manages to heat the water up to about 80˚ or 90˚F. (Remember, flue gasses are condensing this whole time.) Then the thermostat becomes satisfied and the boiler shuts off.

An hour later the thermostat calls for heat again. This time the system warms the water a little more — but still not to the point where condensation is eliminated.

These cycles continue throughout night with each subsequent cycle warming the water a little more, until finally it reaches a point where the return water temperature rises above the flue gas dew-point. Most of the cycles in this particular (but very common) example share the fact that they’re producing low return-water temperatures. It’s the temperature of the returning water that changes from cycle to cycle.

What’s needed to prevent this condensing condition is a control and piping arrangement that can adjust itself dynamically to the changing system conditions. I’ve found that a variable-speed injection-mixing system works perfectly in this situation.

Injection mixing controller

It involves some piping changes near your boiler and the addition of a small, electronic controller to manage the temperatures and control a mixing circulator. The system piping is separated into two loops — a primary loop out of the boiler and back in, and a secondary loop that just circulates water out to the radiators and back. Then I connect those two loops with a piping “bridge,” and hot water from the primary (boiler) loop is “injected” into the secondary (distribution) loop. The controller monitors the temperatures of both loops and adjusts the rate of injection in order to maintain a minimum boiler-loop temperature. It does this by speeding up or slowing down the circulator in the “bridge” to let the boiler catch up to the distribution system’s ability to take the heat away. The beauty of this system is that it can automatically adjust for varying system conditions and provide continuous boiler protection.

Another feature of the electronic controller is its ability to adjust the boiler and the system water temperature in relation to the outdoor temperature. This is called outdoor-reset control. A sensor reads the outdoor temperature and feeds that information back to the controller which then determines the temperature water needed to heat your house at that moment. It can allow the secondary (distribution) water temperature to modulate between, say, 70˚ on a warm day and 160˚ on a cold night. It will also modulate the boiler’s set-point temperature while never letting it drop below its condensing temperature. It’s like cruise control for your heating system — just the right amount of heat at the right time with long, low-temperature cycles.

Outdoor-reset control can save a significant amount of fuel, especially in the “shoulder” seasons when your boiler’s full output isn’t needed.  It will also make your home more comfortable. By lowering the distribution water temperature, each heating cycle is longer and the room temperature swings are minimal, making you more comfortable.

If you have a home that’s heated by a cast-iron boiler and has a high-mass distribution system (such as cast-iron radiators or radiant heat in a concrete floor), you will benefit from this control strategy. Longer boiler life, lower fuel cost and more comfort is a win, win, win!

Heidronically yours,

Wayne