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

How to kill a workhorse

The vast majority of residential hot water boilers in service today are cast-iron mid-efficiency boilers. These are the workhorses of the hydronic industry and have been for many years. Installed and maintained properly, they can provide reliable service for 30 years or more. They typically have efficiency ratings in the low 80% range, meaning about 80 cents of every fuel dollar spent is converted to useable heat. The rest is lost up the chimney.

Older cast-iron water boilers from the early part of the 1900s were larger and held more water volume than today’s models. While this larger volume of water acted as a buffer and helped to smooth out some of the variability in water temperatures, it was at the expense of some efficiency. Today’s cast-iron boilers are smaller, and consequentially more efficient, but are less forgiving when it comes to handling low water temperatures.

Sustained low water temperatures can cause flue gasses inside the boiler to cool to the point that they condense on the relatively cool cast-iron heat-exchanger surfaces. This condensate is corrosive and will attack the bare metal surfaces of the boiler, creating rust and scale that can plug flue passageways and interfere with the operation of the burner. At its worst, this condition will allow dangerous products of combustion to enter your home. But at a minimum, it will shorten the useful life of your boiler. Today’s cast-iron boilers need to maintain water temperatures above the 130° - 140°F temperature range to prevent flue gas condensation.

The key to maintaining these safe water temperatures lies in your boiler's ability to produce heat at a faster rate than your house can use it.

Copper finned-tube baseboard

An example of a system that would work well is a home with copper finned-tube baseboard and small copper distribution piping. Many homes built in the 50s and 60s fit this description. Considered a “low-mass” distribution system, its copper tubing and light-weight baseboard emitters heat up quickly. These systems are usually designed for fairly high operating temperatures—typically 180°F. Assuming the boiler is sized properly to the home’s heat loss, it can come up to temperature quickly and has no trouble staying ahead of the home’s heating load. Water returning to the boiler will remain above the 130° - 140°F range for most of each heating cycle.

Where flue gas condensation problems start to develop are in high-temperature/high-mass systems, or low-temperature/high- or low-mass systems.

Cast-iron radiator.

A very common high-temperature/high-mass system where sustained flue gas condensation needs to be considered is an older (early 1900s) home with cast-iron radiators and large steel distribution piping. There are literally tons of cast iron and steel, and hundreds of gallons of cold water that need to come up to temperature before the radiators can start heating your rooms. This can easily overwhelm a properly sized boiler and cause it to run at sub-130° temperatures for long periods of time.

Another type of system that can overwhelm a boiler is a radiant in-floor system of tubes in concrete. This one is a one-two punch for your boiler. Not only are these systems designed to run at low water temperatures (110°F is typical) but the entire concrete slab must be heated before it can start delivering room heat. Some of these systems take days to recover from set-back. And the flue gasses are condensing the whole time. It’s a recipe for disaster.

One recent trend I’ve been seeing is for radiant in-floor tubes to be stapled to the underside of the subfloor and connected directly to a cast-iron boiler. This type of installation would typically run at 100° - 130°F water temperatures. The installer simply turns the boiler aquastat, or temperature setting, down to 120° and walks away. This system will likely condense for its entire—albeit short—life.

The effects of flue-gas condensation.

I’ve serviced boilers subjected to these conditions, and believe me, they’re not pretty. Sometimes there are piles of rust on top of the burners.

The good news is there are ways to protect your cast-iron boiler from low return water temperatures, extend its life, improve comfort and reduce your fuel consumption. Next week, I'll tell you my favorite solution to this problem.

Heidronically yours,

Wayne