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

What every boiler owner should know about indirect water heaters

Indirect water heaters have been around since the 1970’s in this country, but somehow, even after all this time, they don’t seem to be very well understood. They get their name from the fact that they’re heated “indirectly” by your boiler. They’re connected via piping to your boiler and circulate relatively hot (usually 180 to 200°F) water from your boiler to a heat exchanger within the water heater. The water surrounds the coils of the heat exchanger to produce your domestic hot water. This is in contrast to the typical gas- or oil-fired water heater that heats water through the use of a “direct” flame or heat source within the water heater.

If your home is heated by a boiler — any boiler (hot water or steam, oil or gas, mid- or high-efficiency) — you have every reason to heat your domestic hot water with an indirect water heater. Some of the benefits include:

• High efficiency — When you use your boiler as a heat source to produce your hot water, the water is heated at the same efficiency as your boiler. Some high-efficiency boilers reach 96%. The average gas water heater is about 60 - 70% efficient.
• Reduced heat loss — An indirect water heater is very well insulated and loses very little heat during long stand-by periods (at night or while you’re away). A gas water heater is always losing energy up the chimney.
• High performance — The performance of an indirect water heater is a direct result of the boiler it’s connected to. Given the size of most residential boilers, it’s not unusual for an indirect water heater to produce two to three times the amount of hot water as a standard gas water heater.
• Longer life — Most indirect water heaters have a lifetime tank warranty. It will likely be the last water heater you’ll ever need. The manufacturers can offer this warranty because their tanks are not subject to the abuses that a direct-fired gas water heater is. And many of them are made from stainless steel to prevent corrosion.
• Hot water storage. Available in sizes ranging from 30 - 200 gallons or more, hot water can be stored for high-volume flow-rate usage when you need it.

 The photo at right shows a cut-away view of a typical indirect water heater, with the coil located at the bottom. Boiler water is circulated through the coil and surrounded by the domestic hot water in the tank.

“But Wayne,” you may ask, “doesn’t this mean I need to run my boiler all summer now, too? Won’t that cost me a bundle?” Au contraire, my energy-conscious friend. The only time the boiler fires is when you need hot water. And when it does fire, it’s producing hot water at a higher efficiency rate than a typical water heater. Other than that, it sits quietly and waits. The belief that the boiler is running all summer may be a throw-back to the days of tankless coils in older boilers. They had no storage capacity, so the boiler needed to stay hot all the time to produce hot water on demand.

“This is all well and good,” says the the intrepid Internet traveler, “but I’ve been hearing that the new tankless heaters are the greatest thing since Apple went public. Can’t I save a lot more money with one of those?”

Well, speaking of apples, this is really an apples vs. oranges comparison. A tankless heater usually has little or no storage capacity, meaning it needs to heat the water instantaneously. And it can do that — with some limitations. Older tankless heaters drop the temperature at high flow rates (two fixtures running). Newer models limit the flow rate to maintain the delivery temperature. An indirect heater can handle that high flow rate without reducing the temperature or flow. And, depending on the size of your boiler, it can do it while delivering almost limitless hot water.

Better tankless heaters have efficiency ratings in the 80 - 95% range but typically last just 15 - 20 years. An indirect water heater connected to a 15-year-old cast-iron boiler will deliver hot water at 80 - 82% efficiency. And one connected to a modern modulating/condensing boiler can deliver 96% efficiency. So while tankless water heaters are efficient, they can't beat indirect water heaters.

Indirect water heaters can also reduce your maintenance costs. Having only one gas- or oil-fired appliance means less service. A tankless heater needs to be serviced every year. If not, you can count on reduced hot water production, possible heat exchanger failure and loss of warranty coverage. An indirect water heater requires little additional maintenance beyond your annual boiler safety and performance check-up.

The next time you’re in the basement, take a look at your existing standard water heater. If it's a little worse for wear, you may want to remember that the typical water heater life is 12 years. Wouldn't it be nice to replace it — once and for all?

Heidronically yours,

Wayne

# 7 — Humidity: Why a hydronic heating system is better for your home’s comfort level than forced air

And finally we're at number seven of the Top 7 reasons why a hydronic heating system is a better choice than forced air — humidity.

If you’ve lived with a forced air heating system for any amount of time, you’re quite aware of the drawbacks associated with managing your home's winter humidity levels. You’ve experienced the static shocks, the dry throat and skin, the stuffy nose and even nosebleeds. You might even have grown to accept it as a normal part of the winter heating season. Or maybe you invested in a humidifier to add moisture back into your home.

I’m here to tell you it’s not normal. It’s your heating system working against you by drying out the air as it’s being blown throughout your house. Your furnace acts like a wringer to squeeze the moisture out of the air. The air dries up as it’s repeatedly heated up and cooled down. The furnace blower also pressurizes your house — it forces conditioned (heated and humidified) air out of any little cracks or crevices and draws cold, dry air in to make up for the warm, moist air you’ve lost. Then your humidifier kicks on to add that moisture back in. And the cycle repeats nonstop, every time your heat is on.

A hydronic heating system is better for you (and for your home) because it never dries the air out in the first place. A well-designed hydronic heating system generally operates at a much lower temperature than a furnace,  producing much longer and gentler heating cycles. The lower water temperatures keep the relative humidity at a comfortable level. There's also no blower, so there’s less air infiltration, meaning less of your conditioned air (that you're paying to heat) goes out the doors, windows and cracks throughout your house.

Most experts agree that a comfortable range of winter indoor humidity is 25% - 50%. Below 25% and you begin to experience the uncomfortable effects of a dry environment. Above 50% is concern for mold growth. Most of the homes in our area (upstate New York) outfitted with hydronic heating systems have no need for added moisture to the indoor air. There’s usually enough humidity added through showers and cooking that a supplemental humidifier isn’t necessary. Bathroom exhaust fans and kitchen range hoods help to keep the humidity below the high end of the comfort range. (Some very tight, well-sealed homes may require supplemental ventilation to maintain a healthy indoor environment but these are more the exception than the rule.)

Now that we've come to the end of this series on why a hydronic heating system is a better choice than forced air, I'm sure you can see that there are major advantages — not only in comfort, but also in long-term efficiency. And don’t forget the rest of these advantages:

• Reliability
• Versatility
• Quiet
• Cleanliness

I hope you’ve enjoyed reading this information as much as I’ve enjoyed writing it. And please — feel free to comment. I’d love to hear what you have to say. And let me know if you have a particular question or topic of interest you’d like to see discussed here in the future.

 

For next week, I’ll discuss what every boiler owner should know about indirect water heaters. (Also known as, why a boiler owner should never consider a tankless water heater!)

Heidronically yours,

Wayne

# 6 — Cleanliness: Why a hydronic heating system is cleaner than a forced air system

We've made it to installment #6 of the Top 7 reasons why a hydronic heating system is a better choice than forced air — Cleanliness. I hope you're still with me.

A forced air furnace with its blower and duct system will stir up and move dust, pollen and other impurities throughout your home. Some will remain airborne and inhaled while others will land on surfaces and leave a layer of dust that can again become airborne during cleaning. Then the cycle continues. Some of those impurities come to rest and cling to the inside of your air ducts.

Whole industries have sprung up to prevent and remediate the issues created by the "burner and blower" concept of heating. The sale and installation of HEPA filters, UV lights and electronic air cleaners have produced plenty of revenue enhancement for the furnace guys. And it seems duct cleaning services are being offered by everyone with a shop vac — from handy men to furnace guys to cleaning outfits. You gotta admire them, though — invent and sell a heating system that pushes dirt and allergens around your house, then offer add-on solutions to fix it. Hmmm.

As we've discussed in previous installments of Heidronics, your hydronic heating system uses a completely different heat delivery method. Pipes and tubing deliver heat from your boiler to your rooms without stirring up a trace of dust. The radiators, panels and convectors deliver their gentle heat without blowers or fans. The result is a cleaner indoor air environment with less airborne dust and allergens. You'll also find you need to dust less. And, of course, you'll avoid the added expense of replacement filters and duct cleaning.

Another hydronic advantage, especially for low-temperature radiant systems like in-floor or ceiling radiant, is a reduction in dust mite concentrations. Studies have shown that dust mite populations are significantly lower in homes equipped with low-temperature radiant heating. They've also shown a lower incidence of allergic reactions to dust inhalation. You'll find more detailed information at environmental ergonomics expert Robert Bean's website healthyheating.com.

Next week I'll wrap things up (and maybe dispel a myth) with a discussion about humidity. You may be surprised.

Heidronically yours,

Wayne