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Outdoor Reset Boiler Controls

Conventional boiler controls produce one temperature of heating water, a temperature calculated to keep your house warm on the coldest day of the year. (In Boston, the coldest day’s temperature, or design temperature, is between 9 degrees and 0 degrees F.) Each winter, the design temperature is reached for only a few days and sometimes only for a few hours, but the boiler operates with a water temperature to match this outdoor temperature for the entire heating season, a season in New England which stretches from October to April (or May). Weather sensitive or outdoor reset control on your boiler allows the boiler water temperature to modulate with the outdoor temperature.

Every three degree drop in boiler water temperature equals a one percent fuel saving, so using outdoor reset saves substantial money. For example, if it is a 0° F. design day outside, your boiler will produce 180°water. When the temperature warms to 40°, an outdoor reset control will allow your boiler to produce 150° water, a drop of 30° in water temperature or a ten percent fuel saving.

Outdoor reset controls can be added to your system at any time and do not require the system to be drained. Sensors are added externally and the initial cost is for the control and the electrical wiring. We recommend adding reset to almost any existing forced hot water heating system for immediate fuel savings, system comfort, and efficiency.

Pumps and Zone Valves

Heating system circulators, or pumps, move heated water from the boiler to distribution units (radiators, radiant floors, baseboard, or hydro-air heating coils) and then back to the boiler again. When we design a heating system, we select a circulator that can move the maximum amount of heat- carrying water needed by the building (or portion of the building served by the pump) on the worst day of the winter. But pumps do not come in an infinite number of sizes and also do not come in very small sizes, so we often have to choose one which has more capacity than necessary. Also, the heating needs of a space change depending on many things, including the outside temperature and time of day so the maximum capacity of the pump might only be required a few days or hours a year. Until recently, the pump could only draw electrical energy at one level when it was on- the maximum, over-sized level.

This overuse of energy gets even worse in systems with multiple zones, each served by its own pump. The zones are often small pieces of the whole space’s heat loss, and the smallest pumps available are capable of covering the whole system, so each zone multiplies the overuse of electrical power.

Pumps are now available with ECM (electrically commutated motors), which allows them to change their electrical draw according to the demands of the heating system. When all zones are calling for heat, the pump uses as much energy as necessary to circulate the large load. As zones are satisfied and drop out, the electrical draw also drops. The pump is essentially customizing itself to the system. In a multiple zone set-up, the ECM pump can save thousands of dollars in its lifetime. This is why we choose to pipe our customers’ zoned systems using a single ECM pump and zone valves, rather than pumps on each zone.

Zone valves are motorized valves which open and shut when the zone thermostat instructs them to. They are available with easily replaceable operators and have good reliability. Manufacturers like TACO make relay boxes which allow all the wiring for zone valves to be organized in one place with the proper transformers. The boxes have indicator lights that allow us to track the progress of a call for heat in each zone.

Non-electric TRVs

NON-ELECTRIC THERMOSTATIC RADIATOR VALVES or TRVs are devices which can be added to a steam or hot water radiator to control temperature at the individual radiator. A wax element in the valve expands and contracts with ambient room temperature, opening and closing the valve to maintain whatever temperature is set on the operating dial. These devices are useful in control of over-sized or overheating radiators, as they shut the radiator off when the desired room temperature is reached. They are installed between the radiator and air vents on steam units and in place of the hand valve on hot water radiators.

Rooms can be individually zoned simply by installing trvs on their radiators. The operating head of the valve can be placed directly on the radiator or the valve can be operated remotely with a wall-mounted dial, attached to the valve via capillary tube.

Zoning: Why, why, why?

Heating zones have been touted as energy saving measures, and many customers have shown us their multi-zoned systems, proud of the number of zones they have. But when we look at how the house’s space is divided and used, the “zoning” is often nonsensical. Sometimes the installer has “zoned” in the easiest way the piping will allow. Old gravity heating mains, for instance, were often piped left and right side, or front and back through all the floors in the house, to shorten the distance the water had to travel to and from the boiler. An installer may say he can zone using the existing piping, but it makes no sense to have a left and right division. Customers have been sold zoning for an upstairs and downstairs scheme, but once their children can go up and down stairs themselves, the temperature in the living room and the bedrooms will stay the same as people move back and forth while they are awake, and then the whole house temperature will be lowered when the family goes to sleep. We have customers in whose homes there may be seven programmable thermostats on seven zones and all of them are set to come on and turn off at the same times and temperatures. That is actually one zone with seven expensive control systems stuck onto it.   What is the point of a zone?  Zones need to make sense with the way the house is used, but they have to obey the laws of thermodynamics as well. If the zoned rooms can’t be thermally isolated from one another, there is no point in pretending the btu’s will stay in their respective areas. Heat goes to cold no matter what. You may turn the thermostat down in the dining room, but if there is an open archway between it and the living room, the living room zone will attempt to heat the dining room as well. No savings -in fact, lost money in the equipment wasted in setting up the “zones”, and plenty of discomfort in the drafts caused by wandering btus. A third floor guest room with a door that stays closed, or a basement family room isolated by the cellar door can be zoned successfully- the temperature can be set lower than the occupied part of the house and it will stay that way. Another reason to set up a separate zone is to accommodate heating units with different heating qualities which would not work well together on the same thermostat- for instance, a main house with cast iron radiators and a playroom with fin-tube baseboard. The cast iron will heat more slowly and hold its heat much longer than the fin-tube. If the thermostat is in the room with the radiators, the baseboards will be cold as soon as the thermostat turns off. Put the thermostat in the playroom and the radiator rooms will overheat.   The final factor in deciding whether and how to zone a heating system is the effect of the zones on the boiler’s efficiency. The most efficient way for any boiler to operate is in long runs where the boiler can reach its steady state. Think of a car’s mileage when it is cruising on the highway, vs. stuck in stop and go traffic. Having a system with many small zones can cause a boiler to stop and go, or “short cycle”. If one little office or bathroom is asking for heat, the boiler (sized for the whole house) will quickly make that heat and have to shut down, and the process will repeat itself until a larger space asks for heat. Even a modulating boiler, which is designed to drop its firing rate when less output is required of it will short cycle; there is a limit to how far the boiler can turn itself down. Combining unnecessarily chopped up zones into a larger load can remedy a short cycling problem. On a new installation, fewer zones will have a lower upfront cost and will operate more efficiently over the lifetime of the system than multiple small zones.   See discussion of outdoor reset controls, air sealing and insulation and alternative ways to make domestic hot water for other tools to save energy.

The Hartford Loop

Steam Boilers–The Hartford Loop: Insuring Safety

In the dawn of the Age of Steam, boilers used to blow up with terrible frequency. There was none of that awful government interference with people’s right to be killed by unsafe products, and by goodness, killed they were. Coal fires weren’t easy to control, and systems were allowed to carry high pressures without a good way to relieve that pressure. Safety devices, especially ones to keep the boiler from “dry firing” didn’t yet exist. Dry firing occurs when there is a leak in the system below the water line. Water leaves the boiler and allows the fire to overheat the iron of the boiler shell. If more water is added to the vessel, the water immediately flashes to steam and if there is no way to let the pressure out, a bomb ensues.

Insurance companies began to insist on safer heating equipment, and among other improvements, a simple piping configuration which trapped the water in the boiler, keeping it isolated from any leaks below its water line, became accepted practice. It was called the Hartford Loop after the city central to the insurance industry.

A proper Hartford Loop is taken off the equalizing line of the boiler piping with its center 2″ below the normal water line. The shortest possible nipple and an immediate elbow turn the loop down to meet the wet return, forming a water seal. The connecting nipple must be short since this is the place where steam in the equalizer and condensate from the returns meet and water hammer will occur if there is room for the steam/ water slug to gain momentum. Water hammer from the Hartford loop will typically happen in mid-cycle. If you hear banging in your steam piping, you may well have an improperly piped Hartford loop.

Residential Steam Boiler Maintenance

If you are unfamiliar with any of the devices or procedures listed here and would like a tutorial, we would gladly provide one for a single hour fee. If your boiler was installed by Pipelines, the tutorial is free. Please call to schedule an appointment.

During the heating season:

Check the boiler water level weekly. The water line in the glass tube (gauge glass) should rest between ⅔ and ¾ up when the boiler is cool.

Check the low water cutoff monthly. When the boiler is firing, drain water from the low water cutoff into a bucket until the gauge glass appears empty. The burner should shut off. If the burner continues to run, shut it off with the service switch or thermostat or place the gas valve in “pilot” setting. Do not allow the burner to run without sufficient boiler water. The low water cutoff must be serviced as soon as possible if the burner runs at low water condition. Slowly refill the boiler to proper level – too much cold water on hot metal can crack the boiler. Be patient. Do not refill the boiler or leave the fill valve open when unattended. For float type cutoffs, use the monthly check to flush out the float chamber. Place a bucket under the chamber drain, open the valve and allow the water to run until it is clear of sediment. You may have to refill the boiler several times to get clear water, if this flushing has not been done regularly. Leave the water at the proper level before returning the burner to service.

Be aware of system leaks. The system should recycle its water, going from steam to condensate and back to the boiler to be made into steam again. If vents or radiator connections are expelling water (like a teething baby), or live steam (like a tea kettle), they should be repaired or replaced. If you are adding water to the system more than once a month in the heating season, you have a problem which needs to be addressed. Adding a lot of fresh water to the boiler shortens its life, adding oxygen-induced corrosion to the system, producing more sludge and debris and reducing the boiler’s efficiency.

Your thermostat

Your thermostat is just a dumb switch.

OK, not as dumb as a light switch, because it knows if the temperature in the room has dropped below or risen above its set point, and it can turn itself on and off, and sure, some thermostats have schedules you can program in so they’ll turn on or off at a certain time, and most have a clumsy way of adapting themselves to the type of system you have so they don’t bounce on and off constantly, or overheat wildly, but really, your thermostat, even your sexy Nest, is really dumb. On or off. Those are your choices, and the on choice is ON, full throttle, heat me like it’s January First, 0 degrees Fahrenheit, and there’s a 40 mile an hour gale blowing. Your thermostat is to your heating system what a car with a gas pedal fixed at 60 miles per hour would be. Put the car in gear and away you scream at highway speed, even if you only need to go to the corner store for milk and bread.

About that Nest thermostat by the way; sure it can tell if you’re in the room, and it’s probably told the NSA all about it. Sure you can turn it up from your smart phone. The problem is, it knows nothing about your heating system, NOTHING, including what temperature it is outside. It doesn’t know the first thing about heat, and it doesn’t care. Nest has seduced you through your smart phone, and is codependent with your lazy ass as well. Why should you program your thermostat when the NSA can do it for you? The only efficiency the Nest gives you is that it keeps turning itself off. If you are too benighted to set a schedule on your programmable thermostat, I guess that counts as more efficient.

What’s the alternative? Something like Tekmar’s TN2 system, in which the thermostats are actually sensors, not switches, and the sensors can talk to one another and to the boiler so that the system knows what is going on at many points in the house as well as what the outside temperature is and can optimize the temperature of water the boiler will make, how long the boiler should run and where left over heat should be sent at the end of each heating cycle. Tekmar’s thermostats are the smart ones, and a system with them will also “learn”, but at a much deeper level than the Nest. TN2 thermostats have a touch screen; you can access them with a smartphone or computer, make changes in the system and get reports on what is going on. They were designed by heating engineers, to make heating systems work better. More heat geek than Candy Crush. Your choice.

Boiler Types

Atmospheric Boilers are vented into a chimney. If your boiler is connected directly into your chimney, this is what you have. Atmospheric boilers produce waste gases which are hot enough to be less dense than air. The products of combustion move up through the chimney because of their own buoyancy. Air for combustion is usually drawn from inside the house. This is the least efficient type of boiler, but also the least expensive to buy and install. Atmospheric boilers approach, at best, 85 percent efficiency.

Sealed Combustion Boilers are designed to draw a bit more combustion heat from the flue gas, leaving the gas too cold to move up a chimney on its own, requiring a fan which pushes the products of combustion out of the house through special stainless steel or plastic vent pipe. Air is drawn from the outside to provide for combustion. They are slightly more efficient, and therefore less expensive to operate but more expensive to buy and install than the atmospheric type boiler.

Because the boiler does not use the existing chimney, and because the type of venting it uses has severe length limitations, it can and should be installed near an outside wall in the area of its vent penetration. This type of boiler requires a carbon monoxide detector to be installed in the basement and plug-in detectors to be installed on the floors that have bedrooms. Sealed combustion boilers have 85 percent plus efficiencies.

Modulating-Condensing Boilers capture more heat from their combustion gases than conventional boilers, and they are also designed to condense the water vapor in the combustion gas, which captures the latent heat in the vapor. They have modulating burners which can ramp up and down in response to the needs of the space heating or domestic water heating system. These boilers have efficiencies in the 90 percents. Because they purposefully condense water vapor, they require a way to dispose of the condensate which includes neutralization (the condensate is acidic and dangerous to the drainage system). Condensing boilers can be floor mounted or hung on a wall. They are mechanically vented, usually through a side wall of the house. Condensing boilers have the same carbon monoxide detection requirements as sealed combustion boilers. They are the least expensive to operate but the most expensive to buy and install.

Heating Whaddayagot

Being able to answer a few questions about your heating system will help us come prepared to help you on an emergency call, or can help us save time and take money off your bill on a scheduled call.

What fuel do you use?

Most people know this one. Gas, oil, or electricity are the main choices.

1.oil tankIf you have an oil tank which you have to fill, there’s your clue you use oil to heat your space.
2.gas feedNo oil tank, but a black pipe connected to your boiler or furnace? Likely gas. The picture is of a water heater, but you get the idea.
3.electric bbd jpgElectric baseboards or perhaps hot air made by a heat pump.

What type of heat maker do you have?

A furnace heats air which is delivered to your space through ducts and registers. This is forced hot air heat.
Hydro-air is a hybrid system in which a boiler heats water which is sent through coils in air handling units. Air passing over the coils is heated and sent through the ducts and registers.
A boiler is a vessel in which water is heated and sent through pipes to the heat emitters in the space. The water can stay as a liquid or be heated until it forms steam. Either way, it’s a boiler and you have what is called hydronic heat (hydro for water).

What type of boiler/ hydronic heating system?

SteamHot waterForced or Gravity
Steam boilers have gauge glasses which allow you to see where the water line is in the boiler. If you have a gauge glass, you have steam.
Hot water boilers have expansion tanks. They provide room in the system for water to get bigger as it gets hotter5.expansion tank

4.steam gauge glassHot water systems can be either forced or gravity (this is extra credit stuff). Very old gravity systems, which were put in before electricity, were designed to move by virtue of the differing densities of hot and cold water. They have no pumps. They will have large pipes in the basement and radiators for heat emitters. Some former gravity systems have now been adapted with pumps to become forced systems. More modern forced hot water systems have pumps and smaller pipes.

What type of heat emitters?

Hot Air / HydroHydronicSteamHot water

6.hot air vent

Hot air and hydro air systems will have registers.

Hydronic systems may have radiators, fin tube baseboard, convectors, fan coil units including kickspace heaters, or radiant floors, walls or ceilings.

8.fin tube 9.open fin tube 10.convector


Steam radiators will have steam air vents  or steam traps.

12.steam vent
Steam Vents
13.steam trap
Steam Traps
15.steam only
Steam Radiator
14.one pipe steam rad
Steam-only Radiator

If you have just one pipe attached to your radiator, it is most probably a steam radiator.

If your radiator has a connection only on the bottom of all its sections, it is a steam-only radiator. See how there is a “pipe” through the bottom, but only spacers at the top?


Hot water needs to be able to make a circle through the radiator so hot water radiators have a connection through the top of all the sections, like this. Steam can use this radiator as well, so check for steam vent or steam trap for positive identification.

Hot water radiators have air vents like this. Note the vent is at the top of the radiator, where the air in a hot water system collects. Steam vents should be in the middle of the radiator.


How is your system controlled?

For gas fired systems, do you have standing pilot or intermittent ignition?

Standing pilotIntermittent ignition
Standing pilot systems have thermocouples which sense the pilot flame. The gas valve will have a thin copper tube or silver, braided wire which runs into the firing chamber of the boiler, and the top dial on the gas valve will show the words “on”, “off” and “pilot”.




  • Intermittent ignition systems will only show “on” and “off” on the gas valve. They have systems which light the pilot each time the boiler has to fire. This version uses a spark to light the pilot gas. The orange rubber coated spark wire is the clue.


For gas fired systems, what voltage is used to operate the primary control (the gas valve)?

Milivolt24 volt120 volt

Milivolt– These older systems generate their own power- enough to operate the safeties, thermostat and gas valve via the pilot flame. Needless to say, they are all standing pilot systems. Instead of a thin copper tube, the gas valves will have a silver, braided cable attached to the top terminals of the gas valve.

120 volt– Pre-war systems may have 120 volt controls, and also often have mechanical safety devices with mercury in them. Oil fired equipment may have 120 volt controls as well.

On-Demand Water Heaters

On demand water heaters heat water as it passes through them. Opening a hot water faucet starts water flowing through the unit, turning its burners on. Once flow stops, the burners stop. The unit stores no hot water. These facts account for the efficiency and fuel savings found in on demand heaters, and also for the differences in their behavior. Following is a list of pros and cons, then a longer explanation follows.

Click to view larger image
  • More compact, wall hung, space-saving
  • No stand-by loss. Saves energy.
  • Option available to those who have hot air furnaces.
  • Provides an unlimited source of hot water at a limited rate.
  • Unit is made of high quality material. Less likely to end up      leaking and sent to a landfill.
  • Most models qualify for rebates from the gas company.
  • Needs large gas feed.
  • It takes time to get hot water to the tap. The column of cold water between the unit and the opened tap must be exhausted before hot water reaches tap. There is no gravity “creep” in the hot water line bringing hot water closer.
  • A minimum flow is required to start burners- you cannot get hot water at a very low flow.
  • There is a limit to the volume of hot water you can run at once; there is no “dump load”. You cannot open your tub faucet wide and fill the tub.
  • You need to accept that to get hotter water, you have to run the water more slowly.  

Because the unit must make the hot water needed the moment the water is passing by its “doorway”, the engine required to make that energy transfer is very large. By large we do not mean dimensionally, we mean large energy output. The more hot water it must make at one time, the larger the energy input has to be, to keep up. Burners will modulate with change in flow up to the maximum rate the unit can achieve. The units are rated for gallons per minute (gpm) at a listed temperature rise. Temperature rise means the difference between the incoming water temperature and the temperature at which you would like your hot water to be. The listed temperature rise is important to pay attention to because we live in a climate where the incoming water can be colder than 40° F. in the winter. Manufacturers of many on demand heater brands have southern headquarters and factories and at first advertised their outputs with 60° temperature rises. We need to see at least a 90° rise for hot enough water from an on demand system. When comparing performance of indirect heater brands, look for the 90° rise column to see how much water the unit can make at any moment in our climate.

The maximum output of the on demand unit will determine how many taps can be opened at once. To calculate the number of gallons per minute your heater must supply, you should know that in Massachusetts, shower heads are supposed to operate with no more than 2.5 gpm flow, and new sink aerators hold flow to 1.25 gpm. An open tub spout can draw 5 gpm or more, as can a top loading clothes washer. An on demand heater with a 5 gpm rating at a 90° temperature rise would allow you to run 2 showers at once, or to wash dishes and shower simultaneously, but you could not start the top loading washer while someone was in the shower. Well, you could, but one of you would be sad.

On demand heaters store no hot water, and therefore cannot waste energy as the stored water loses its heat either up the chimney, or through the pipes. A negative effect of this efficiency is the amount of cold water which has to run out of the tap before the hot water reaches you. In Europe, where indirect water heaters are common, the units are put as close to the fixtures they serve as possible, usually in the same room, which alleviates the foot tapping, water waste problem. Also, because the unit shuts down when the tap is shut, if you want to run small amounts of water repeatedly, as in washing and then rinsing dishes, you will get slugs of cold water mixed in with the hot.

2.norman on demand
Click to view larger image of the on demand high efficiency water heater

Some new on demand heaters are very efficient and well made. (Some are literally disposable- they are designed to be unrepairable [!] and must be completely replaced if any part breaks). A modern, condensing on demand heater can be much like a high efficiency boiler both in energy savings and in complexity and installation cost. They are vented through the side wall of the building, have acidic condensate that needs to be neutralized and disposed of, and need proper combustion set up and periodic maintenance.