HVAC Boiler Page 2 Quickpage Jump links:

Boiler Expansion Tanks

Boiler Aquastats

Boiler Thermal Shock

The Steam Boiler

Steam Boiler Safety

Common Problems with Steam Boilers

Behaviors of the Furnace/Boiler Flame

Boiler Reviews

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Expansion tanks found on an Apollo (or water to air) Heating System.

Boiler Expansion Tanks

All hot water boilers must have an expansion tank. Water expands when it is heated and there must be a place for it to expand to so that the pressure in the boiler loop system remains constant. Without an expansion tank the pressures in the boiler loop system would vary wildly and most likely will exceed the maximum pressures designed for the boiler loop system. This will result in setting off the pressure relief valve. There are two different types of expansion tanks that can be used on hot water boiler systems. The most modern is the bladder type expansion tank (see picture to the right). The other type of expansion tank is a basic cylinder or barrel looking tank that hangs from the ceiling or is located in the attic. These tanks need to be drained from time to time to make sure they are not full. If they fill up with water and have to be drained constantly there is a problem with your boiler that should be looked at by a boiler technician. The bladder type expansion tank does not need draining on a regular basis like the cylinder or barrel type tank.

Boiler Aquastats

Aquastats are like the typical HVAC thermostats except it controls the temperature of the water inside the boiler and is used mainly in residential applications. Again there are many different configurations for the way the boiler aquastat is hooked up so I'm going to stay with the basics. The aquastat has a sensing bulb that is inserted into a well in the side or front of the boiler. This bulb senses the temperature of the water inside the boiler. When the temperature drops below a certain set point (usually 140 degreesF.) it turns the burner on so that the water can be heated to the maximum temperature setting on the aquastat. The burners are then shut off until the temperature again drops below the set temperature. These temperature settings are usually manual and are different from one type of boiler to the next. When you turn your thermostat up you are turning the circulator pump on (on hot water systems) so the hot water can circulate through the pipes. The hot water leaves the boiler and makes the loop. By the time it gets back to the boiler it has lost some heat through the heat exchange process in your radiators or baseboards (and any un-insulated pipes under the house or in the loop). The cooler tepid return water is sensed by the bulb in the aquastat and the aquastat again turns the burner on to reheat the return water so it may make the trip through the loop again when the thermostat calls for heat.

Boiler Thermal Shock

Boiler Thermal Shock is a big problem for some boilers especially when the loop piping is not installed correctly or the loop flow and/or boiler firing is not controlled properly. Boilers using steel and cast iron can crack under constant or heavy thermal shock conditions. If you take a piece of steel or cast and heat to a high temperature and then throw cold water on it the steel or cast iron will most likely crack depending on the quality of the steel or cast. This happens to a lot of boilers when they are not set up properly. Many manufacturers recommend never exceeding a 20 degree Fahrenheit delta T between the supply and return. Consult an HVAC professional to ensure your boiler doesn't suffer from boiler thermal shock. Boiler thermal shock will reduce the life of your boiler system drastically.

The Steam Boiler

The residential steam boiler is still alive and well despite the fact that some HVAC professionals refer to it and the steam boiler profession as the dead mans club. However, there are not a lot of HVAC boiler technicians or HVAC companies that specialize in steam boilers because there are not a lot of the steam boilers used for residential and light commercial applications. People have either converted their old boiler steam systems over to hot water or have gone with forced air systems. The concept is simple but the mechanics of it can be frustratingly complex when a problem occurs unless you have the experience and knowledge that comes with working on the residential or light commercial steam boilers. Before we dive into some of the problems that arise with steam boilers, we'll cover safety first. After all, HVAC safety is the most important aspect to any home heating system.

Steam Boiler Safety

The steam boiler must super-heat the water inside the water cavity to more than 212 degrees F. If the proper controls are not in the system a steam boiler can reach dangerous temperatures and possibly cause a fire. For a steam boiler to operate at the proper temperatures it needs some basic safety controls. Some of these are:

• Limit switches which will open if the temperature rises above a certain set point need to be in the system. Limit switches (manual reset) located near draft diverters will shut the system down if the chimney or flue gets plugged up. This prevents carbon monoxide poisoning or conditions. Limit switches near the burner manifold detect roll-out and prevent fires or worse.
• Low water cut-off. If the water in the water jacket falls below a certain point a dangerous condition will exist. The water level in a steam boiler should never go below half-way in the sight glass. Dirty sight glasses should be cleaned. Additionally they should be checked to ensure that they are not plugged with crud of some sort. A regular visual inspection of the sight glass is good practice especially in the heating season.

  Steam to Water Heat Exchanger

• Automatic water feeder. An automatic water feeder, when functioning properly, will keep the boiler water level at the necessary levels needed to maintain good steam heat. Minor adjustments are necessary so that the right level is maintained in the boiler. Too much water and the system will not heat. Not enough water and a dangerous condition can exist.
• Residential steam boilers operate under very low pressure conditions. However, a large volume of low pressure steam can be a danger. For this reason there are pressure controls on the boiler. Normally set at 2 P.S.I., the pressure control will shutdown the system if too much pressure is sensed by the pressure control. There are redundant features to the pressure cut-off control. A pressure relief valve should be maintained on the boiler. If the boiler exceeded the pressure of the rating on the relief valve, the relief valve would open and release pressure. The steam boiler should be blown down once a week during the heating season or while it is in use. Blow down valves are found on the water feeders and low water cut-offs. It looks like a lever and most are yellow (some are red). These valves should be opened allowing the water to flow into a bucket or sump. Water is normally drained until it is clear or you fill a five gallon bucket. This ensures fresh water is introduced into the steam boiler on a regular basis and can help keep the cruddy deoxygenated water out.

Common Problems with Steam Boilers

• Water Hammer or banging pipes. This is a common problem that can be reduced if not eliminated with the proper devices. Water hammer occurs when the condensed water meets the steam. Cool water meeting hot steam causes the pipes to expand and contract rapidly. This makes it sound like someone is down in the basement beating on your pipes with a hammer. The system could have a problem with the return condensate water getting back to the hartford loop and eventually the boiler. It could mean a strainer or trap is plugged with sediment and limiting the return of condensate water returning to the boiler. The automatic water feeder will continue to keep the water level in the boiler but the system will have too much water in it and the mains will flood. It is important to keep the return piping system efficiently returning water to the boiler to prevent water hammer. Consult an HVAC professional about reducing or eliminating this problem.
• Steam Vents leaking water. Steam radiators on one pipe systems should not be level. Often the floor will settle and the radiator will begin to leak water from the steam vent. A shim can be placed under the legs of the radiator opposite the pipe. This will allow the condensed water to drain out of the radiator back to the return line to the boiler. If after placing a shim under the radiator and the leaking continues, consult an HVAC professional.
• Radiators not heating. This can be the result of several things depending on if the radiators are two-pipe or one-pipe. First check to make sure the valve is open at the base of the radiator unless it has a thermostatically controlled valve. If it is one-pipe the thermostatically controlled valve will be at the steam vent. If it is two-pipe this valve will be on the supply pipe coming into the radiator. If you turn the thermostat all the way down the valve should shut. Turn it all the way up and the valve should open. If none of these things work it is probably best to consult an HVAC professional.
• Total Dissolved Solids (TDS) can build up in the boiler if a blow-down of the boiler is not done on a regular schedule or set frequency. Dissolved solids in the water that are caught in the steam settle in the boilers. Without regular blow-downs the solids build up and cause problems. Additionally, they reduce efficiency and create safety issues. This is something usually done by the homeowner or maintenance person (if it is a business) but should be done according to the advice of an HVAC professional as far as blow down frequency is concerned.
• The build-up of carbonic acids can eat through hydronic piping and needs to be addressed by either venting oxygen and CO2 or having water treatment. Consult and HVAC Boiler expert for more information.

Learn more about hot surface igniters on the Furnace Page. The following is an excerpt from the furnace page and is related to boilers.

Characteristics and Behaviors of the Furnace Flame

The flame that your furnace or boiler produces can effect the performance and efficiency if it is not properly burning the fuel1. A professional HVAC Technician will have high tech specific HVAC tools so that your furnace can be tuned to where the mixture, fuel and air, is proper ensuring the furnace is getting the maximum amount of efficiency possible. This is super important considering that the cost of fuels, whether it is natural gas, propane, or oil, is continually rising. The first test that can be performed is simple and requires no tools, but does require knowledge of what a proper flame should look like. Natural Gas should have blue flame with a very minimum amount of yellow tips on the top of the flame. Too much yellow tips would mean there is not enough primary air or a combination of some other problems where a simple adjustment can be made to correct the problem and get the most out of the combustion problem. Problems other than yellow tipping are rollout, flashback, lifting flames, and floating flames. All these problems should be corrected to get the most out of your furnace or boiler. The next test that can be performed is the combustion analyzer test. An expensive HVAC tool must be used for this test. An electronic device that is capable of reading and analyzing several processes of combustion can give the HVAC technician clues about any potential problems so that some sort of action can be taken to correct it. A combustion analyzer will tell the HVAC technician the stack temperature, carbon monoxide level, carbon dioxide level, and oxygen levels in the exhaust gases leaving the furnace. This combustion test should also be done in conjunction with a furnace or boiler draft pressure test. It is important that all the byproducts of the combustion process are exhausted outside the home. A furnace or boiler draft test can determine this will happen. A visual test using a mirror can also be done to ensure there are no blockages of the chimney or the flue. All the byproducts of the combustion process are analyzed and measures taken to correct any abnormal readings by making adjustments according to manufacturers recommendations. Annual combustion testing is especially important for oil burners. Finally, the last test that can be performed by an HVAC technician is a gas pressure test. These pressures are different from one furnace or boiler to another and from natural gas to propane to oil. Propane and Natural Gas are measured in water column inches using a manometer. It is important that the proper fuel pressures are present in the furnace or boiler not only for efficiency purposes but also for complete combustion. All these test can performed by a qualified HVAC technician and can make a big difference in the amount you pay for the heating fuel you use over the colder months. Call an HVAC contractor to schedule your furnace or boiler tune-up today. They can send a qualified HVAC technician to make sure you are getting the most out of your furnace or boiler heating system.