High Performance HVAC: Hot Water and Steam Boilers Pages

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The Boiler Water Source

The Boiler Loop

Boiler and Heating Related Links

Boiler Basics

gas boilers, steam boilers, hot water boilers

People who use boilers for their main source of heat report that they love the kind of heat that a boiler provides, whether they have a steam boiler or hot boiler. If you were thinking that there really is only one type of heat, you may want to reconsider. All you have to do is add adjectives to the beginning of the word heat. There is dry heat, wet heat, drafty heat, even heat, and uneven heat just to mention a few. The heat provided by a boiler will give you even, draft-less heat. That is opposed to drafty, uneven heat many forced air systems provide. There are many different combinations and configurations for boilers. There are gas boilers and oil boilers and a combination of the two with dual fuel burners. A boiler is simply a pressure vessel where water is heated for the purpose of providing heat somewhere for something. There are high pressure boilers and low pressure boilers. There are steam boilers and hot water boilers. The focus of the boiler page is not on high pressure boilers or the type of fuel used to heat the water, but on the mechanics of how the loops work and hot water and steam controls on low pressure boilers. Many of the same controls found on gas boilers and oil boilers are the same controls used on furnaces and water heaters. Refer to the furnace page for more info. on gas controls. Oil controls will be introduced later. Boilers can either be hot water or steam (either of those can be gas boilers or oil boilers) and the piping systems are different for each type. The hot water system boiler uses a pump to circulate the hot water while the steam boiler uses its own pressure to circulate the steam throughout the system. They also have some similar and some different controls to run the boiler. Whatever system you may have I will offer maintenance tips for each type of boiler system. Welcome to hot water boilers and steam boiler consumer information.

Boiler Efficiencies

It is important to understand that a boiler in a commercial application (can also apply to residential boiler applications) is going to consume a large amount of the energy budget therefore it is important to track efficiencies because a slight drop in the boilers efficiency can cause a sharp increase in the energy costs. Understanding that, we all know it is important to have good HVAC boiler preventive maintenance practices and regular HVAC boiler system tune-ups including maintenance on boilers. When we talk about boiler efficiency we talk about two related types of boiler efficiencies and both can have an impact on the boiler energy budget. There is boiler efficiency and there is combustion efficiency. A decrease in boiler combustion efficiency will decrease boiler efficiency but not necessarily vice versa.

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Boiler Combustion Efficiency is measured by dividing the usable heat produced in the boiler by the fuel input to the boiler in BTU/h content. This calculation is based on the actual heat available produced by the boiler system after heat loss up the boiler stack and other boiler heat losses which do not provide usable heat. Boiler excess air is partially responsible for the boiler heat loss but it is necessary to complete the boiler combustion process. Therefore it is important that the boiler burner system is tuned and monitored on a regular basis. Using combustion analyzers the O2 and boiler stack temperatures can be monitored for spikes which will alarm the boiler maintenance crew about possible boiler problems. It is important to maintain good boiler combustion efficiency for overall boiler efficiency.
Boiler Efficiency is measured by dividing boiler combustion losses, radiant heat losses from the boiler jacket and near boiler piping, and boiler unknown losses (losses from tube scaling, soot build-up (exchanger thermal efficiencies)) by total fuel input to the boiler in BTU/h. Much of boiler efficiency is determined by combustion efficiency and a lot of boiler maintenance departments focus on boiler combustion efficiencies and ignore the other boiler heat losses.
Boiler Thermal Efficiency is related to how efficient the boiler heat exchanger is working. Things like boiler soot build-up or boiler water scaling can reduce the efficiency of a boiler.
Boiler Steady-State Efficiency is the efficiency of the boiler running full blast under maximum load.
Boiler Overall Seasonal Efficiency is important to track to see if there is an annual degradation in boiler efficiency. It gives you the big picture from year to year for comparison of boiler efficiency.

To get the most out of your boiler system it is necessary to implement a complete boiler maintenance/efficiency plan to maintain every aspect of boiler efficiency. Providing that a boiler maintenance/efficiency plan is put into place and qualified boiler technicians perform the boiler tuning with the appropriate tools a cost savings in energy use can be realized.

Boiler Types

Of the boiler types there are two definitive boiler categories of the most commonly available boilers for residential and commercial use. The first and most efficient boiler is the Condensing Boiler (for more information on how a condensing boiler or furnace works please see the Furnaces Page 2. The next type of boiler is referred to as the Conventional Boiler and among these boiler types there are three boiler sub-categories.

Commercial Fire Tube Boilers (Approximately 24 Million BTU/h Combined)

Cast Iron Boilers - from the smallest residential boiler all the up to 10 million BTU/h boiler the cast iron boiler is the most durable. Cast iron boilers are commonly sectional boilers so the boiler can be torn down to get into tight spaces and then rebuilt. These boilers are most typically used in residential or light commercial heating applications and can be retrofitted with a domestic heat coil for domestic hot water use. For heavier commercial boiler use several of these boilers can be set up on a manifold piping system and offer boiler step-up sequencing to meet higher boiler heating demands when needed. Cast iron boilers can provide either steam or hot water heat in low pressure boiler applications. Boiler combustion efficiencies range from 75% to 93% depending on (whether it is steam boiler or hot water boiler) and boiler combustion controls, boiler flue dampers, sequencing of multi-stage boiler set-ups, frequency of boiler tune-ups and/or air and/or water pre-heaters (boiler economizers).
Water Tube Boilers - from 10 million BTU/h boilers all way up to 300 million BTU/h boilers these boilers are generally found in medium to large commercial/industrial use and can be either steam boiler or hot water boiler in low to high pressure boiler applications. These boilers can be either oil boiler, coal boiler, or gas fired boiler and pass hot flue gases around tubes filled with water. Boiler combustion efficiencies depend on several factors for these boilers including: whether it is steam boiler or hot water boiler, boiler combustion controls, boiler flue dampers, frequency of boiler tune-ups and/or air and/or water pre-heaters (boiler economizers).
Fire Tube Boilers - from .6 million BTU/h boiler up to 50 million BTU/h boiler these boilers use hot flue gases passing through tubes submerged in water generally found in medium boiler to large boiler commercial/industrial use and can be either steam boiler or hot water boilers in low to medium pressure boiler applications. Again as with the water tube boiler combustion efficiencies depend on several factors as noted above.

One other type of boiler not mentioned above is the electric boiler. One could say that an electric hot water heater is an electric boiler although there are electric boilers that can heat water to steam temperatures. Boilers can use many types of fuels (oil, gas, coal, wood, and electric power) to heat water (or other liquids) but the main focus of Boiler Types article was to categorize the mechanical aspects and differences of boilers.

Boiler Control

There are many ways to control a boiler and the boiler controls can be layered. For example: when many people talk about boiler control they talk about the local controls on the boiler itself. These local boiler controls can be set for stand alone operation to maintain desired boiler set points and some boilers do operate stand alone in that fashion. Another, more complex, layer of control can be added to the system with the addition of Direct Digital Control (DDC). DDC control offers boiler control strategies which are only limited by the imagination and boiler equipment. DDC control, a separate system unto its own, is especially good at controlling multi-stage boilers for boiler lead/lag, boiler lead/lag with demand assist (demand load management), boiler multi-pump rotation/operational sequencing, and domestic hot water boiler services. Additionally, DDC control offers a plethora of boiler monitoring and boiler data collection that you would otherwise have to spend a large number of man hours collecting and disseminating. Residential boilers are typically controlled by an aquastat (which can be found on Boiler Page 2) but commercial boilers, especially the higher horsepower boilers use other, more complex control systems. The following boiler control descriptions are for local boiler controls only. The local controls are usually built into the boiler straight from the boiler factory depending on the spec of the boiler system and customer boiler efficiency requirements.

Multi-boiler staging - when you have several boilers and stage them on and off according to demand. Several cast iron boilers are prime candidates for this option. Electro-mechanical and/or solid-state controls are available for this type of control. Solid-state controls measure the delta-T of the supply and return and stage the boilers on based on the difference.
Modulating Control - based on demand this system will limit the amount of fuel and air to the burner. It ranges its boiler firing rate from low fire to high fire and everything in between based on specific input temperatures that determine demand such as delta T.
On - Off Control - The boiler comes on and goes straight to high fire and stays there until it satisfies a pre-determined set point.
Step-up/Step-down - with this boiler control you have two, three, or more firing rates that change based on heating demand. Typically low fire to high fire or low fire, medium fire, and high fire.
Oxygen Trim Control - monitors the amount of O2 in the flue gases and trims the excess air for more efficient combustion.
Excess Air Regulation Control - maintains a specific amount of excess air to fuel ratio for maximum combustion efficiency.
Air/fuel cross limiting control - allows more air to be added to the combustion process before more fuel is added and when stepping down reduces the fuel before reducing the air. This adds safety while optimizing fuel consumption.
Drum Level Control - best used on boiler systems where water level in the drum is critical.

DDC, in addition to everything described above, can offer you a hot water reset schedule that will reduce boiler cycle rate as compared to outside air temperature.

To determine which boiler control option is best for your facility it is best to talk to boiler manufacturers and/or engineers with specific boiler experience.

The Boiler Water Source

Commercial Back flow Preventers

Hot water boilers and steam boilers alike need a water source. This can either be city water or well water supplied by a pump. This is the source for water that is fed into the boiler. Barring any leaks, drips, or weeping in your system, the supply from the well or city is necessary to keep the water level up in the boiler. A dry fired boiler is very dangerous and it is important to keep the water in the boiler at proper levels. For hot water boiler systems the boiler should be full. Steam systems are different. Too much water and the boiler mains flood. The steam boiler will not function properly with flooded mains. A boiler control at the supply should keep the proper water levels in the system. The beginning of the supply (city or well) should have a gate or ball valve to turn the water off to the boiler for maintenance and/or to stop major leaks. When this is shut off the power and fuel source for the boiler should be shut down also. After the shut off valve there should be a back flow preventer, however there may be some older boilers that do not have back flow preventer's. Current local and national codes require back flow preventer's to keep the boiler supply water from being contaminated by back flow water from the boiler. Older boilers are grand fathered so they do not require a back flow preventer. After the back flow preventer there should be a pressure reducer valve (PRV). Many city water sources (and well sources) have pressures exceeding 40 P.S.I. A residential boiler should not operate at pressures over 20 P.S.I. The pressure reducer valve reduces the supply water pressure down to 12 P.S.I. After the pressure reducer, the boiler source supply water for the boiler should be fed into the boiler return or boiler supply loop.

The Boiler Loop

Near Boiler Piping is Important

The beginning of the boiler loop is inside the boiler at the water jackets where the water is heated. The pump kicks on when the thermostat calls for heat and the heated water is pumped up the boiler loop supply line where it flows through the baseboards, radiators, or coils. As it flows through these heat exchange devices the water loses heat and flows back to the boiler in the return line. As it gets near the boiler the water goes through the impeller which is part of the circulator pump. *I have seen a few boilers where the circulator pump was on the loop supply side but this is uncommon. Most boilers in a residential boiler hot water system have the circulator pump on the return side of the loop.

Back Flow Preventer and Pressure Reducer Valve

The picture on the right is of an oil fired boiler my co-worker and I installed in March 2002. After making several minor adjustments it started up on the first shot. The boiler piping design is ideal for servicing the boiler. The boiler isolation valves can be turned off so that the circulator pump can be changed without bleeding the entire loop. Maintenance can be performed on the boiler without introducing air into the systems loop. Additionally, the air purge drains eliminate the need to bleed air from all the baseboards or radiators on start-up. As a service technician, I often wish all boilers I work on were installed with these features. Problems with hydronic loops develop because air gets trapped in the pipes. It's called hydronic lock by some and air block by others but it prevents the water from circulating in the pipes. The solution is to drain the air out of the pipes and with some systems it is easier said than done. With the proper piping, gate valve, and boiler drain arrangement in the near boiler piping this is a cinch. You simply hook up a water hose to the drain in the piping, close a gate valve, and allow the fast fill (or bypass) to flush the air from the loop. Set up properly, you can even back flush the air from the loop. In both residential and commercial boiler loops there air purges installed in the loops to prevent build up of air. Sometimes these air purges malfunction and the air builds up and causes hydronic block. The customer then goes without heat until a technician is called in to find the block and purge the air. Some boiler loops are more complex than others but it is nice to know that the proper valves and drains were installed strategically in the boiler loop system to accomplish the purge quickly and efficiently.

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