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 hot water steam 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 will give you even, draft-less heat. That is opposed to drafty, uneven heat that heat pumps and forced air systems provide.
We all know it is important to have good preventive maintenance practices and regular system tune-ups. There are many different combinations and configurations. There are gas or 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 low and high pressure boilers. There are steam and hot water boilers. The focus of the this page is not on the systems 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 steam systems. Many of the same controls found on gas boiler systems and oil boiler systems 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. Systems can either be hot water or steam and the piping systems are different for each type. The hot water system uses a circulator pump to circulate the hot water while the steam system uses its own pressure to circulate the steam throughout the system. They also have some similar and some different controls. Whatever system you may have High Performance HVAC will offer maintenance tips for each type of system.
The following is geared more to commercial boiler control rather than residential boiler control. For more on residential boiler control see our boiler sequence of operation article.
There are many ways to control a boiler and the controls can be layered. For example: when many people talk about control they talk about the local controls A dry fired boiler is very dangerous and it is important to keep the water at proper levels. on the boiler itself. These local controls can be set for stand alone operation to maintain desired 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 control strategies which are only limited by the imagination and equipment. DDC control, a separate system unto its own, is especially good at controlling multi-staging for lead/lag, lead/lag with demand assist (demand load management), hot water multi-pump rotation/operational sequencing, and domestic hot water services. Additionally, DDC control offers a plethora of monitoring and 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 but commercial boilers, especially the higher horsepower boilers use other, more complex control systems. The following control descriptions are for local controls only. The local controls are usually built into the system straight from the factory depending on the spec of the system and customer 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 the cycle rate as compared to outside air temperature.
To determine which control option is best for your facility it is best to talk to manufacturers and/or engineers with specific control experience
The Boiler Water Source
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 at desired levels. A dry fired boiler is very dangerous and it is important to keep the water at proper levels. For hot water boiler systems the system should be full. Steam systems are different. Too much water and the mains flood. The steam boiler will not function properly with flooded mains. A 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 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 systems that do not have back flow preventer’s. Current local and national codes require back flow preventer’s to keep the supply water from being contaminated by back flow water from the hot water loop. Older systems are grand fathered so they do not require a back flow preventer even though it is smart to go ahead and install a backflow preventer to prevent illness from fresh water contamination. 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 source supply water should be fed into the return or supply loop depending on the application and type of system. PRV’s are not necessary for steam boilers as water pressure is not an issue with steam boilers. Water level is of importance for steam boilers so a float type of system is used to control the water level in stream boilers. Typically these float type systems are integrated into the low water cutt-off for the steam boiler. The low water cutt-off is a necessity for all steam boilers as a dry fired steam boiler is very dangerous.
The Boiler Loop
The beginning of the loop is inside 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 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 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 systems in a residential boiler hot water system have the circulator pump on the return side of the loop.
It is important that the temperature difference from the return to the supply never exceeds what the boiler manufacturer recommends. It is different from boiler type to boiler type but for most condensing boiler this temperature is around 20 degrees Fahrenheit while for cast iron boiler this temperature is around 30 degrees Fahrenheit. Depending on the boiler manufacturer if this temperature differential is exceeded then the boiler can experience thermal shock and with thermal shock big problems can happen to the boiler. This is part of the reason High Performance HVAC always recommends a competent boiler technician or contractor perform any boiler work for you. It is important the boiler is sized properly along with the hot water loop piping and that the controls are set up properly. Boilers are not cheap so make sure you get a competent contractor to do your boiler work.
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 piping design is ideal for servicing. The isolation valves can be turned off so that the circulator pump can be changed without bleeding the entire loop. Maintenance can be performed without introducing air into the systems loop. Additionally, the air purge drains and automatic air vents 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 loops the there are air purge devices installed in the loops to prevent build up of air. Sometimes these air purge devices 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 water loops are more complex than others but it is nice to know that the proper valves and drains were installed strategically in the loop system to accomplish the purge quickly and efficiently.
Important note about boilers: All boilers should carry a certification from ASME certifying the system meets or exceeds all specifications from ASME. If you have a commercial boiler chances are very good that the inspector for that boiler has been certified through ASME which sets standards for boilers and pressure vessels.