Boilers and Hydronics 101


The descriptions and pictures below are designed to foster a basic understanding of the parts of your boiler and hydronic (hot water heat) system.  When you call for service, we hope they will help you to identify those parts of your system that may need service.  When you receive an invoice from us, we hope the information below will help clarify which parts and systems we worked on during our service visit.
A boiler is a device that uses combustion to heat water for space heating and domestic hot water (via an indirect tank).

IMG_1429rotatedThe propane regulator reduces the high pressure found in the propane tank to a safer pressure found in the pipe from the tank to the equipment inside.  The regulator is either on the house or near the tank.  Sometimes in the winter it can freeze up and get stuck, resulting in no propane to the house and no heat.



To check the level of propane in the propane tank, lift the lid and look at the percent gauge.



If your boiler is oil fired, then you can look at the level gauge on your oil tank (inside or outside).  We do not service oil burners but we do service the boiler part (the wet part).






This is a non-condensing (old school) propane boiler that has a steel flue pipe.  This means that it has a hot exhaust and is less efficient.  It also means that it may last longer because it does not develop acidic condensate inside the boiler because the exhaust temperature is so high.




These are non-condensing (old school) oil boilers that have steel flue pipes.  This means that they have a hot exhaust, are somewhat less efficient, but will last a very long time because there is no acidic condensate.  Again, please bear in mind that we do not service oil burners.



These are the propane burner tubes found on the non-condensing propane boiler.






The gas valve admits propane into the burner tubes and is controlled electrically.  This particular model has a pilot light that is lit by a spark during every heating cycle.





Here is the spark ignition system that lights the pilot light





Here is the “roll out” safety switch which shuts down the boiler if it detects fire outside of the burner tubes (such as when the flue pipe is obstructed and the fire is not moving through the boiler correctly.




This is a condensing boiler that has a plastic PVC flue pipe.  The exhaust gas is quite cool because the hot water vapor has condensed to squeeze out almost all of the heat.  This also means that the condensate is quite acidic, which is hard on boilers.



This is the draft inducer that pulls the air into the boiler so that it can mix with propane and combust.  This draft inducer modulates to adjust the output of the boiler to match with the heating load.




The ionization rod is used to detect the presence of a flame because the burning propane air mix is plasma which is conductive.  If there is ignition failure, then no current will conduct and the ionization rod will tell the gas valve to shut off.

31eboi8UbtL._AC_UL160_SR160,160_hot surface

The hot surface ignitor glows red hot to ignite the propane-air mixture.  It is quite delicate and breaks easily.   Sometimes boilers have spark ignitors instead of hot surface ignitors.


The gas valve opens when there is a call for heat and will stay open as long as there is plasma present (burning propane-air mixture).






The pressure gauge shows the pressure of the water inside the boiler.  It is typically at least 15 psi.  Buderus condensing boilers will not work below 15 psi so we set them a bit higher.



The boiler fill station reduces the pressure from the plumbing system (usually between 30 and 80 psi) to the 15 psi used by the boiler and hydronic system.





The pressure relief valve is set to open at 30 psi to relieve an unsafe buildup of  pressure in the boiler and hydronic system.  These valves usually start to dribble at 25 psi.





Sometimes the pressure gauge is combined with a thermometer that shows the system temperature.






The boiler and its hot water is connected to the space to be heated by hot water piping.





The flow of hot water through the piping to different zones is controlled by zone valves.  Here is a Taco gold power head which has been the industry standard for decades.





Sometimes the zone valves are motorized so that they open more quickly.  Here is the Taco “Sentry” motorized valve that we use.




To move the hot water through the heating system, circulators are used.





Thermostats in each zone talk to a zone valve control box (made by Taco) that turn on the zone valve, the circulator, and the boiler.




426-3-5Taco can vent


Hot water will not flow correctly through the heating system is air is present.  To purge or remove the air, a small “can vent” is used on high points of the system.




Even better at removing air is the Spirovent air separator.  They are highly effective at removing dissolved air bubbles.




Before the Spirovent was invented, many systems had these really ineffective “air scoops” with small can vents at the top.





T8775-2In each heating zone, thermostats are used to measure the current air temperature and tell the heating system to send hot water to that zone.  This thermostat is an example fo the traditional round electromechanical type.  The older ones had mercury, the newer ones are digital instead.TH6110D-c2


Here is an example of a typical digital thermostat.  For more choices, please visit our thermostat pages (conventional and internet based).




In the heating zone, the hot water might go through radiators to release its heat.  We commonly think of hydronic heating in the conventional sense to mean water at 140F or hotter.







In other zones, the hot water might go through baseboard fin tube.




A third way to heat a space is through a radiant ceiling or floor using lower temperature water (85F to 120F).  For more information, please see our Radiant Heating 101 page.


Our goal is to be Martha’s Vineyard’s premier plumbing and HVAC outfit – we appreciate the opportunity to service, install, and rescue your boiler and hydronic system!