Mechanical system explained

While writing the last post about our boiler going into lockout mode, I realized that after all these years I never fully explained our mechanical system.

I have posted a time lapse showing the utility room setup and described the PEX and radiator installation, including the difficulties we encountered along the way, but I have yet to show the components of the mechanical system in the utility room and how they work together.


The diagram above does exactly that, except most of us will have some level of difficulty deciphering it. Let me try to put it into a format that is easier to follow, starting with a list of the main components (I won’t list all to keep it reasonably simple):

Boiler (Trinity LX 150)

This pieces of equipment is basically heating the water for the hydronic heating system (radiators and radiant floor slab) as well as indirectly heating the domestic hot water (DHW).

Boiler pump


This pump is moving the water through the boiler into the buffer tank while the boiler is firing.

80 gallon buffer tank (insulated)

This tank is feeding the hydronic heating and also (indirectly) heating the domestic hot water system. The tank temperature is set to 140 degrees Fahrenheit.

Due to our low energy load on the space heating and DHW side, we run the risk of short cycling the boiler. The buffer tank prevents that by providing the initial thermal energy. In that process, the temperature in the tank drops and is elevated again through hot water supplied by the boiler. But at this point the energy load is large enough for the boiler to run efficiently and without short cycling.

Main manifold

This is a three zone manifold supplying hot water to (1) the radiant floor slab in the basement, (2) the radiators on the first floor, and (3) the radiators on the second floor.

Zone pumps

In the manifold are three zone pumps, supplying hot water to each zone once the thermostat of that zone turns on.

Mixing valves

Also in the manifold are mixing valves for each zone. The mixing valves reduces the temperature from 140 degrees Fahrenheit to the supply temperature of 120 degrees Fahrenheit.

System pump



Whereas the zone pumps push the hot water into the hydronic heating system, the system pump sits right under the manifold and pushes the water from the system back into the buffer tank. This pump is activated whenever there is a heating signal from any of the three zones.

120 gallon domestic hot water storage tank (insulated)

This tank supplies domestic hot water to the kitchens and bathrooms throughout the building. The tank is heated with hot water from the buffer tank that flows through a double walled heat exchanger. This way, the non-potable water from the hydronic heating system does not mix with the potable DHW. Because the tank has no gas or electrical powered heating element, it is also referred to as an indirect water heater.

DHW mixing valve

The temperature in the DHW tank can also get up to 140 degrees Fahrenheit, which is dangerously hot. To prevent scalding, the temperature is mixed down to 120 degrees Fahrenheit, which is also the supply temperature to the various faucets and showers.

DHW pump


This pump is feeding the water from the buffer tank through the heat exchanger in the DHW tank whenever there is sufficient hot water demand.

To make things a little easier to follow, I put a simplified diagram together that is animated and shows how the system is working. I hope this will do the trick.

Related posts:

Utility room installation

The heat is on

Wrestling the unruly…

Connecting the unruly…

Baseboard radiators delivery

Radiator connection preps

Radiator installation

Peak stress

Radiator déjà vu

Radiator start up

About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

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