What is a combi boiler?

A combi boiler is a high efficiency boiler modulating/condensing boiler that can heat your home and DHW (domestic hot water) all in one unit. Combi boilers usually are wall hung that will open up floor space in your basement of utility room.

At this time all combi boilers are natural or LP gas and very few are designed for fuel oil and operate above 90% efficiency. They are vented using PVC, CPVC, polypropylene, or Stainless steel through a sidewall or vertical through the roof. Some manufacturers allow air for combustion from inside the home or from the outside. Other manufacturers only allow outdoor air for combustion. My choice is outside air for combustion to reduce cold air infiltration into the home.

A combi boiler depends on a 3-way valve to heat your home or make DHW. A 3-way valve has one input port and two output ports. The water will flow through the boiler heat exchanger and absorb heat and enter the inlet port of the 3-way valve. Depending on which exit port the water exits through the heated water will be directed to the heating system or the homes hot water system

When you open a hot water faucet you will create a demand for DHW. Potable water will pass through the flat plate heat exchanger. A flat plate heat exchanger is multiple layers of metal brazed together with minimal amount of space between the layers. Every other layer of potable water or boiler water. The heated water will exit the heat exchanger and go to the homes hot water system.

DHW in a combi unit will always take priority over central heating. This means if you are running for central heat and open a hot water faucet the demand will leave central heat and the 3-way valve will move to redirect hot water to the flat plate heat exchanger. The demand will stay in DHW until the faucet is turned off or the DHW priority time expires. Then, and only then, will both heat and DHW run at the same time. Hot water temperature may be affected.