Guide · Boilers
Combi or system boiler for radiant plus hot water.
One boiler can heat your floors and your water. Here’s how a combi boiler and a system boiler with an indirect tank compare, and how to choose between them.
The short version
- A combi boiler heats your home and makes domestic hot water on demand from one appliance.
- A system boiler with an indirect tank heats your home and keeps a store of hot water ready.
- Both can run radiant floors well. The choice comes down to how your household uses hot water and the heat-loss numbers.
- Either way, the boiler is sized from a heat-loss calculation, never from the old one’s label.
01 · Two ways to do both
Heating and hot water from one boiler
Most homes with radiant floors don’t need a separate boiler just for the floors. A properly sized boiler can handle radiant heating and domestic hot water together, with towel warmers or garage heat on the same plant as extra zones.
Natural Resources Canada calls appliances that do both jobs combination systems, and notes they suit homes that need less heating, such as well-insulated, airtight homes1. There are two common ways to set one up with a boiler.
02 · Side by side
Combi boiler vs system boiler
03 · Choosing
How to choose between them
The heating side is the same either way: the boiler is sized from a room-by-room heat-loss calculation on your home. What separates the two is hot water.
- Smaller households, one bathroom in use at a time: a combi often fits, and saves space.
- Several bathrooms, showers at the same time, a big tub: a system boiler with an indirect tank keeps a store ready for peak demand.
- Tight mechanical room: a combi takes less space, if the hot water demand suits it.
- Planning extras like snow-melt: the design sizes the plant for everything it serves, which can tip the choice.
Efficiency matters too. Condensing boilers run at 90 to 96 percent AFUE, and they reach their best when the water coming back to them is below about 54 °C (130 °F)2, which radiant floors make easy.
04 · Ratings
A note on comparing ratings
NRCan points out that rating a combination system by its water-heating efficiency alone gives an incomplete picture, because it works differently when it’s heating the house. It suggests looking for systems rated as a whole under CSA P.9, and checking that whoever designs the system is qualified to1.
05 · Boiler questions
Boiler and hot water questions
Do I need a separate boiler for radiant floors and hot water?
Usually not. A properly sized combi or system boiler can handle radiant heating and domestic hot water together, with extra zones like towel warmers on the same unit.
Is a combi boiler enough for a family?
It depends on how you use hot water. If several showers run at once, a system boiler with an indirect tank usually suits better. The design looks at your household’s peak demand.
Which is more efficient?
Both can be high-efficiency condensing boilers. Condensing boilers run at 90 to 96 percent AFUE, and radiant floors help them stay in their efficient range.
Can a combi boiler run a snow-melt zone?
The boiler has to be sized for everything it serves, including snow-melt. The design checks the total load before choosing the boiler.
How do I know what size I need?
From a room-by-room heat-loss calculation on your home, plus your hot water needs. Never from the old boiler’s label.
Sources
- Natural Resources Canada, Combination systems.
- U.S. Department of Energy, Building America Solution Center, Condensing Boilers.
Sources checked October 2026. Programs, amounts and codes change, so confirm current details with the source before you plan around them.
Keep reading
More radiant guides
- Service
Boiler installation & replacement
Condensing boilers sized from a heat-loss calculation, never the old label.
- Guide
Heat-loss calculations, explained
What goes into one, and why every system is sized from it.
- Guide
Radiant and boiler maintenance
A simple yearly checklist for the boiler, pressure and controls.
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