Solar Energy for Radiant Floor Heating in Ontario: Solar Thermal vs PV

The two ways to heat a floor with the sun

Both routes end at the same place – warm water in the floor – but they get there through entirely different hardware, and that difference is the whole decision.

The plain-English overview – components, what it costs, whether solar is worth it at all – lives on solar radiant floor heating. This page is the part where you actually choose between the two. For the heat-pump side of things, see hydronic heating.

The Ontario winter problem (this is the crux)

Here’s the uncomfortable truth that decides most of these projects: your heating demand peaks exactly when solar output collapses. December and January bring the highest heat loss and the least sun, plus short days and snow on the panels. A solar thermal array sized to do real work in deep winter will then dump a surplus in summer when you don’t need floor heat at all. That mismatch is why, in much of Ontario’s limited-winter-sun climate, it’s often more practical – and frequently more cost-effective – to run an efficient heat pump on PV power than to feed the floor directly with solar thermal. PV keeps producing useful electricity on cold clear days, and a heat pump multiplies it into several units of heat. Solar thermal still earns its keep where you have strong exposure and a big domestic-hot-water or pool load to soak up the summer surplus.

Collector types: flat plate vs evacuated tube

If you do go solar thermal in Ontario, the collector choice matters more here than it would in a mild climate, because our winters punish heat loss.

Why you need a glycol loop in Ontario

You cannot run plain water through rooftop collectors in this climate – one cold night and the loop freezes and splits. So an Ontario solar thermal system is almost always an indirect (closed) loop: a food-grade propylene-glycol antifreeze mix circulates between the collectors and a heat exchanger in the storage tank, and the building’s potable and radiant water never go up to the roof. The glycol loop is your freeze insurance, and it’s one more reason evacuated tubes shine here – their low heat loss makes the indirect loop more freeze-tolerant and more productive on cold, clear days.

The modern setup: PV-driven air-to-water heat pump

The configuration we see winning more often in efficient Ontario homes skips rooftop plumbing entirely. PV panels make electricity, an air-to-water heat pump uses it to warm water into a buffer/thermal-storage tank, and the radiant floor draws from the tank – storing heat when demand is low and releasing it when the house calls. It pairs beautifully with radiant because radiant wants low water temperatures, which is exactly where a heat pump is most efficient. The PV doesn’t have to power the heat pump in real time, either: any electricity it makes offsets your bill, and the grid acts as your battery unless you choose to add real ones. The trade-offs, sizing, and where a heat pump fits among boilers and combis are all on hydronic heating.

Off-grid and battery considerations

Going fully off-grid with radiant is the hardest version of this. Heating is a big, sustained winter load, and batteries that can carry a heat pump through a dark, cold stretch are expensive. Most “solar” radiant homes in Ontario stay grid-tied: the panels offset consumption, the grid covers the gaps, and a conventional backup handles the coldest days. If true off-grid is the goal, the design leans on a tight, low-load envelope (ICF helps a lot here), generous storage, and an honest backup such as a wood stove or propane – solar shrinks the load, it rarely carries January alone.

Sizing, summer surplus, and the backup you still need

Two rules carry over from any solar radiant design. Size for winter, plan for summer: an array big enough to matter in January overproduces in July, so route that surplus to domestic hot water or a pool rather than letting it cook the tank. And always include a backup heat source – a combi or boiler, a heat pump, electric resistance, or a wood stove – because in an Ontario winter solar is an offset, not a guarantee. Realistically, solar can offset on the order of 30% or more of the fuel a conventional forced-air system would burn; the rest is the envelope and the backup doing their jobs. The full sizing and cost picture is on solar radiant floor heating, with per-square-foot radiant numbers on the Ontario cost page.

Building new? The HST rebate can cover a big slice

If this radiant system is going into a new build, that home likely qualifies for Ontario’s enhanced HST rebate – up to $130,000 back if your build contract is signed before the deadline. Check your number before you commit.

Solar radiant pairs best with ICF

Whether you go thermal or PV-plus-heat-pump, the single biggest lever is the envelope. An ICF home loses far less heat, so a smaller array and backup carry more of the load - and a bigger share of your heat comes free. It's the combination we build into our own homes. See what ICF is, browse our ICF house plans (every one offered with the ICF + radiant package), run the ICF cost calculator, or check code with the OBC Code Navigator.

All radiant guides

Solar energy for radiant floor heating: frequently asked questions

Note: general guidance for Ontario homeowners and builders, not a quote. Solar output and the right thermal-vs-PV choice depend on your roof, climate exposure, load, and heat-loss design.