Sizing is a surface area calculation, not a guess
A pool heater is sized to the pool's surface area, because that is where the heat leaves. A shallow wide pool loses more than a deep narrow one holding identical water.
The working method multiplies surface area in square feet by the temperature rise you want in Fahrenheit degrees, then by twelve, to give BTU per hour. A 16 by 32 foot pool has 512 square feet of surface, so a 20 degree rise puts it near 123,000 BTU per hour before any allowance for wind exposure, shade or an overnight cover.
Real installations round up from that figure, and the reason is recovery time rather than capacity. A heater sized exactly to the steady loss will hold a temperature but takes days to raise one. Sizing up shortens recovery after a cold week, which is the complaint that generates callbacks. Residential gas units are commonly available at 200,000, 266,000, 336,000 and 400,000 BTU per hour.
We do not size a replacement to the outgoing unit. The old heater was frequently sized to a different pool, a different cover arrangement or a different owner's expectations.
The gas supply decides what you can actually install
This is the constraint that stops more Guelph replacements than budget does.
A 400,000 BTU per hour heater burns roughly 400 cubic feet of natural gas an hour, about 11 cubic metres, which is close to four times the draw of a mid-size furnace. That load sits on top of the furnace, the water heater, the range and the dryer already connected. Older Guelph services were never sized for it.
We measure the total connected load and the pipe run before quoting. If the meter or the line needs upgrading, that appears in the quote rather than in a phone call afterwards. A heater starved at full fire will nuisance-trip for the rest of its life, and the fault will read as an ignition problem every time.
Gas work in Ontario falls under the Technical Standards and Safety Authority, working to the CSA B149.1 natural gas and propane installation code. Only a certified gas technician may connect the appliance.
Where the heater can physically go
Vent terminal placement is the second constraint people meet late.
CSA B149.1 sets minimum clearances from a vent terminal to windows, doors and mechanical air intakes. An outdoor heater tucked against a wall, under a deck or beside a basement window frequently cannot meet them where the old unit stood, which means either relocating the pad or changing the vent configuration.
Condensing models add a consideration. They pull flue gas below its dew point to reach the mid nineties in efficiency, so they vent in polypropylene or stainless rather than metal B vent, and they produce acidic condensate that has to drain somewhere appropriate.
The heater also needs service clearance. A unit boxed in by decking or landscaping costs more to work on for its entire life, and we would rather move it at installation than charge for the access every visit.
Replacing gas with a heat pump, or the other way
Replacement is the moment the fuel decision reopens, and it is worth taking seriously rather than defaulting to like for like.
A pool heat pump moves four to six units of heat per unit of electricity at mild conditions, so its running cost is far lower than a gas heater's. It can only move heat that is in the outdoor air, and its output falls sharply below about 10 °C. A gas heater costs more per hour and works on demand in any conditions.
The practical Guelph consequence is seasonal. A heat pump is excellent through July and August and close to useless in the first and last three weeks of the season. Pools used for a fixed season lean gas. Pools used opportunistically lean heat pump. Some properties run both.
Switching to a heat pump also moves the load from the gas service to the electrical panel, and a large unit may need a dedicated circuit the panel does not have spare. We check that before quoting, for the same reason we check the gas meter.
Full detail on the heat pump option is on the pool heat pump page.
What installation actually involves
- Site visit and measurement. Pool dimensions, existing pad, plumbing runs, gas meter and connected load, electrical supply, vent clearances.
- Written quote. Heater model and rating, any gas or electrical upgrade, pad and plumbing work, removal and disposal of the old unit, all at a fixed price.
- Removal. The old heater is disconnected, drained and taken away. Refrigerant-bearing units are handled separately, because refrigerant must be recovered rather than vented.
- Set and connect. Pad, plumbing unions, gas connection and vent, electrical and bonding.
- Commission. Manifold gas pressure read under load, temperature rise measured across the heater at rated flow, safeties tested, controls set, and the owner walked through the sequence.
Commissioning is the step most often skipped by a low quote, and it is the step that establishes the heater was installed correctly rather than merely connected.
Cost and timing
These are typical Guelph-area ranges, not a quote.
| Job | Typical range |
|---|---|
| New gas heater supplied and installed | $4,000 to $7,500 |
| New pool heat pump supplied and installed | $5,500 to $9,500 |
| Electric resistance heater, small pool or spa | $1,800 to $4,000 |
| Gas meter or line upgrade | Quoted by the utility and the installer separately |
| Dedicated electrical circuit for a heat pump | $600 to $2,000 |
| Removal and disposal of the old unit | Usually included, quoted separately if refrigerant recovery is needed |
Booking matters more than most people expect. Heaters and their parts are tight through June and July, and lead times on specific models stretch. Replacement booked in May or September is a different experience from replacement booked in the second week of a hot July.
Frequently asked
What size pool heater do I need in Guelph?
Can I put a bigger heater on my existing gas line?
How long does a pool heater installation take?
Do I need a permit to replace a pool heater in Guelph?
Should I replace my pool heater with the same model?
Pool heater not working?
Replacing a pool heater in Guelph? We check the gas supply and the vent path before we quote the heater. Serving Guelph and Wellington County.
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