Why does my boiler provide hot water but no heat?

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Why This Happens: Two Circuits, One Boiler

Most residential boilers in Metro Vancouver homes — especially in older houses with hydronic (hot water baseboard or radiator) heat, and combi boilers in condos and townhomes — actually run two separate jobs from one heat exchanger. One circuit sends hot water to your taps and shower on demand. The other circuit pumps hot water through your radiators or in-floor loops to heat the building. When you get hot water at the tap but cold radiators, it tells a technician the burner and heat exchanger are working fine; the fault is isolated to the space-heating side of the system, usually at the point where the two circuits split.

This split point is typically a diverter valve (also called a three-way valve) on combi boilers, or a set of zone valves and a circulating pump on a system/regular boiler paired with a hot water tank. Because the fault is mechanical or electrical rather than combustion-related, it's rarely an emergency — but it does mean your heating won't come back on its own.

Most Common Causes

• Stuck or failed diverter valve — the valve that should redirect flow to the heating circuit is jammed toward the hot water position, common as these valves age or accumulate mineral scale from Metro Vancouver's moderately hard water.
• Low system pressure — hydronic heating loops need adequate water pressure to circulate; a slow leak or an unbled radiator can drop pressure below the point where the pump can push water through the loop.
• Failed or seized circulating pump — the pump that moves hot water to radiators can fail electrically or mechanically while the domestic hot water side (which often uses a separate flow path) keeps working.
• Faulty zone valve or actuator — if your home has multiple heating zones, one stuck-closed zone valve can leave an entire floor or wing cold.
• Trapped air in the system — air pockets in radiators or baseboards block water flow and heat transfer, even when the pump and valves are functioning.
• Thermostat or control board fault — a miscommunicating thermostat, dead batteries, or a failed printed circuit board can prevent the boiler from ever receiving the 'call for heat' signal, even though the domestic hot water function operates independently.

What You Can Safely Check Yourself

Before booking a service call, a few checks are safe for any homeowner and can sometimes resolve the issue or at least help the technician diagnose faster over the phone.

1. Check the thermostat: confirm it's set to 'heat' (not just fan or off), replace batteries if it's battery-powered, and confirm the display is responding.
2. Check the boiler pressure gauge: most residential boilers should read roughly 1.0–1.5 bar when the system is cold (check your manufacturer's plate or manual for the exact number). If the needle sits near zero, low pressure is a strong suspect.
3. Feel the radiators: if some are cold at the top and warm at the bottom, trapped air is likely and the radiator may need bleeding with a radiator key — a task most homeowners can do safely by opening the bleed valve slightly until water (not air) escapes, then closing it.
4. Check for an error code or fault light on the boiler's control panel and note it before calling — this speeds up diagnosis significantly.
5. Confirm the boiler hasn't simply been switched to 'hot water only' or 'summer mode' at the control panel, which some models offer.

What Requires a Licensed Technician

Anything beyond the checks above involves the boiler's internal plumbing, gas components, or electrical controls, and under BC law this work must be performed by a licensed gas fitter or technician. Diverter valve replacement, circulating pump replacement, zone valve repair, control board diagnosis, and any work involving the gas train or heat exchanger all fall into this category. Attempting to open a sealed valve body or wire in a new pump yourself risks water damage, electrical hazard, or voiding both the manufacturer's warranty and your home insurance in the event of a related claim.

CANRO's technicians are Red Seal certified and hold Technical Safety BC gas licence LGA0043870 and boiler licence LBP0046450, so this type of diagnostic and repair work is within scope for a standard same-day response visit.

What to Expect From a Professional Diagnostic Visit

A technician will typically start by confirming your symptom (hot water present, heat absent) and checking system pressure, then use a multimeter and the boiler's diagnostic menu to test whether the diverter valve or zone valve is receiving power and moving correctly, whether the circulating pump is drawing current and spinning freely, and whether the thermostat signal is reaching the control board. On older cast-iron radiator systems, they'll also check for sludge or scale buildup in the heat exchanger's heating-side ports, which is more common in homes with original 1970s–1990s hydronic systems that have never been power-flushed.

Most diverter valve, pump, or zone valve repairs are completed same-day once the part is identified, since these are common failure points and parts are typically stocked. If the boiler itself is aging and repeated component failures are stacking up, the technician will walk you through repair-versus-replace math rather than just quoting the next fix.

Repair vs. Replace: When It's Time for a New Boiler

If replacement makes sense, high-efficiency boiler replacement in Metro Vancouver typically runs $7,500 to $15,000 or more installed, before rebates, depending on sizing, venting requirements, and whether gas line or panel upgrades are needed. A written quote after an in-home assessment is the only way to get an accurate number for your specific home — CANRO provides upfront pricing with no obligation.

Rebates and Financing to Know About

The Canada Greener Homes Grant closed to new applicants in early 2024, so it's no longer available for new projects. However, homeowners upgrading heating equipment may still qualify for the interest-free Canada Greener Homes Loan (up to $40,000) or, for eligible households moving off oil heat, the Oil to Heat Pump Affordability (OHPA) program. CleanBC, BC Hydro, and FortisBC also periodically offer rebates tied to specific efficiency upgrades. Program rules and amounts change, so it's worth confirming current eligibility and figures directly at quote time rather than relying on older information.

Frequently asked questions

Reviewed by the CANRO Heating & Cooling service team — June 30, 2026. CANRO holds Technical Safety BC gas licence LGA0043870 and boiler licence LBP0046450, employs Red Seal certified technicians, and carries WorkSafeBC and liability insurance. 4.9★ from 85+ Google reviews · serving Metro Vancouver since 1999 · (604) 359-1081 · info@canroheat.ca.