Water heating coil repair
Heating coil repair for air handling units and unit heaters: frost-burst tubes, leaking bends and corroded headers. We repair on site or in our workshop, pressure test every coil and check why the frost protection failed.
F-gas company certificate. A report with measurements after every visit. Mobile service in Poland and the EU.
How heating coils fail
Water expands by about 9 percent when it freezes. In a heating coil, ice forms first in the tubes closest to the cold outdoor air and in places where water does not flow, such as return bends of inactive circuits or the lowest points that cannot be drained. The pressure splits copper tubes along their length or pushes out a bend. The leak often appears only days later, when the coil warms up and the system is refilled. Besides frost, coils leak from vibration at the supports, erosion from high water velocity, and corrosion from aggressive water or from air with chlorides.
Why frost protection failed
A heating coil repair without finding the cause is only a delay before the next one. Typical reasons we find:
- Frost thermostat capillary mounted only in one corner, not across the whole face of the coil.
- Heating circuit pump stopped by a schedule or a summer mode that nobody switched back.
- Control valve actuator seized or with a broken coupling, so the valve stays closed while the controller shows it open.
- Outdoor air damper not closing or with worn seals, so cold air flows through a stopped unit.
- Supply temperature from the boiler room dropped at night or during a fault.
- Stratification: cold outdoor air and warm return air not mixed, leaving one part of the coil in sub-zero air.
- Unit heaters in gates and loading bays exposed to cold air when the doors stay open and the fan runs.
We check each of these points and describe the result in the report, together with recommended changes to the controls. See our air handling unit service page for the full frost protection checklist.
Repair options
| Option | When it makes sense | Limitations |
|---|---|---|
| Brazing a bend or a joint on site | Few accessible leaks on bends or header connections | Space around the coil, drained and dry section required |
| Plugging damaged circuits | Splits inside the finned part, small share of circuits affected | Each plugged circuit reduces capacity and risks frost in neighbouring tubes |
| Workshop repair of the removed coil | Many leaks or poor access in the unit | Removal and transport, downtime of the unit |
| New coil made to dimension | Widespread frost damage, corrosion, old coil with repeated leaks | Lead time for manufacture |
As a practical limit, once about 10 percent of the circuits would have to be plugged, or the coil has burst in several places along the tubes, a new coil is the better investment. For a replacement we measure the coil dimensions, connection sizes and positions, the number of rows and circuits and the fin spacing, and we check the heating capacity against the design conditions so that the new coil is not undersized.
Workshop repairs
Coils removed from units or complete unit heaters can be repaired in our workshop in Wałsnów. The coil is cleaned, filled with compressed air or nitrogen and submerged or sprayed with leak detection fluid to find every leak, including small ones that would not show when the system is cold. After brazing to EN 13585, the coil is tested again at a pressure above its operating pressure and held for a set time. Fins damaged near the repaired areas are straightened and the coil is cleaned before return.
Refilling and recommissioning
- Installation of the repaired or new coil and connection with new gaskets.
- Filling and venting at the highest points of the coil.
- Check of the system pressure and leak tightness under operating temperature.
- Test of the frost thermostat by cooling the capillary, and check that the unit reacts correctly: fan stop, damper closed, valve open, pump running.
- Check of the supply and return temperatures at design air flow.
Glycol in heating circuits
For coils that cannot be protected reliably, for example in units with long standstills or in exposed locations, a glycol mixture in the heating circuit is an option. It reduces heat transfer slightly and requires checking pump capacity, and it can only be used where the heating system allows it. We discuss it case by case.
Seasonal inspections of AHUs and rooftops, including frost protection tests before winter, are part of our AHU and rooftop service. Remote alarms for low coil temperature can be added through our controls and remote monitoring work.
Questions from maintenance teams
Can a burst coil be repaired in winter while the unit is needed?
Often yes, by isolating and plugging the damaged circuits as a temporary measure. The final repair or replacement is then planned for a period when the unit can be stopped.
How do we know if a coil is leaking inside the unit?
Signs are falling pressure in the heating system, water in the AHU floor or drip tray in heating mode, and corrosion marks below the coil. A pressure test confirms it.
Do you repair unit heaters?
Yes, both the heat exchangers and the fans of water unit heaters used in halls and warehouses.
Is a frost thermostat enough protection?
Not on its own. It must work together with the damper, the valve, the pump and the supply temperature, which is why we test the whole sequence.
Coil burst after a cold night?
Send photos of the coil, its nameplate and dimensions, and we will propose repair or replacement.