Tiger Loop on a Basement Conversion with a Top-Feed Boiler
A two-storey house has had its garage converted into a utility room, and the new condensing oil boiler sits on the far wall with the top of the tank outlet roughly 900 mm below the burner pump inlet. The single-skin plastic tank is outside on a plinth, 12 m of 10 mm soft copper away, with a head of only 600 mm above the tiger loop when full and 250 mm when low. The installer proposes fitting a de-aerator at the boiler and running a two-pipe supply/return from the tank. On first firing the burner runs for 40 seconds then locks out; the oil line was not flushed and the tank is a 20-year-old steel-to-plastic replacement with visible sludge at the outlet.
1. Assess whether the proposed tiger loop arrangement is acceptable, and state what must be verified before it is fitted.
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A de-aerator (tiger loop) is only acceptable where the pump can draw the oil, because the loop does not create suction. The critical checks are:
- Suction capability of the pump: the burner pump must be able to lift the oil from the tank outlet to the pump inlet. With the tank outlet 900 mm below the pump, the pump is lifting, not gravity-feeding, so the manufacturer's maximum suction lift (typically around 3.5 m for a standard Suntec-type pump, less for a long, small-bore line) must be checked, not just the head above the loop.
- Line sizing and length: 10 mm soft copper over 12 m plus fittings gives significant pressure drop; the pump's quoted suction lift assumes a correctly sized single-pipe line. A two-pipe run to a tiger loop defeats the purpose unless the loop is at the tank end.
- Air ingress: every joint must be pressure-tested; a de-aerator will mask a small leak by venting it, but it cannot compensate for a line that is drawing air faster than it can vent.
- Sludge and water: a 20-year-old tank with visible sludge at the outlet must be cleaned and the outlet filter replaced; sludge will block the pump strainer and cause the 40-second lockout far more readily than air.
The 40-second run-then-lockout is consistent with either air reaching the pump or a blocked strainer/filter; the line must be flushed and the pump strainer cleaned before any conclusion is drawn about the loop.
2. The installer wants to fit the de-aerator at the boiler. Explain the consequence and the correct alternative.
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Fitting the de-aerator at the boiler on a two-pipe supply/return means the loop is at the discharge side of the suction line, so the pump still has to lift oil 900 mm plus overcome 12 m of 10 mm pipe. The loop will vent air that has already passed through the pump, which does not solve a suction problem and can cause cavitation and erratic firing.
The correct approach is one of:
- Single-pipe supply with the de-aerator at the tank end (or as close to the tank as practicable), so the pump draws a short, well-vented line and the loop handles any residual air.
- A two-pipe gravity system with a properly sized return and a non-return arrangement, only if the tank can be raised or the boiler relocated so the outlet is above the pump.
- A booster/transfer pump where lift cannot be achieved within the burner pump's limits.
Whichever is chosen, the line must be flushed, the tank outlet filter replaced, and the suction lift and pipe sizing confirmed against the burner manufacturer's data before commissioning.