Heat Pump Retrofit in a 1985 Bungalow

The installer is wrong on both counts.

1. Sizing and running cost. A rough heat-loss estimate for this house at design conditions (20 °C inside, 0 °C outside) gives: walls about 110 m² at roughly 0.55 W/m²K ≈ 1.2 kW; roof 140 m² at about 0.25 W/m²K ≈ 0.7 kW; windows 20 m² at 1.4 W/m²K ≈ 0.6 kW; floor 140 m² at about 0.45 W/m²K ≈ 1.3 kW; ventilation and infiltration at about 0.5 air changes per hour ≈ 1.2 kW; plus thermal bridging. Total roughly 5–5.5 kW, so the heat pump would be sized around 6–7 kW with margin. Taking the attic from 100 to 300 mm cuts the roof loss by roughly 60% (about 0.4 kW) and, more importantly, reduces the temperature the system must run at in cold weather.

2. Radiator output is the real constraint. Radiator output scales approximately with the mean water-to-room temperature difference raised to about 1.3. Existing radiators rated at 75/65/20 (ΔT 50) produce only about 0.35 of that output at 45/40/20 (ΔT 22.5): (22.5/50)^1.3 ≈ 0.35. To deliver the same heat at 45 °C you need roughly 2.8 times the emitter surface. Reduce the heat loss by 20% and you still need well over twice the area - larger K2 panels, or more of them.

The risk. Install the heat pump on the existing fabric and the system will run at 55 °C or more in a cold snap, dragging the seasonal COP from around 4 towards 2.5, with electricity at three to four times the price of oil per kWh - the owner ends up with higher bills and a colder house. Fabric-first is cheaper than an oversized heat pump plus oversized radiators, and it makes the 45 °C design achievable.

Sequence - fabric, then airtightness, then system, then ventilation.

1. Attic to 300 mm with a raised deck or proprietary eaves vent trays so that the 50 mm ventilation path at the eaves is not blocked; insulate and draught-seal the attic hatch.
2. Airtightness and draught-proofing at skirting, service penetrations, the front door and any pipework through the ceiling, aiming for around 3 m³/h/m² at 50 Pa rather than the existing figure of perhaps 7. Do the airtightness work before the heat pump is commissioned, because a leaky house wastes the low flow temperature.
3. Re-run the heat-loss calculation on the improved fabric and size the heat pump from that figure, not from the original quote.
4. Radiators sized room by room against the revised heat loss at the chosen flow temperature; where a radiator cannot be enlarged, the design flow temperature is raised and the COP concession accepted for that zone only.
5. Cylinder. Replace the small indirect cylinder with a heat-pump cylinder of about 250 L with a large coil (around 3 m²) so it can reheat at low flow temperature without the heat pump going to a high temperature, with a timed legionella cycle.
6. Open fire. A heat pump house is far too airtight for an open fire: it will draw combustion air back down the flue and spill smoke, and it undermines the airtightness. It should be removed and the chimney capped and vented, or replaced with a room-sealed appliance with its own air supply. Fit CO alarms regardless.
7. Commissioning with the manufacturer's representative, a full heat-loss and radiator schedule handed to the owner, and a documented weather-compensation curve.

Grant and contractor check. Confirm SEAI grant eligibility and that the installer is registered before signing, since the grant conditions may require specific fabric measures or a BER assessment before and after.

Yes, but for the right reason. Before the airtightness work, the house ventilates itself through its leaks - expensively and uncontrollably, but it does ventilate. Once the airtightness is improved, background infiltration falls away, and the moisture from cooking, showering, laundry and occupants has nowhere to go. The evidence would appear as condensation on windows, mould in the corners of the bedrooms and a persistent damp smell. Airtightening without providing purpose-designed ventilation is a common and avoidable retrofit failure.

The options:

• Demand-controlled intermittent extract in the kitchen and bathrooms with background ventilators in the habitable rooms. Cheapest, meets the requirement, does not recover heat, and does not rely on long duct runs.
• Whole-house MVHR with continuous balanced supply and extract. It recovers most of the heat from the extract air, filters it, and suits a house that is now fairly airtight; it is the better fit here because the bungalow has a simple plan and a roof space for ductwork, and because a heat pump benefits from a low continuous ventilation rate rather than intermittent bursts of cold air.

Practical conditions. Ductwork must be short, insulated and sized properly; a poorly installed MVHR is noisy, uses more fan energy than it saves and gets switched off. The unit needs a condensate drain and a serviceable filter location. Commissioning must include measured airflow to each room against the design figures.

Summary for the owner. Attic first, airtightness second, heat pump sized on the improved fabric third, radiators and cylinder sized to the chosen flow temperature, open fire removed or room-sealed, and ventilation provided as an integral part of the package rather than an afterthought. The €3,000 attic job is not optional; it is part of making the heat pump work.