Choppards Mill radical retrofit: Planning for a heat pump

How it all began

This project is an unusual one for us in several respects. Usually, most of our building retrofit clients come to us and we discuss the merits of a fabric first approach. At Choppards, however, our client had approached Bill Meadowcroft at Green Source Heat, to discuss a heat pump for the property before they approached us.

Bill established that the heat loss from the house was far too high for a heat pump to work even at its maximum peak capacity. As such, the house would require significant insulation and airtightness work to enable the heat pump to be feasible, and gratefully, Bill recommended Green Building Store to our clients. Thus the project is heat pump led, rather than being fabric first.

The property

Choppards is built over three stories, constructed from sandstone and gritstone, with several extensions that have been added over the years. The oldest part of the house was built around 1800 as a cloth mill. Whilst it has now been a residential dwelling for many years, it has had little modernization or repair.


The three stories of Choppards and Bill Butcher.

The mill ground floor had a stone flagged floor laid onto the ground; the kitchen floor, however, was made from timber joists with a subfloor underneath, so was draughty with no insulation. At the other end of the house were bedrooms and a bathroom, and below them garages and a porch, between which were uninsulated and leaky timber floors.

The mill’s external walls are around 400mm thick with stone inner and outer leaf and rubble fill. On the internal face of the walls were either gypsum plaster onto stone or older lime plaster with timber studs carrying plasterboard.

In the newer part of the house built in the 1970s, there was a masonry cavity wall with a dense block inner leaf , sandstone outer leaf and an uninsulated cavity. In a one room the walls were damp and mouldy. We found this wall was getting damp from leaking gutters outside and the cold air was getting into the cavity at the eaves and around the joists’ ends, making the internal leaf blockwork cold, resulting in surface condensation and mould.

We investigated the roof spaces which revealed that none of the roof had any insulation. Our clients told us their house was cold and draughty and now it was clear why!


Floor Plans of Choppards courtesy of A+DP Architects

The water source heat pump

The site once had a working water mill, now since demolished, but it retains a sizeable mill pond adjacent to the house. So the question was asked: could a water source heat pump provide heating and hot water to the house?

Choppards' Mill Pond

The mill pond at Choppards

Water has a high specific heat capacity 4.2J/g-K (joules per gram, kelvin), compared to air 1J/g-K and granite 0.76 – 0.85J/g-K. This means that water has a high heat retention capacity and is a good source of heat to tap into for a water body which has flow, which the mill pond has.

Two identical heat pumps were specified to meet the peak demand. This has the benefit that for most of the year, only one heat pump will be required to meet the load, saving energy and prolonging the life of the pumps by using them alternately.


Spot the difference! The two identical heat pumps

Two collector options were considered, either coils of plastic pipe laid at the bottom of the pond (similar to what is used for a Ground Source Heat Pump) or a stainless steel plate collector system.


Collector plates

The former option would require 2.25km of pipe to be submerged in the pond in nine loops requiring a manifold to combine 18 pipes! It was felt that this would be too complicated and invasive to the pond and surroundings, so the steel plate collector was chosen.

The stainless steel plates bring several benefits: they occupy a small volume of only 5m x 1.2m x 0.6m and they require only two pipes to be connected. They give an excellent thermal transfer between the water and the circulating liquid within the sealed system, which in turn raises the heat pumps’ efficiency.

The heat pumps were installed in one of the garages underneath a mezzanine floor. This allowed the two pipes from the collector to pass underground and through the garage wall directly adjacent to the heat pumps. With multiple heat pumps, it is important to correctly connect the flow and return pipes and install a non-return valve in the line to each heat pump. Non-return valves are a £20 part but one that causes severe problems if omitted!

Building project scope 

The project scope was to install insulation to at least achieve the peak heating load of the heat pumps. This would result in lower heating bills, a significant improvement in thermal comfort and reduced draughts. The architect’s and engineer’s drawings also included the following works:

  • Demolition of the lean-to porch
  • Replacement door and windows
  • Creating a new staircase providing better access to the lowest level by repurposing existing rooms
  • Underfloor heating in selected rooms
  • Full electrical re-wire
  • Replastering walls
  • Insulating the roofs
  • Re-roofing part of the house
  • New WC in basement and enabling work for a new house bathroom
  • Structural works on the lowest level to remove the internal walls and open up the room to create one big space
  • Create a new patio window opening, with patio door
  • Masonry wall structural repairs
  • Repointing stone walls in lime mortar

SAP modelling

We commissioned our colleague Rob Stubbings at our sister business QODA Consulting to undertake SAP energy modelling to examine three scenarios:

  • Existing heating demand – important to understand our current benchmark
  • Option 1: Insulating all ceilings/roofs and suspended floors over garages to a U-value of 0.16W/m2K, replacing doors and windows with GBS windows
  • Option 2: Insulating all ceilings/roofs and floors over garages, replacing doors and windows with GBS windows AND insulating the ground floors, ground floor level external walls, master bedroom walls

SAP gave an existing heating energy requirement of 52,123kWh/year.  The reality is that the existing heating demand was more than this, as some of the assumed U-values are better in the model than in reality.  A more accurate assumption would be closer to 60,000kWh/year!

The most ambitious of the upgrades showed that heating demand would be reduced to 38,402kWh/year. A 26% reduction in space heating demand. This specification also reduced the peak heating load to that required for the heat pump. This was the specification chosen by the client to proceed with.

We have subsequently insulated a number of additional external walls, which will further reduce the space heating demand by >30%.

Whilst it’s not the most ambitious retrofit we have undertaken in terms of U-values and airtightness, the scale is significant and our work here will transform our client’s home from a cold and draughty one, into a much warmer and comfortable home, with a highly efficient heat pump to boot.

4th November 2022

One response to “Choppards Mill radical retrofit: Planning for a heat pump”

  1. blank Adam Harper says:

    Please post comments and questions below

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