18th century watermill replaces oil heating with water source heat pumps

The Grade II listed 18th century watermill is situated on the picturesque River Stour. The river was originally diverted to feed the millpond and water was controlled by a sluice gate under the property, where the original millwheel is still housed.

The owner, knowing that his 20 year-old oil-fired heating system would need replacing before too long, had been thinking about using the river water as a heat source. “I couldn’t bring myself to replace our oil boiler with another oil boiler,” he said. After a conversation at the Ecobuild exhibition, the owner was able to confirm that ground source heat pump technology using river water as a heat source, was indeed viable.

One of the main challenges was deciding on the best location from which to abstract the river water. Ultimately, it was decided that the millpond at the entrance to the mill wheel was the optimal place. To prevent build-up of river silt and pond weed wrapping itself around the equipment, the owner decided to have the pond dredged and to modify the existing pond level retaining wall. He also built a second protective wall, so that the filter is housed between both walls, with a small volume of continuous water flowing through the bases to disperse silt.

The two Vitocal heat pumps were located in the existing plant room, taking the place of the old oil tank and boiler, with separate heating and hot water flow / return underground pipes to the house. The owner was pleased that through a carefully worked plan, the new installation while keeping the old system operational, and so maintaining an uninterrupted supply of heating and hot water.

How the system works
The system operates via a high volume, low pressure pump that draws water from the river, pumping through the water / brine plate heat exchanger to extract heat. About 20 percent of the water is returned to the river water intake filter unit as part of the self-cleaning process. The filter removes river sediment, down to extremely fine particles. The cooler water is then returned to the river about 10m downstream. The installation operates in a master slave configuration and distributes heating via a Vitocell 950 litre buffer cylinder and hot water storage is in a Vitocell 300-V 500 litre cylinder.

The property has three existing heating circuits, including underfloor on the ground floor, which are now managed by Viessmann’s Vitocom 100 controls, and which can be fully accessed over the internet by the owner from a mobile app. A Vitotrol 300 wall-mounted control in the house also allows for instant operation of the system.

Compared to the previous oil-fired system, it is expected that about 10 tonnes of CO2 per year will be saved thanks to the high efficiency heat pump in stallation. Wi th net savings on running costs, due to the switch to renewable energy, together with RHI payments the mill owner expects a payback period of less than 5 years on the initial investment cost of the heat pump installation.

“We’re very pleased that we have a complete heating and domestic hot water system running by extracting heat from our river, and are no longer dependent on fossil fuels. I have absolutely no regrets about my choice!”

Project year: February 2017 
Equipment: 2 x Vitocal 300-G
Rated output: 
Ground source heat pumps 34 kW 
?Buffer cylinder 950 L
DHW cylinder 500 L
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