To install a completely new central heating and domestic hot water system in a three bedroom semi-detached house making best use of a variety of heat sources. To maximise energy efficiency and minimise environmental impacts and running costs.

Heat Sources

Solar Panels Wood Burning Stove Natural Gas Boiler Electric Immersion heater

Heat Availability

Solar Thermal Panels Wood Burning Stove Mains Gas Boiler and Electric Immersion Heater

Heat Source Winter Spring Summer Autumn
Electricity YES YES YES YES

System Requirements

Domestic Hot Water (DHW) Must always be available, with peak demand during evenings and mornings. Hot water (60 degrees C) is required for Shower (thermostatic mixer), Bath, Bathroom washbasin, Kitchen Sink, Dishwasher, Washing Machine.

During spring / summer / autumn solar energy is used to heat HW cylinder. If water does not reach 60 degrees C then some gas / electricity is used.

Heat Source Winter Spring Summer Autumn
Solar - 25% 50% 25%
WBS 60% 25% - 25%
Gas 40% 50% 50% 50%
Electricity backup backup backup backup

Central Heating (CH)When the wood burning stove is in operation, gas central heating will only be needed in the coldest winter weather, or in mornings after the stove has gone out. Heating (WBS or Gas) is generally only required in winter, but occasionally in autumn and spring. The requirement for the whole house is approx 11kW (38000 btu) for 3 bedrooms, bathroom, hall, kitchen/diner, living room.

Heat Source Winter Spring Summer Autumn
WBS 75% 90% - 90%
Gas 25% 10% - 10%

The Designed System

Heating Circuits

1.Solar circuit (cyan and magenta) currently operates without the need for a pump or controller, but these may be used to improve the efficiency. In the future, the system will be controlled using a differential temperature controller (T3 & Ts). The circuit is constructed in 15mm tubing to keep flow speed through the house (between panels and cylinder) high to reduce heat losses. However, the four solar panels are plumbed in parallel in 15mm tubing so the flow speed through them is one quarter of that in the house. This allows more time for solar heating to occur. Within the cylinder, a 22mm high exchange indirect coil is fitted - the flowrate through this coil is approximately half the speed of that in the house (between panels and cylinder).

2.Wood Burning Stove (WBS) circuit (red and blue, to the right of the two cylinders) - gravity circulation in 22mm tubing.

3.Gas circuit (red and blue, to the left of the two cylinders) - pumped CH and HW circuit with flow controlled using standard 7 day programmer and 3 port mid position motorised valve (A).


The three heating circuits are independent. When solar energy is available (Ts>T3), pump A runs to cycle the fluid in the solar circuit, heating the lower of the two cylinders. This cylinder is the cooler (often cold) cylinder, thus the solar system runs at the highest possible efficiency (Solar collectors are more efficient heating water from 10 to 30 oC than from 30 to 50 oC).
When the wood burning stove is operating, the circuit thermosyphons to provide heating to the upper cylinder (with the two way valve (shown above) closed). The WBS is a high grade heat source easily capable of boiling water and needs a safety release for excess heat. This is provided by a second mode of operation. When tank thermostat T2 exceeds 70 degrees C, the two way valve is opened and simultaneously a neon indicator illuminates beside the WBS. Water is now allowed to circulate through the coil in the lower cylinder which will be much cooler than the upper cylinder. When this cylinder becomes warm, a second neon indicator illuminates beside the WBS, indicating that the WBS needs to be allowed to go out, or that some hot water should be used. Obviously, when the lower cylinder is heated the solar circuit is only able to operate at much lower efficiency (if at all). However, this case will generally only occur in winter when the WBS has been running for a long period. In this case there is only a small amount of solar energy available anyway.
The Gas system drives the CH circuit and provides heat to the upper HW cylinder provided the temperature of this cylinder is below 60 degrees C (T1). Gas can easily be turned on and off, unlike the wood burning stove, so is suitable for top-up water heating. All except one radiator are controlled by TRVs so will be shut down when the WBS is providing space heating.
When hot water is drawn from the upper cylinder, this water is replaced by the hottest of the pre-heated water from the top of the lower cylinder. This causes cold water to be supplied to the base of the lower (pre-heat) cylinder. Thus the solar circuit always has the coolest water on which to operate.

Alternative System Configurations

You really must read the excellent solar system design and plumbing guide for domestic solar hot water heating systems provided by the CAT, Wales. It covers a great number of issues and provides invaluable information. Another site giving useful information on solar system configuration is Thermomax, manufacturers of evacuated tube (expensive) solar collectors. See what the Austian government has done to promote a scheme of DIY solar energy for rural communities, and then write to yours.

Why not have a go? I didn't know how to plumb when I began my system?

Environmental Science Lancaster.