Controls For Domestic Heating Systems
The following is a discussion on types of control available for domestic heating systems.
It is divided into three main sections:
- Oil or Gas Fired Systems
- Systems with underfloor heating
- Solid Fuel Systems
Oil & Gas Fired Systems
Introduction
The HEATING
section recommends that systems are pressurised.
The diagrams below are suitable for
pressurised and open vented systems.
The following diagrams show five typical systems, which may be used:
1. Two-Port Motorised Valves
2. Mid Position Motorised Valve
3. Thermostatic Control Valves
4. Separate Circuit Control with Two-Port Motorised
Valves
5. Separate Circuit Control Using Pumps
The lines on the diagrams between electrical
components indicate a control link and not
electrical conductors.
Oil and gas fired boilers have a control thermostat built in, so that the operating
temperature can be set.
The settings may be between 65oC
in mild weather to 80oC for very cold outdoor temperatures.
The operator must be careful to keep to
manufacturer’s recommendations since operating a boiler at too low a flow
temperature can be inefficient.
Also boilers have a built–in high limit thermostat
as a safety feature. These are set at about 90oC so that water will
not boil if the control thermostat fails.
1. Two-Port Motorised Valves
The room thermostat and hot water cylinder
thermostat switch 2-port motorised valves on and off.
A pressure relief valve is required to
operate if both valves are closed at the same time.
The photographs below show 2-port motorised valves, a time clock and a pressure relief valve.
If purchased as a manufacturer’s package then the pressure relief valve is not required since the closure of both 2-port motorised valves will switch the pump off.
Advantages:
1. Can be purchased as a package.
2.
Meets building regulations (
Disadvantages:
1. No individual room control
2. Mid Position Motorised
Valve
Advantages:
1. Can be purchased as a package.
2. Meets building regulations
Disadvantages:
1. No individual room control
2. The flow through the mid-position valve is not as good as through 2 No. 2-port valves.
NOTE: The system is wired so
that if both thermostats are satisfied the pump will stop.
3. Thermostatic Control
Valves
Advantages:
1. May be more reliable since there are no electrical control items except time clock.
2. Individual room control.
Disadvantages:
1. In summer the radiator valves need to be turned off manually.
2. There is no separate Hot Water and Heating time settings.
NOTES:
A 3-port Thermostatic valve with by-pass can be used at the hot water cylinder instead of a 2-port Thermostatic valve, thus the pressure activated relief valve would not be required.
If the Bathroom radiator in a house has no Thermostatic radiator valve, then this may also act as a pressure relief if other thermostatic valves close down.
4. Separate Circuit Control
With Two-Port Motorised Valves
Advantages:
1. Upstairs heating can be timed differently to downstairs.
Disadvantages:
1. Small motorised valves may not be as robust as domestic heating pumps.
NOTES:
In this system each thermostat operates the 2-port Motorised
valves through the wiring centre. See notes on wiring diagrams.
The pump and boiler are switched on and off by the time clock. Relays may be required if more than one time clock is used.
5. Separate Circuit Control
Using Pumps
Advantages:
1. Upstairs can be heated at different times to downstairs.
2. Pumps are robust.
Disadvantages:
1. Only suitable for large houses or small/medium commercial premises.
2. More expensive to install than simpler systems.
NOTES:
In this system each thermostat operates a pump.
If a 3-channel time clock
is not available then two time clocks will be required, a 1-channel and a
2-channel.
Relays will be required if more than one time clock is used.
See notes on wiring diagrams.
Electrical running costs
are not a problem since domestic heating pumps require only about 80 to 100
Watts of electrical energy.
Non-return valves are required at each pump to stop water reversing in circuits that are switched off.
