Programmable room thermostats and zone controls
The building regulationsThese are the legal regulations which govern how a house is constructed. (not to be confused with Planning Permission which is about whether you are allowed to build the house at all or what it might look like) see Building Regulations) insist that thermostats are fitted on all radiators. For the last thirty years or so these have usually been thermostatic radiator valves (TRVThermostatic radiator valve.)
However, using separate zone controls allows you to set different heating regimes for different rooms in the house. These are programmable for both time and temperature. So for instance you could set the bedrooms for a low temperature during the day and for a higher one when you get up, whereas the living areas could be warm during the day and drop to a lower temperature at night. With this method you would use electric valves to control the radiators rather than TRVs. There are four aspects which are different from a non-zoned system
- The system can remain running all the time but the boiler and pump are only running when a radiator (or cylinder) is calling for heat. This saves on pumping costs. It also means that hot water will always be available.
- you can have a low but fixed night time temperature (more pleasant if you have to get up or work during the night)
- You can over-ride the programmer to increase or decrease the temperature at any time and it will only affect the zone you are in.
- It nearly always removes the need for an optimiser because the warm up period on a morning is predictable, rather relying on the weather.
Note that in the example shown in the diagram the living and dining areas are open plan so that they can share one programmer. Note also that if you tried having two bedrooms sharing one programmer it would be difficult to get it to work properly because although the timing would work, the temperatures might not match up.
Note also that the cylinder always gets priority of hot water and does not require a two way valve. The differential pressure valve is necessary for when all the radiators are closed and the cylinder calls for heat. This setup works for a well insulated house since even if the cylinder is taking most of a boiler’s heat output for an hour or so there will be plenty remaining for space heating and the air temperature in the rooms will drop very little.
Zoning can save a good deal of energy because only the areas that need heat are getting it. It is particularly useful for rooms where someone has a special requirement such as an elderly relative who may need heating when others don’t, possibly also people who work a lot at night.
There are a couple of drawbacks: some people find the programmers difficult to adjust (although most people can work the over-ride button). Also the equipment is a bit more expensive and the electric valves may be prone to failure if the circulant is not well filtered.
Programmable timer with thermostatic radiator valves
This is probably the commonest arrangement for central heating circuits in the UK but not the best one in terms of energy efficiency or comfort. It ensures that the heating is switched off when you want it off and that no room is warmer than it needs to be. What it does not do is allow you to maintain a minimum temperature (say during the night or during the day when you are out at work) and it does not allow you to vary the temperature up and down in different rooms depending on the time of day unless you are willing to run around adjusting the TRVs all the time.
By far the craziest approach is to have TRVs on all the radiators and then to have an electrical thermostat in the hall to override the whole system including the TRVs. This is still not uncommon to see. The problem is that the hall is probably the least representative room in the house for having a thermostat. For instance a gust of cold air from the front door may cause the heating for the whole house to kick in, even if all the other rooms are warm enough. Alternately the thermostat will react to heat nearby and prevent a radiator functioning properly somewhere else.
One of the reasons people still do this is when they want the heating to be able to shut down at night and use a combined thermostat and timer rather than just a timer. The answer then is to turn the thermostat up high and let the TRVs do the temperature control and only use the timer function.
The boiler thermostat
The thermostat built into the boiler determines the temperature of the water that the boiler supplies. The level to set this at is a subject of considerable debate. See the section on gas boilers
Boilers need to put out more heat in cold weather. Optimisers are small computers which monitor the outside temperature in order to tell a modulatinga modulating boiler is able to gradually turn itself up and down depending on how much heat is required. This is more efficient than cycling on and off. boiler how low a temperature it can run at to do its job effectively.