Central heating design
This article is about Central heating systems using hot water as a heat-carrying medium. (Warm-air systems are sometimes found in the UK but their design and installation is not covered here. There is a discussion on updating existing warm-air systems here)
The article is intended as a guide to:
- choosing a design for, and installing a new central heating system
- understanding how an existing system is designed, for the purposes of maintaining and fault-finding
There are separate articles about:
- Central heating operation for help maintaining a working CH system
- Central Heating Repair for diagnosing and fixing a faulty system.
Parts of this article are in skeleton form with main points listed but needing to be expanded
In order to get a heating system which works effectively and economically it is important to calculate how much heating power will be required, into the building as a whole (in order to calculate the required size for the boiler or other heat source) and into each room (to calculate sizes of radiators or other heat emitters).
Whole house/boiler sizing
For boiler sizing there is a relatively simple yet sufficiently accurate calculation known as the whole-house boiler sizing method. The method is described in the
Online calculators implementing the method are available at:
- IDHE (Institute of Domestic Heating & Environmental Engineers) calculator (site not available - 10th July 2008)
- Energy Saving Trust calculator This calculator is rather cumbersome to use and is incorrect for flats (it doesn't discount roof losses)
A spreadsheet implementing the calculator is available at:
The spreadsheet allows easy what-if calculations showing the effect of, say, cavity wall insulation on the house heating requirements.
For calculating the heat requirements of rooms an elemental approach is taken.
- The area of walls, windows, doors, floor and ceilings is calculated
- U-values for the materials of these elements are found from tables
- Temperature differences across the elements are multiplied by the above figures to calculate total fabric loss
- The volume of the room is calculated
- The number of air-changes per hour expected for the room is found from a table
- The above two figures are multiplied together with a factor for the heat capacity of air to calculate total ventilation loss
- The fabric and ventilation losses are added to calculate the total heat requirement of the room, and therefore the size of radiator or other heat-emitter(s) required.
A (Microsoft Windows) computer program implementing this method is the
* Discuss + links to energy conservation articles
The most popular fuels for central heating systems are (in order of increasing expense):
- Natural Gas
- LPG (Liquefied Petroleum Gas). Propane and Butane are LPGs but for heating propane is mostly used. It is often known generically as "Calor" gas (in the same way that vacuum cleaners are known as "Hoovers").
- Electricity can be used for central heating systems but where it is used for heating it is generally found used with storage heaters using off-peak rate electricity. Where no other fuel is available a system using a heat bank heated by off-peak electricity with Underfloor Heating or radiators is likely to be more economical to run than one using storage heaters or any sort of peak-rate heaters. An example of such a system is the "Electramate" made by Gledhill. This is a ready-made package, but similar systems can be designed using other manufacturers' components.
- solid fuels - coal, anthracite etc, and wood or woodchips - are sometimes used to contribute to space and/or water heating. Nowadays they are not usually used as main fuels since most domestic appliances for using them cannot be automatically fed and regulated.
Renewable sources such as:
- solar thermal
are also increasingly found contributing to heating systems rather than providing sole energy supply.
The most common appliances for supplying heat are boilers using natural gas, oil or LPG to heat "primary" water. Primary water is water intended for heating rooms via radiators etc or heating "secondary" water for washing etc (see DHW). Some boilers - known as combi boilers - heat DHW directly.
For further discussion of types of boiler, combi/conventional choice etc see:
Electric boilers perform the same function as non-combi boilers using electricity. They can be expected to have very high running costs compared to natural gas, oil or LPG boilers.
Ranges e.g. Agas and Rayburns usually heat DHW as a by-product of their cooking functions. They may use natural gas, oil, LPG or solid fuels.
Combined range/boilers appear outwardly almost identical to ranges but contain a separate central heating boiler sharing the flue of the cooking range. They may use natural gas, oil or LPG. They are generally non-condensing appliances and therefore less efficient than current central heating boilers (one exception being the Rayburn 480 CD which has a condensing boiler section: this gas-only appliance is only available with a balanced flue).
They generate electricity with much lower efficiency than fossil fuel generated electricity supplied by conventional central power stations, but they only generate when heat is wanted, which means all the heat and electricity output is used. This makes the overall picture more efficient than a central power station, where over half the input energy is wasted as heat. So overall the method works out more energy efficient.
CHP requires non-trivial arrangements for connection into the domestic electricity supply, and financial and administrative arrangements to sell surplus electricity back to the supplier.
CHP is a well established technology for large facilities, but domestic CHP generators are not readily available in the UK at present (Feb 2007), partly due to concerns about some aspects of the systems and lack of a solid proven track record of domestic CHP or the products on offer.
Traditional coal fires or more modern wood-burning stoves with back boilers can contribute to domestic space or water heating. Their heating output is sometimes combined with that of a main heating boiler by means of a Dunsley Neutraliser, although thermal stores can also be used.
Solar thermal panels are usually used to provide Domestic Hot Water (If considering this technology one might also investigate solar warm air systems which may give better energy returns for a given cost: see Solar Thermal.)
Ground-source heat pumps provide energy at lower temperatures than are required for DHW and are generally used in space heating systems, often with under-floor heating which can make better use of the lower temperatures generated.
Both systems, as well as waste water heat recovery, can be used with thermal stores, combining their output with other systems including conventional boilers and/or electric backup heaters to provide space heating, via UFH and radiators, and DHW.
Air-source Heat Pumps - similar benefits as ground-source heat pumps and easier to install; however in locating these units, the potential noise of the fan must be taken into account as some people are very sensitive to such noise, especially at night.
Emitters are means of heating spaces: radiators, under-floor heating etc.
"Radiators" actually emit heat mostly via convection rather than radiation: they heat the air which heats the fabric of the room and its occupants.
They come in a number of types -- standard panel radiators, Low Surface Temperature (LST), "designer" radiators and towel warmers -- and shapes, sizes and colours/finishes. Radiators must be chosen and located so as to provide sufficient output to heat the spaces they are installed into.
There is more detail in the article:
These types use forced convection, compared to natural convection employed by radiators.
Sometimes known as kickspace heaters, these have a fan to distribute air warmed by a water-to-air heat exchanger (typically tubes with fins attached) which transfer heat from the central heating primary water.
- particularly suitable for small rooms with limited wall space for rads (e.g. kitchen) or too-high heat-loss/floor-area ratio for UFH (e.g. bathroom)
- fast warm-up
- less localised heating effect than radiators; can be effective at heating larger areas
- may feel uncomfortably cold when shut off by thermostat (like electric fan heaters)
- may be too noisy for domestic use in lounges and bedrooms
This gives radiant heat which warms occupants and fabric directly rather than warming the air.
- Requires less total heat output (figures of around 20% are quoted) for a given comfort level compared to radiator-based systems.
- Tends to give warmer feet and cooler heads giving a more comfortable, less stuffy feeling.
- Good for heating large spaces where it would be hard to install sufficient radiators, and spaces with high ceilings e.g. halls where the output of radiators would be lost to the higher parts of the room.
- Limited heat output due to limitation on maximum comfortable floor temperatures which may be insufficient for small rooms with large heat requirements & large losses e.g. bathrooms (although the warmer floors, in conjunction with extra heating from radiators or kick-space heaters, can make for a more comfortable room than one with a cold floor).
- Heat output dependent on floor coverings which need to be chosen to work with the UFH system.
- Slower to heat & cool than radiator based systems, so need better control systems.
- Slow thermal response leads to lower overall efficiency for spaces occupied for relatively short periods, due to the heat lost during the longer warm-up and cool-down periods.
- Hydronic (hot-water) systems generally require lower water temperatures than radiator systems leading to extra complexity and expense (extra pump & thermostatic mixing valve) to run in mixed system with radiators.
- The lower water temperatures required by pure UFH systems enable condensing boilers, solar collectors and heat pumps to operate more efficiently than with radiator-based systems.
- Generally expensive & disruptive to retro-fit to existing building due to need to remove & relay floors (or possibly ceilings below for upper-floor installations).
- Electric UFH is cheaper to install but has higher running costs: popular choice for small bath or shower rooms.
More on Underfloor Heating
Walls can also be used for radiant heating. Usually this is acheived by embedding heating pipework into a solid wall surface. A discussion of the possibility of heating via stud walls can be found here.
Heating ceilings has the obvious disadvantage of unwanted heat loss upwards; even so one (singularly ineffective) installation is known to one of the authors.
Filling arrangements: sealed or vented
The traditional arrangement for maintaining a body of water in the system comprises a feed and expansion (aka header) tank above the highest point of the system. The tank is kept topped up by a float valve similar to that in a main cold water storage tank or WC cistern.
Modern systems are usually sealed with water introduced to the system by a temporary filling hose (or special key-operated device built-in to the boiler). More info can be found in Ed Sirett's Sealed System FAQ.
For a completely new system a sealed arrangement is generally preferred, unless it is wished to use a directly heated Thermal Store. Where an old and poorly-constructed existing system is converted to sealed operation there is the possibility of minor leaks from old radiator valves and poorly-made compression joints leading to relatively rapid loss of pressure and the need to top up the system frequently. For this reason if there is no compelling requirement to convert the system it may be better left alone. Conversely if an open vented system suffers from scaling up of the feed pipe, pumping over of the vent pipe into the feed and expansion tank, microbial sludge growth in the F&E tank or air air-locks when filling up it may be worth converting to sealed operation (provided the boiler is a type for which this is permitted).
Configuration: Controls and Zoning
An important aspect of the way a heating system is designed is the way heating is divided into physical zones, and the controls used to regulate heating. These are discussed in a separate article:
Available in various grades and sizes. Those found in domestic CH installations are:
- Rigid ("Table X") in small-bore sizes: 28mm, 22mm, 15mm
- Fully-annealed (soft) ("Table Y") in micro-bore sizes: 10mm, 8mm
- Material usually more expensive than plastics
- Available in lengths 1m, 2m, 3m (also 6m?). 2m and 3m are most common.
- More time-consuming to install
- Requires more lifting of flooring when retro-fitting to existing building
- Small-bore pipes must usually be run in notches in the top of joists: susceptible to damage by nailing and contrary to building regulations.
- Micro-bore pipes may be threaded through holes in joists out of reach of nailing
- Micro-bore pipe may be "cabled" through floor and wall spaces with less disruption in existing building
- May be noisy (e.g. clicking noises) as pipes expand & contract when heating & cooling
- Surface runs can be done neatly avoiding need for boxing-in in certain locations
- Can be joined with solder, compression or push-fit fittings
- Micro-bore may be bent by hand (with external spring) or by small machine for neater bends
- Small-bore may be bent by hand with spring for 15mm (and possibly 22mm if pipe annealed or fitter very strong)
- Small-bore may be bent with large hand-held machine for 15 & 22mm, larger machine on stand for 28mm
Some older installations using small-bore (15-28mm) pipework in PVC and ABS may be found but these materials are no longer used for CH pipework.
Moderm materials (used for last 2 decades or so in UK) are
- PB (Polybutylene)
- PEX (Polyethylene cross-linked)
Sizes available are:
- Pipework usually cheaper than copper
- Pipe available in long rolls e.g. 25m, 50m and 100m
- Easier & quicker to install than rigid pipe
- Pipe may be "cabled" with minimum lifting of flooring in existing building
- Pipes may be run through holes in joists out of reach of nailing
- Pipes expand and sag when hot requiring boxing-in if run on surface
- Can be joined with compression and push-fit fittings.
- Long runs possible with bends in pipework and fewer fittings
Barrier and non-barrier
Conventional Wisdom is that only barrier pipe should be used for CH systems as the metallic barrier layer prevents oxygen diffusing through the plastic walls of the pipe into the primary water and causing corrosion in ferrous and possibly other metallic parts of the system - boilers, radiators etc. However Hepworth Plumbing Products have stated in the uk-d-i-y newsgroup that:
If Hep2O Standard pipe has been installed in accordance with our instructions in a central heating system and one of the recommended inhibitors used there is no technical reason why it should not continue to give good service for many decades. []
It is now considered by British Gas that central heating systems that include plastics pipe manufactured to the appropriate British Standard (such as Hep2O) do not represent a potential corrosion problem from oxygen ingress where the system water includes an adequate strength of inhibitor. This applies equally to Barrier and Non-Barrier pipes. []
However in real life not all central heating systems have effective corrosion inhibition at all times, so barrier pipe is still the preferred option.
Even in a system using plastic pipe for the main pipe runs the boiler manufacturer usually requires the first 600mm or 1m of pipework connected to the boiler to be in copper.
Also many installers and/or clients prefer copper tails to radiators rather than plastic. For "designer" radiators or towel radiators in bathrooms chromed radiator tails are often preferred. Since chrome is very hard it is necessary to remove the chrome from the part of the tail pipe being connected into a push-fit fitting since the grab ring of the fitting may not bite securely into the chrome and the fitting may become detatched. It is also necessary to remove the chrome when connecting into a solder fitting since solder may not adhere properly to chrome. If using a compression fitting a brass olive is preferable to a copper one since the olive has to slightly compress the pipework to secure the fitting and the chrome may be too hard for a soft copper olive to acheive the necessary pressure.
pipe sizes v. heat-carrying capacities + noise
Single pipe loop
This arrangement is not used for current designs but may be found in older installations (e.g. 1960s and early 1970s), and it may be required to extend such systems.
The general topology is a single pipe run in a loop from the boiler's flow back to its return (via a circulating pump) with the radiators connected into the pipe loop via plumbing Tees (sometimes swept Tees). This arrangement is usually immediately obvious as the main loop pipe runs underneath the radiator. Although one might expect all the heating water to take the more direct route through the pipe, enough rises into the radiator, travels along it (losing heat as it goes) and sinks back to the main pipe at the other end to keep the radiator hot and emitting heat.
This arrangement means that radiators further from the boiler's flow / closer to the return end of the loop run at slightly lower temperatures than those at the flow end so should be oversized to give their desired output at their reduced temperatures. Also the circulating pump for a single pipe loop is doing a certain amount of work just circulating water through the pipes so less of its effort is available for circulation through the radiators.
Where it is necessary to add, say, one or two extra radiators to such a system they can be added into the existing loop (allowing for over-sizing if they are at the cooler return end of loop). However if many extra radiators are to be added it may be better to divide the system into two (or more) separate loops if possible (or to add a separate normal 2-pipe loop, perhaps as a separate zone).
Tree: trunk + branch
A good pipework arrangement has 'trunk' pipes from the boiler in 22mm (or 28mm depending on the output of the boiler and the manufacturer's instructions) with branches in 15mm (or microbore: 10mm or 8mm) to individual radiators. Pipe sizes can be stepped down from trunks through branches e.g.
The topology of a microbore installation is usually a tree with trunks of 22mm and branches of 10mm and/or 8mm pipework, and is inherently well-balanced.
The fully-annealed ("Table Y") copper pipe (or flexible plastic pipe) used for the microbore sections can be bent and threaded through stud walls and joists unlike rigid copper in small-bore sizes.
Some clients may prefer to have 15mm tails to radiators (joined to microbore under the floor or in the wall) for the sake of appearance.
There is an article on retro-fitting 15mm radiator valves to microbore.
Limited power throughput (2.5kW-ish)
Narrow bore is more vulnerable to sludge.
inherently balanced but rarely practicable
An arrangement with radiators connected with widely varying lengths of narrow-bore pipework is bad for balancing but sometimes necessary especially when extending an existing system where access under floors etc is limited.
The choice of routes for the pipework may have to incorporate a number of criteria. These may be aesthetic, cost or performance based. For instance, the performance considerations will urge you avoid routing through unheated spaces or a zone differing from the one to be heated. The cold space below a suspended ground floor is not a natural performance choice and requires effective insulation but is an overwhelmingly good choice from the cost and aesthetic aspects.
Typically there are general approaches depending on the structure of the floors.
- All floor(s) suspended: Main pipes run vertically between floor(s) in one place (often a corner near the boiler). Plastic pipe is a great choice as it can be cabled through joists and readily installed in runs between joists; it is also out of sight, has fewer joints and lower heat losses. Copper/chrome tails emerge through floor to supply radiator from below.
- Ground floor is solid other floor(s) are suspended: The upper floors or even the loft space (for a bungalow) are used as above and also to supply the radiators in rooms below. Groups of one to three radiators (or perhaps more) are supplied from above by a pair of pipes. Each group will need a drain off point. Pipes drops are on show but can be hidden in a duct but this can make a feature out of a necessity, there will still be runs under the radiator.
- All solid floors and ceiling: Copper pipes are run round at skirting board height from room to room, going through internal walls as needed. This usually leads to a inferior layout topology so needing extra care to balance the system. The entrance door is often an obstacle; it can be unsightly to go over and across and down, and that leads to the needs for air bleed points, ideally at the top of each down flowing pipe, this is a further draw back. It is often best to bite the bullet and dig the concrete up to cross under the doorway and accept that such sites have difficulties.
- New build: Apart from the possibility of underfloor heating there is also the oportunity to install microbore plastic behind dry lining boards.
- installation in solid floor
Building regs impose restriction on the places and sizes for notches and holes in joists. Notches must be no more than 12.5% of joist depth installed between 10% and 25% along the spans length. The best way to cross joists is through holes bored in the middle of joists. Microbore can be cabled through, about 4 or 5 joists seems to be the maximum number that can be crossed by any single length. Flexible plastic pipe is a very welcome material in these cases, even then a good right angled joist boring drill or cordless equivalent is a must for this part of the job. Rigid pipework has to be installed in notches, which should be placed in the middle of the floor board.
It is certainly good practice, if not manadatory, to install drain off points on the lowest points of the pipework. Ideally the best place is outside over a gully. Some radiator valves include a built in drain tap. There are two types of fittings on is known as heavy and light pattern. The heavy type have a seal around the spindle they are a little less messy when in use. There is no reason why you could not use ball-o-fix service isolators although this does not seem to be common practice.
- plastic v. copper or chromed pipetails
- play in tails
- pressure testing