Central heating design

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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:

This article is a skeleton: the main points to be covered are listed but some or all still need to be expanded

Heat requirements


  • Discuss + links to energy conservation articles

Heat Sources

Fuels

The most popular fuels for central heating systems are (in order of increasing expense):

  1. Natural Gas
  2. Oil
  3. 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").
  4. 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
  • geothermal

are also increasingly found contributing to heating systems rather than providing sole energy supply.

Appliances

boilers

The most common appliances for supplying heat are boilers using natural gas, oil or propane 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 Ed's Boiler Choice FAQ

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

Ranges e.g. Agas and Rayburns usually heat DHW as a by-product of their cooking functions. They may use natural gas, oil, propane or solid fuels.

Combined range/boilers -- which may be outwardly almost identical to ranges -- contain a separate central heating boiler sharing the flue of the cooking range. They may use natural gas, oil or propane. They are non-condensing appliances and therefore less efficient than current central heating boilers.

CHP

Combined Heat and Power (CHP) generators (e.g. Microgen, Whispergen) generate electrical power whilst heating primary water.

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.

solid-fuel back-boilers

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.

renewable sources

Solar thermal panels are usually used to provided Domestic Hot Water, although solar warm air is lower cost, more efficient, and can return more energy.

ground-source heat pumps provided 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 can also be used with thermal stores to combine their output with other systems including conventional boilers and/or electric backup heaters, to provide space heating via UFH and radiators, and may also provide DHW.

waste water heat recovery can also be used with thermal storage to contribute to space and water heating.

Heat Emitters

Emitters are means of heating spaces, such as radiators, under-floor heating etc.

radiators

output mostly via convection: heat air to heat fabric of room & its occupants Radiators comes in a number of types, shapes and sizes. Most are rectangular and painted white. Designer radiators can be more visually attractive and certainly will be much more expensive, they will give very much less heat than a similarly "budget" radioator. 

Ordinary Radiators

come in a range of heights typically between 300 and 800mm; most are 500-600 high unless they are 'dwarf' models usually 300mm high. Lengths are from 200mm upwards to 3m or more, but most radiators are under 1.8m long. Radiators have one or more panels,usually only one or two)


output specifications

Apart from the size (length and height) and type (one, two or even three panels each with or without fins


locations

  • Where in a room should a radiator be placed? In some rooms such as a bathroom the position of a towel warmer is decided by the layout of the room. In most other rooms there will be more choice. The decision mostly hinges on whether to place the radiator under a window or not.

The arguments for putting it under a window are:

  • it's not in competition with larger items of furniture as the latter won't be placed so as to block the window.
  • it will convect warm air up against the descending air cooled by the window so stopping convection draughts.
  • The outside wall may well be a stronger block wall better for hanging a radiator and than an internal stud wall.

The arguments against are:

  • Where there are excessively long curtains there is a temptation to draw them in front of the radiator, blocking heat to the room.
  • The wall behind and the window above will be warmer than the rest of the room and will increase heat losses to the outside.
  • Installation costs will usually be a little higher than a radiator on an inside wall due to longer supply pipe runs.

Even when a radiator is not fitted directly under a window it should be located close by and preferably on an outside wall. If the radiator is located at the opposite end of the room from windows and outside walls it is likely to result in the room being uncomfortably warm near the radiator and/or cold near the window & outside wall(s). This will be particularly so with large areas of single-glazed windows and solid or uninsulated cavity walls (although for energy-efficiency, and economy in the long-term, it is also advisable to improve insulation).

Fitting reflective insulating sheet behind radiators fitted on outside walls can help reduce heat losses directly throughh the wall.

fan-assisted e.g. kickspace

forced convection

  • 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

underfloor

radiant: heat occupants and fabric directly

  • require less total heat output (e.g. 20%?) than radiators for a given comfort level due to better air temperature distribution
  • more comfortable: warmer feet, cooler head; less stuffy
  • better for heating large spaces e.g. halls
  • limited heat output due to limitation on max confortable floor temperatures: may be insufficient for small rooms with large heat requirements & large losses e.g. bathrooms
  • heat output dependant on floor covering
  • slower to heat & cool than radiator based systems: need better control systems
  • Slow thermal response causes lower efficiency operation, since heat is given off when not needed
  • need radiant-sensing instead of conventional air-temperature-sensing thermostats? no
  • hydronic generally require lower water temp than rad systems - really need extra pump + thermostatic mixing valve to run off mainly rad-based system
  • better suited than radiators to lower-temperature flows from condensing and renewable sources, as the lower water temp enables solar collectors and heatpumps to operate more efficienctly
  • expensive & disruptive to retro-fit to existing building: need to remove & relay floors (or poss. ceilings below for upper-floor installations)
  • electric UFH has high run cost and is prone to failure due to corrosion.

More on Underfloor Heating

other radiant

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 using heated stud walls can be found here.

Heating ceilings has the obvious disadvantage of unwanted heat loss upwards but one (singularly ineffective) installation is known to the author.

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.

Configuration: Controls and Zoning

Zones

A zone is an area whose heating is under control of one time and one temperature controller, i.e. one timer and one thermostat, or one programmable thermostat. For example heating in a large house may be divided into one zone comprising living rooms and another zone comprising bedrooms, with a timer and room thermostat (or programmable thermostat) for each zone.

Underfloor heating is usually run as a separate zone from radiators.

Where Domestic Hot Water is heated by the boiler the water heating may be considered as a zone.

always-on/bathroom radiators

In some older systems it was common for the bathroom radiator to be connected across the flow and return pipework to the hot water cylinder, keeping the bathroom warm and towels hung over the radiator dried and warmed at all times. Where DHW is heated by gravity circulation the pipework to this radiator had to be arranged to allow gravity circulation through the radiator also: typically the radiator would have a flow connection at the top and return at the bottom.

In some fully-pumped systems one radiator (often in the bathroom) was arranged to be open at all times (with lockshield valves on both connections) so as always to provide a path for hot water even if all other radiators are closed. In S-plan systems or where the boiler manufacturer specifies a bypass loop such an always-on radiator may be connected directly across the boiler flow and return, before the zone valve(s).

As a matter of design choice a bathroom towel radiator can be connected directly across boiler flow and return, before zone valves, to allow towels to be dried and warmed when either space heating or DHW re-heating is in operation. As long as the radiator is not acting as a mandatory bypass for the boiler it can be fitted with a TRV so that it does not waste heat in hot weather.

Zone control configurations

Y-plan, S-plan etc

Honeywell diagrams

Pump Plan

Timers, programmers, thermostats, programmable thermostats

  • location of thermostats
    • hall or living room - no external heat sources

Additional Thermostats

Some houses have areas with quite different heat loss profiles. One example would be a partly underground building, where the upper storeys are exposed to exterior air temperature, but the underground floors are exposed to near constant soil temp all year round. Conventional heating control setups can not deal effectively with wide variations in relative heat output between areas. TRVs improve the situation, but they can not balance a system effectively, as too large an air temp variation is needed to achieve the large degree of flow modulation these situations require.

For such applications, 2 or more thermostats are required, one controlling the heat in each zone. Generally one timer will control both areas.


TRVs

A Thermostatic Radiator Valve varies the flow of heating water through a radiator according to the temperature of a sensing element in the head of the valve (usually: a few models are available with remote sensing elements). This gives an approximation to control of the room temperature. Although the control is far from perfect TRVs are a great improvement over manual valves for maintaining comfortable room temperatures in an energy-efficient way.

Bidirectional TRVs

A result of the design of TRVs is that they are liable to cause water hammer if fitted the "wrong" way round with respect to the flow of water through them. Most modern TRVs are designed to minimise the probability of this occurring and are descrbed as bi-directional. One model (Danfoss' RAS-C revolver) has a design which allows the flow through the valve to be easily switched to the correct direction.

Where to fit TRVs

TRVs may be fitted to all radiators except at least one in the area sensed by the zone thermostat. Where only a few TRVs can be fitted (for whatever reasons) the greatest benefit is likely to be gained by fitting them to bedrooms (with the largest, hardest to heat first) followed by large halls and stairways, and WCs. All these can ideally be run at lower temperatures than main living areas so using TRVs to maintain lower temperatures saves more energy.

In a standard system if TRVs are fitted to all radiators and there is no room thermostat then energy will be wasted by the boiler cycling when all rooms are up to temmperature. (There are, however, unusual systems employing thermal stores or other measures in which an all-TRV setup is intentional and energy-efficient.)

TRVs and balancing

Fitting TRVs to radiators is not a substitute for properly balancing a system. They can mask, and to some extent ameliorate, the effects of poor balance.

mixed rads + UFH layouts

UFH needs to be run at temperatures (circa 30°C) well below those used in radiators (c. 80°C). The standard way of reducing the temperature of water in mostly radiator systems uses a thermostatic mixing valve and a separate pump to circulate the UFH loop. Various cheaper schemes exist using e.g. a thermostat which blocks flow from the main CH pipiework to the UFH when the UFH is up to temperature. Where a thermal store or heat bank is used to supply space heating it is often arranged with a take-off loop towards the bottom of the store supplying water at lower temperatures suitable for the UFH.

Special controls

Generally available controls are On/Off: they control the boiler with Demand or No-Demand signals.

Proportional controls (with analogue sensors) are available with some high-end boilers. They tell the boiler what the current temperature is so the boiler can determine how much heat to produce to acheive the desired temperature.

Weather compensation controls are also available with some boilers: an outdoor temperature sensor provides advance warning to the boiler of the likely heat demand of the building.

Pipework

Pipework materials

Copper

Traditional material.

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

Features:

  • 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
  • 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

Plastic

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:

  • 28mm
  • 22mm
  • 15mm
  • 10mm

Features:

  • 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. [[1]]

and

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. [[2]]

However in real life not all central heating systems have effective corrosion inhibition at all times, so barrier pipe is still the preferred option.

Tails

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.

pipework layout

pipe sizes v. heat-carrying capacities + noise

Single pipe loop

Obsolete - not used for current designs but found in some old installations.

Single pipe means that radiators are plumbed in series. The problem is that as water passes through each radiator, it loses heat, so radiators at the far end of the chain have to be oversized, and run at reduced temps.

Where necessary to extend (add extra rads) can either add new rad into existing loop (allowing extra size for rad if at cool end of loop) or, especially if several new rads to be added, divide system and add a seperate 2-pipe loop - perhaps as a seperate zone if it makes sense.

Single pipe systems may underperform, as heating expectations are higher now than they were when these old systems were installed. Replumbing the radiators in parallel is a logical option, but there is a simpler and cheaper way to improve total heat output to some extent, and that is to increase pumping rate. The faster the pumping, the less temp loss occurs along the chain. A more powerful or 2nd pump can thus be a low cost way to increase system output.

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. 

                            15 --- RAD
                          /
                   == 22 =- 15 --- RAD
                 /        \
                |           15 --- RAD
                |
                |           15 --- RAD
                |         /
BOILER == 28 ======== 22 =- 15 --- RAD
       (flow    |
        and      \
      return)      == 22 =- 15 --- RAD
                          \ 
                            15 --- RAD

Microbore

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.

Dual loop

inherently balanced but rarely practicable

Random

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.

Installation

  • routing
  • installation in solid floor
  • joist notching
  • drain-off points
  • plastic v. copper or chromed pipetails
    • play in tails
  • pressure testing
  • flushing
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