Central heating design

This article is about designing and installing 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

Heat requirements

• Discuss + links to whole-house heatloss calculators
  • IDHE (Institute of Domestic Heating & Environmental Engineers) calculator
  • Energy Saving Trust calculator

• Discuss + links to energy conservation articles

Heat Sources

Fuels

The most popular fuels for central heating systems are:

• natural gas
• oil
• propane ("Calor" gas)

The latter fuels are rarely used when natural gas is available since they are more expensive.

• 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.

• 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 for any significant heating load.

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.

• Lots of solar designs
• Solar system expertise

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

output specifications

• Delta-50 and Delta-60

locations

• wrt heat losses - under windows v. inside walls

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.

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.

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

Thermostatic Radiator Valves vary the flow in each radiator according to room temperature.

Fit to all radiators except in area with zone thermostat

TRVs are not perfect thermostatic controls, but they do reduce any amount of imbalance in the system by dynamically varying the balance according to room temperature.

TRVs alone are not capable of maintaing a preset room temp, and heating systems with no room stat, relying solely on TRVs, have inadequate control, and can be considered defective.

mixed rads + UFH layouts

UFH requires reduced circulating water temperature. This is usually achieved with a pipe thermostat which blocks flow from the main CH pipiework to the UFH when the UFH is upto temperature, plus a separate pump to circulate the UFH loop.

Blending valves + pumps

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]]

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

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.

microbore

Easier installation, as pipe can be bent rather than cut & joined.

It is often preferred to fit 15mm tails to rads, and join these to microbore under the floor, to maintain a nicer appearance. Another option is to use microbore all the way to the rad, and cover this with chromed plastic radiator tail wraps.

+ balancing

Limited power throughput

Narrow bore is more vulnerable to sludge.

tree + branch

([28]-22-15-[10/8]) - good for balancing

dual loop

inherently balanced but rarely practicable

random

bad for balancing but sometimes necessary

radiator on HW circuit

This is never an advisable option. It is found with some existing heating systems, and is almost always due to mistaken connection. Unless the radiator is a combined towel warmer and radiator in use in a bath or shower room.

Signs of a radiator on the HW circuit are:

• Radiator comes on sometimes in summer
• Radiator sometimes fails to come on when other rads do
• Insufficient heating in the affected room

The solution is to replumb. Unfortuntely this means cost and time inputs, and sometimes these are not available. There are workarounds which may prove adequate, but are never ideal.

Installation

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