Central heating design

This article is about designing and installing Central Heating systems.

Heat requirements

• Discuss + links to whole-house heatloss calculators
• Discuss + links to energy conservation articles

Heat Sources

• single source
  • gas or oil boiler
    Discuss types of boiler, combi/conventional choice etc as per Ed's Boiler Choice FAQ with additional discussion of
    • combined range+boilers
    • CHP (Microgen, Whispergen)
  • electric
    • peak rate v. off-peak
    • standalone electric boilers
    • thermal store

• multiple sources
  • sources (additional to above):
    • ranges (Aga, Rayburn etc) with back boiler
    • wood burner with back boiler
    • solar thermal hydronic
      • Lots of solar designs
      • Solar system expertise
    • ground source heat pump
  • combining sources:
    • thermal store
    • Dunsley Neutraliser
    • controls

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
• may feel uncomfortably cold when shut off by thermostat (like electric fan heaters)

underfloor

radiant: heat occupants and fabric directly

• more efficient (e.g. 20%?) than radiators
• more comfortable: warm feet, cool 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
• need radiant-sensing instead of conventional air-temperature-sensing thermostats?
• 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 to lower-temperature flows from condensing and renewable sources
• expensive & disruptive to retro-fit to existing building: need to remove & relay floors (or poss. ceilings below for upper-floor installations)

other radiant

e.g. in wall, in ceiling (I kid you not!)

Controls and Zoning

• Zoning
• Y-plan, S-plan, pump-plan
• Timers, programmers, thermostats, programmable thermostats
  • location of thermostats
    • hall or living room - no external heat sources
• TRVs
• mixed rads + UFH layouts
• On/Off v. proportional control (aka Digital v. Analogue)
• Feedforward aka weather compensation etc

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 throug 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

microbore

easy install + balancing

tree + branch

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

dual loop

inherently balanced but rarely practicable

random

bad for balancing but sometimes necessary

Installation

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