Heating units compared

Press, Volume CXIII, Issue 33251, 14 June 1973, Page 13

Heating units compared

' By

JOHN CRANKO

8.E.. C.ENG., M.I.MECH.E... MNZIE.. MNZIHVE.,)

(A Consulting Engineer)

Heating of the whole house — usually called central heating—may be undertaken with a wide range of heating equipment. The number of types and models of equipment available, together with claims made for their special abilities can be confusing. Let us look at some of the factors which will help in designing an efficient heating aystem, and thus aid in the selection of suitable equipment. Please appreciate that psychometrics and human comfort are complexly interrelated, and an article of this length cannot attempt to give more than some general guide lines. The most important piece of “heating equipment” gives out no heat at all—it Is thermal insulation. Minimum Insulation in new houses in some council areas is now mandatory, but better insulation than the mandatory offers greater fuel savings—a U valve of 0.1 Btu/sq. foot hour degree F. may be easily and cheaply achieved for ceiling roof combinations and similarly values of 0.25 or better in walls and floors. Insulation is available in two basic forms: cellular fibrous materials whose structure encloses tiny air pockets, e.g. vermiculite, fiberglass, fire resistant macerated paper and foam plastics, or reflective foils which are used in conjunction with an enclosed air space. Insulation installation is linked with the necessity to provide vaper barriers — a subject now much better under stqod by the building trade. Data on this subject and that of insulation requirements for new dwellings is available to builders

from one Christchurch area county council in its publication "Manual for the Installation of Insulation.” Before looking at the various types of heating equipment, their fuels and their methods of control one must appreciate that there is a constant interchange of heat by radiation between the human body and its solid surroundings. If one is in a room were the air temperature, air movement rate and humidity are all at comfortable levels, one may still have a “chill” feeling if any wall or other surface is even a few degrees cooler than the air temperature-— this is the so called “cold wall” effect. Conversely, one may be quite warm and comfortable even with low air temperatures if one is surrounded by warm solid surfaces radi. ating heat. Thus, comfort requires not only correct air temperature, but a’so that either all “cold walls” are eliminated or the “cold wall” effect is countered by the right amount of radiant heat. Room linings of low mass, well insulated, reduce the “cold wall” effect to where a well designed hot air heating system can become comfortable, but ideally some form of radiant heat should still supplement the warm air. Running cost» Running costs include three principal areas: Cost of fuel used. Cost of heat wasted caused by inefficient control. Cost of annual maintenance. Fuel costs Typical figures for the cost of fuel for each therm(100,000 Btu) of useful heat

delivered into the house are: All forms of electric heaters (except air conditioners and night rate storage heaters assuming power 1 cent per unit), 29.3 cents/therm. Night rate storage heaters (5/8 cent per unit), 18.4 cents/therm. Dieselene, 75 per cent efficiency, 16.5 cents/ therm. Home heating oil, 75 per cent efficiency, 19.9 cents/therm. Gas, 75 per cent efficiency at $1.17 per 1000 cu. ft, 35 cents/therm. Coal, 45 per cent efficiency at $1.82 per 1401 b delivered, 27 cents/ therm.; at 65 per cent efficiency, 19 cents/ therm. Coke, 45 per cent efficiency at $17.50 per ton, 14 cents/therm. Heat wastage A room not being used wastes heat if it is kept at — an over-ride switch permits the heat to be turned on should the room or zone be required to be heated at an unusual time. It can be readily appreciated, if one thinks of one’s living pattern, that good design temperature all the time. Whilst the simpler centrally-placed air furnaces may be “zone controlled” by shutting doors to rooms which are not in use, this is seldom achieved in the average family situation. The more sophisticated hot air or hot water systems ■where heat is ducted or piped to the individual rooms are readily controlled both by zone and time clock. Electric systems, either skirting convectors or underfloor heating, can likewise be zone and time

clock controlled. It works simply by the time clock shutting off the heating tn rooms or zones when they are not normally occupied “Zone Control” in an average house could save up to one-third of the fuel billls providing it is coupled with proper thermostatic control of each zone. Such zoning and time control does require that the heating units should have a rapid response, so that once the heat is introduced into the room it quickly becomes habitable. Maintenance All equipment must be maintained. Generally oilfired equipment will need at least one, often two maintenance visits each year by qualified persons. Static electrical equipment, e.g. underfloor heating, skirting convectors require almost on maintenance. Summing up Insulation is most important. You must select a heating system to match the design of the rooms. Remember the “cold wall” effect. Shop around and get all the data on several types of system and don’t forget, that unless a heat pump is involved, one kilowatt will, no matter what is claimed, only produce 3412 Btu/Hr. of heat. Remember the old rule of thumb; lived in area times 40 gives output heat in Btus required from the heating unit(s). Do think about maintenance costs as well as capital and fuel costs. Do look at other peoples’ systems before you buy and judge for yourself how good they really are.

, house heating may be classThe more commonly jged under a number of available equipment for headings, and tabulated to

CENTRALLY-PLACED HEATING UNITS show the principal means Such a table is given below, of distribution and control. The first section deals with

centrally placed heating individual units, which ma' units and the second section be linked to form a whole considers various types of house heating system

Name 1. Gravity air under floor; free furnace in wall; standing console. 2. Gravity air furnace as above, but incorporating a fan. 3. Hot air, fan blown furnace. 4 Hot water boiler.

Usual Fuels and Burners Blended oil. vapourising burner. Gun burner, diesel or blended oil. Coal, coke- oil or gas.

Control Wall thermostat and high low burner carburettor. Water thermostat controlling boiler, room thermostat(s) controlling circulating pump and motorised valves. Radiator thermostatic valves controlling heat out from each unit.

Heat Distributed By Room and interroom convection. „ -. plus ability to supply a short length of duct and one/two room grilles. Duct work which must be insulated. Small insulated hot water pipes.

Heat Delivered in Rooms By Room Convection. and through grilles off duct. Air delivery grilles. Skirting convectors radiators, etc.

Characteristics High temperature air leaving units Doors must be open to transmit heat through house. Rooms furtherest from unit tend to be colder. Hot air leaving unit. Some doors must be left open to permit heat to flow through house. May include air filter, fresh air control, automatic motorised duct dampers controlled by room thermostats a neb time clocks, dividing house into zones. If well designed, provides an excellent heating system. A very controllable and sophisticated heating system which coupled with good zoning and appropriate ventilation can provide optium comfort conditions and the correct balance between convective and radiant heat. Can also provide domestic hot water at minimum cost.

Order of Cost S3OOS6OO S4OO SBOO SIOOO- - SISOOS3OOO

In general these individ- through portable fan heatual heaters are powered by ers, electric fan heaters electricity and range from placed either in wall fitthe simplest electric fire, ments or (less readily acces-

INDIVIDUAL UNITS sjble for maintenance) in skirting convectors, electric boxes under the floor, fixed heating elements in concrete in place radiant heaters, floor; wall, or ceiling plaster oil filled tubular heaters, panels, (the last two a new

development) electric storeage heater units, to window type air conditioning units. Only fixed appliances which

may be integrated inte a whole house system have been listed below.

5 Name Fan heaters in cabinet boxes: wall or underfloor 5 placed. j Electric skirting _ heaters. z • Radiant heaters. 1 Underfloor heating. Heating cables buried in concrete floor slab. t' In wall or ceiling , panel heating. 1 Electric storage ‘ heaters. ' Window or through wall air conditioning I units.

Usual Fuels and Burners Electricity at normal rate. Electricity at normal rate. Electricity at normal rate. Electricity at night rate. Electricity at normal rate.

Control Room Thermostat. Individual thermostat built in the unit. Usually none. Room Thermostat. Thermostat. Thermostat and manual or automatic damper. >» >»

Heat Distributed By Room forced convection. Natural thermal convection within room. Radiant heat. Convection and radiation. Long wave radiation. Natural convection and radiation. Forced air.

Characteristics Generally not greater output than 4 k.w. (13.6488tu hr), and hence only suitable for heating an area of between 350 and 450 sq. ft of average house. Unit sizes 1-2 k.w. (3412 to 6824 Btu hr), one or two units will heat the average room. May be readily controlled by time clocks. Zoning the house saves power bills. Ideal in bathrooms, etc., where high radiant heat desirable. May be linked with light switch. Often rather slow response rate; generally very satisfactory in well insulated houses. May be zoned and controlled by time clocks. Only possible if installed at time of building of house. General intensity 10-18 watts/sq. ft (34 to 62 Btu/sq. ft.). Comments generally as above. System must be designed to have sufficient storage capacity. Consult with Supply Authority Supplies. Good blend of convective and radiant heat. May include reverse cycle (heat pump) heating cycle. Generally units quiet enough for bedroom use. In-coming fresh air damper controlled. Air filtered. Provides complete air conditioning. Units capacity generally about 12,000 Btu/hr on both heating and cooling cycles.

Order of Cost sl2fl-$250/Un!t. $4O-50/Unit installed $3O-50/Unit installed About 50 cents/sq ft of floor area. $l5O-$250/Unit. $6OO-$BOO/Unit.