Understanding Heating and Ventilation – Rev 1

Originally posted on October 3rd, 2010. (In the last few weeks it disappeared for some unknown reason)

Understanding mechanics is nothing to do with mathematics, it is the reality of what is actually happening and why it is happening.  Note:- I have been asked “what has heating and ventilating got to do with mechanics?”

Mechanics is about moving things, and relates to anything that moves, whether levers, steam engines, ships, aircraft, spacecraft, arms and legs, water, sewage, blood, air, heat, light, magnetic radiation, radio waves, electricity etcetera.

Heating and ventilating is primarily about moving air, water and heat.

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As one of the jobs that my wife has added to my very long list of things more important that physics, I am now in the process of redoing my loft insulation, a most uncomfortable job.

This has brought to mind an occasion many years ago when one of my duties was to arrange for ‘Air Curtains’ (An industrial version of the air curtains found in many shopping centres and stores.) to be fitted to the industrial doors in a large production area.

I arranged for quotations from 6 manufacturers.  Over the next few days I took manufacturers engineers around the production area to enable them to produce their quotations.  These visits normally took about 2 hours.  However, one manufacturers engineer was there for two days.

The following week  I received the quotations and arranged separate interviews with the manufacturers to discuss their quotations.

The company whose engineer had taken two days I left till last, because their quotation had been the lowest and I was curious.

When I finally asked them how they had arrived at their price, I was given a technical file of about 30 pages.  That included the details of every single source of heat production within the manufacturing area.  This included machinery, pipe-work, workers and normal heating devices.  It even included variations allowing for summer/winter and night/day.  They got the job, but I had difficulty with our directors, who had a practice of throwing the highest and lowest quotes out, and then choosing between the remainder.  They did not understand that being a real expert allowed you to bring your prices down.

What finally convinced them were the running costs and guarantees.

The same problems arise with home heating and ventilation.  Every item in your house affects the temperature and ventilation requirements.  Heating costs are higher in a ‘minimalist’ environment,  (all goods and furniture are like storage heaters.) An empty room can lose all its heat in a few minutes, but an over furnished house will take hours to drop to the same temperature.  I have been looking after an old terraced cottage for a lady in hospital for 12 months.  The cottage is stuffed with furniture and other goods, but over the severe winter in which outside temperatures hardly ever got above zero, the temperature inside the cottage never felt uncomfortably cold.  There is no central heating in the cottage and the only form of heating was a gas fire, and the gas was turned off.

Like our manufacturing area, every item in a house either stores heat or produces heat.  The heat storage includes walls, furniture, goods and even items down to cutlery and children’s toys.  The heat producers include lighting, electrical equipment, bodies (including animals) sunlight and finally your main heating source whether a central-heating system or electric fires, gas fires or solid fuel burning.

Basically your central heating system compensates for reductions in the overall heat input into your house.

If you decide to switch off all your lights, the central heating system will compensate for the heat loss.  (The energy dissipated by lighting is nearly all heat, only a  small amount is radiated as light, and most of this is converted to heat when striking some object.  In the olden days when I was a lad, it was not uncommon for people to put an electric light bulb in a biscuit tin or similar metal container to use as a heater)

If everyone leaves the house, the central heating system will make up for the heat loss.  If you let the cat or dog out, the central heating will make up the heat loss.  If you switch off all your electrical equipment that is in standby mode, the central heating system will make up for the heat loss. However, if the outside temperature is high, switching off ‘heat producing items’ will save you from discomfort and save you money.  During the summer we had about 8 days in which the central heating was off, but the radiators were still very warm due to the heat accumulated in the roof-space, and we had ‘free’ hot water for a week.

Normally, in Britain, you will only save energy by ‘energy saving light bulbs’ or switching off electrical goods that are normally on standby mode’ when the outside temperature is higher than the set temperature of your central heating system, and that doesn’t happen very often.

I have saved a little bit of energy by adding external insulation to our freezer, (its under the stairs in my computer room so it doesn’t worry about looking pretty).  I used under-floor insulation which is only about 2.5mm thick but is a good insulator.  If you put your hand on anything and feel the cold immediately, then it is a serious source of heat loss.  The only problem is that fridges and freezers circulate heat.  Any heat removed from inside the fridge is returned through the radiator at the back.  Obviously, if your fridge or freezer is outside in a garage or shed you should save energy and money.  However, if it is inside the house you will not save much money however ‘energy efficient’ it is.  My computer room is about 5 degrees Fahrenheit cooler than the rest of the house therefore I save some heat.

Generally, it is better to look at more obscure methods of saving heat.  Many people will quite happily insulate the hot water pipes in a house, but ignore the cold water ones.  However, the cold water pipes in your house absorb heat from the general house temperature.  In winter, flushing your toilet loses more heat than boiling a pint of water.  (I will be carrying out some experiments to verify the actual amount lost, and trying a method of insulating the cistern on the inside to reduce heat loss into it, and also reducing the condensation on the outside which can be a problem sometimes)

When you run your cold water tap, the water in the pipe-work has been absorbing heat from your house, which is then generally flushed down the drain.  (This depends on how close to the main water inlet  to your house is to the particular tap you are running, complicated isn’t it?)  Water can absorb a lot of heat, which is why it is used in central heating systems.  The ideal solution for showers is a heat exchanger in which the draining water heats the incoming water feeding the shower.  This can save approximately 40% of the cost of running the shower, but can be difficult to construct in many houses.

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How HOT Is It?

Many people get confused when attempting to estimate whether something is hot or cold.  This, in many cases, is because of the ‘rate of heat flow’ between different materials.  In between the two extremes of freezing and boiling (or burning) there is a ‘comfort zone’ in which no damage is caused to the human body.  At freezing point damage can be caused due to the high water content of the human body.  At boiling point the water in the human body starts to boil.  Both are obviously dangerous situations.

However, if you have both a cushion and a piece of steel at freezing point it will be fairly obvious, if you touch it, that the steel is at freezing point.  You would probably argue, on touching the cushion, that it was a lot warmer than the steel.  This is because the rate of heat transfer (RHT) between you and the cushion is a lot less that that between you and the piece of steel.  There is energy passing from your hand to the cushion or the piece of steel.  The human ‘machine’s’ heat detection system uses this ‘rate of heat transfer’ to assess danger levels.

When the RHT is too high, damage is done to the body.  This applies whether the heat is travelling out of the body or into the body.

There is a similarity between the action of low temperatures and the action of low pressures.

See Understanding Pressure and Vacuum.

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Ventilation

Ventilation is a major source of heat loss, whether by open windows, open doors or just draughts.  Unfortunately we cannot manage without ventilation.

All ventilation is a source of heat loss.  With doors the airlock principle works quite well, but is subject to the fact that the door is open for particular reason, and generally the reason takes your mind off saving heat, and the door is left open longer than necessary.  There is not a lot you can do about human nature.

Windows have a tendency to be opened when a room becomes stuffy.  Unfortunately, it gets left open until someone claims that it is too cold!  Your central heating then has to make up for the lost heat.

Most houses have a heat store that is ignored, and that is the roof space.  This heat can be re-used both for heating, and reducing the heat loss due to ventilation.  Its use for heating is used in quite a few industrial buildings, where the warm air at high levels is ducted down to lower levels with the aid of small fan units.  Note:-  It is advisable to thoroughly clean the attic with a vacuum cleaner before using it as a source of warm air, alternatively bring fresh air into the attic via a galvanised duct (preferably rectangular, it gives a better heat transfer) to isolate the air in the attic from incoming air.

It can also be used to warm-up ventilation air by running a separate duct at high level in the roof space (the fan unit should be at the ‘fresh air’ end of the ducting so that the duct is at positive pressure).  Note that a fine mesh filter should be fitted to the air intake to keep out wasps and other undesirables.  You would not be pleased if a swarm of bees arrived in your living room by way of your heating duct.

All ducting should be of galvanised steel, not stainless steel or plastic.  The zinc on galvanised steel is fatal to many bugs and viruses.  I have a feeling that many of the cases of ‘Legionnaire’s Disease’ may be caused by the use of plastics or stainless steel for the ducting.  These materials are relatively inert, so a warm ducting is an ideal environment for many ‘nasties’.  This problem may also occur in water systems where storage is by plastic or stainless steel tanks.

SEE ALSO “Boil or Carbuncle?”

Tip from my recent attic insulation antics. I had to carry out some extra electrical work and therefore needed to clear some of the insulation. I decided to cover the area of insulation I had to remove, with a thin plastic dust-sheet. Folding the insulation back using two pieces of wood, after doing  the electrical work the insulation folded neatly back without sticking to the rest of the insulation. I followed this by doing the rest of the attic in the same way. It keeps the dust off the insulation and in the case of any small water leaks will help to isolate the damage. Note: do not try to cover the entire attic with one sheet, multiple sheets help to retain some necessary air flow.

More to come on this.