Power at Work: Don't oversize your heating units

Thursday, April 2, 2009

If you are buying a space heating unit for your house, shop or poultry/livestock building, remember that oversizing can waste energy and cost you big dollars

RALPH WINFIELD

There was a time, especially when we were young, when we held steadfast to the view that bigger is better. And, yes, even later in life we were told by sales personnel that we could get a bigger ice cream cone or tractor for just a few cents or dollars more.
But do be careful with that mindset if you are buying a space heating unit for your house, your shop or your poultry/livestock building. An oversized heating unit can waste energy and cost you big dollars.

First, let's assume that you are going to heat a non-ventilated building, such as your house or shop, with a forced-air heating system. As the outside temperature decreases during the fall, only a small quantity of heat is required to bring the inside (room) temperature up to the desired level. Thus the heating unit will be cycling ON-OFF-ON in an attempt to maintain the desired set point or inside temperature.

Do remember that heat losses from a reasonably well-sealed building, without planned ventilation, are directly proportional to the temperature difference between inside and outside temperatures. The heating system should be sized based on the temperature difference, using the January design temperature for your location.

In addition, some air infiltration (exchange) will occur, even in newly-constructed buildings with a good vapour barrier acting as an air exchange seal. Thus, you need to add about 20 per cent to the temperature difference heat loss calculation.

What will be the result if you add 50 or 100 per cent to the temperature difference heat loss when your heating unit is selected? You will be very uncomfortable with the very short ON cycles and long OFF cycles of your heating system, especially if you are trying to relax in your favorite chair after supper. So uncomfortable, in fact, that you will get up from your chair repeatedly to raise the thermostat setting during those prolonged "OFF cycles."

The result will be increased energy usage in an attempt to maintain a comfort level. Even a sweater or throw is insufficient to overcome the effect of temperature cycles.
For any of you old enough to remember the wood-fired cook stove in the kitchen, you will recall that it was a great heat source. The reason is that it was constantly on and you could move closer or away to find your comfort level. However, you will also remember the cold drafts around the windows in that old farm house!

Excessive air exchange from any building can be a real energy robber, as well as a creator of an uncomfortable environment. Note the word "excessive." Virtually all buildings, including new air-tight houses, require some air exchange to control humidity and the buildup of undesirable gases.

Buildings housing combustion heating devices must have a source of combustion air that will prevent negative pressure in the building. Many new air-tight houses have special heat exchangers installed to provide fresh air, while minimizing the heat losses associated with direct ventilation of a building.

Ventilated livestock/poultry buildings.
All buildings housing livestock or poultry must have air exchange provision, again to control both humidity and other gases produced by the animals or their excrement.
Most mature animals can tolerate some temperature cycles between night and day, because they have fur or feathers to provide a thermal barrier. But young animals do not have that tolerance, so we must provide warm, draft-free environments for chicks, poults and piglets.

When heat is added to ventilated buildings, we must ensure that oversized heating units do not cause over-ventilation. If that happens, energy (and money) will be wasted. But, just as importantly, if young animals are subjected to cyclic over-ventilation, the relative humidity of the air will also cycle.

This can and will create respiratory problems in those young animals. To prove my point, have any of you had a multiple-day winter stay in a hospital lately? The air is often so dry that the respiratory system literally dries up. Ditto for many hotels.

Heating systems with mass. In recent years, since the rise in energy costs, have you noted the increased interest around "in-floor" heating systems? Any mass – be it water, concrete or other material – attached to the user end of a heating system will effectively dampen the system's ON-OFF cycle. With sufficient mass attached, the heat supplied can be from any source – natural gas to wood or straw – and can be oversized and not cause the uncomfortable cycling effect.

Units burning outdoor wood, corn or pellets are appearing on many Ontario farms. These relatively high heat output units can serve two or three buildings with hot water for space heating or heating potable water. Space heating systems can vary from coils (radiators) in a forced-air ducting system, to individual baseboard-type hydraulic heaters, to piping in concrete or other floors with sufficient thermal mass.

So, if you are designing or purchasing a heating system for direct air heating, do be careful not to oversize the heating systems. Buy a staged system or, alternatively, have a backup supplemental heating system for those few cold days in January or February.

Before you spend money on a new or supplemental heating system, do make sure that you have sealed up those cracks around windows and doors. Infiltration air above that needed for combustion or relative humidity or gas and odour control can be a serious energy and dollar waster as well. BF

Agricultural engineer Ralph Winfield farms at Belmont in Elgin County.