Space Heating

The design of a home's heating system involves decisions about the fuel to be used, AC or not, method of distribution, and the system for heating domestic hot water (DHW). The choice of any one element, the type of heater, for example, is related to another choice, such as the method of distribution. Some of the so-called choices are forced on the designer by area conditions. Others tend to fall into place after the designer, or the owner, decides that one of the mechanical elements can be met in only one way.

Also, Design of the building will dramatically affect the heating/cooling load that the mechanical system must handle. Departure from the guidelines under Siting and Design can double the heat load, or require air-conditioning in an area that should not need it, or impose other demands that we can not foresee.

So, in this section, we don’t try to pick the total system or size that is best. We simply present information that might help you in your decision.

Equipment sizing, incidentally, starts with a calculation of the building’s winter heat loss (and summer heat gain). For this, see the section on Figuring Heat Loss.

When oil is the fuel of choice:

Starting in the late 1930s,  #2 heating oil became firmly entrenched in the Northeastern United States, long before the gas lines had reached that far from the Southwest or from Canada. Today, oil may still be chosen, in homes and other buildings, because it provides more BTUs per dollar than any other common fossil fuel.

Owners using oil will probably want hydronic heat, from a boiler, piped to room radiators. The burner in that boiler should use flame-retention technology to achieve 80% efficiency, and any burner that does not provide that efficiency should be replaced.

For the homeowner, that oil-fired boiler can also heat the household’s domestic hot water (DHW) for showers, laundry, etc. That obviates the need to buy, and buy again, a separate water heater. Oil burns so hot that, with a separate internal heat exchanger, the boiler can heat the domestic hot water for your shower as fast as the water runs through the exchanger.  That is why it is called the "tankless" water heater.

When gas is the fuel of choice:

For most of the US, street gas is the preferred fuel for space heat and DHW. Gas can, of course, fire a boiler for hydronic space heat, and the higher burner efficiency that is possible with gas will tend to reduce oil’s price advantage.

As for the “tankless coil” method of heating DHW, gas does not burn as hot as oil, and may not be able to keep up with the rate at which the domestic water will flow through the water-heating exchange coil.
However, with gas, the space heating system probably would use a furnace, (hot air, not hydronic) and distribute the heat through air ducts, because:

• With no oil storage required, the gas furnace will generally cost less than oil-fired.
• The gas furnace can be configured for upflow, or downflow, or horizontal flow, adapting to a wide variety of HVAC designs.
• The ducts for distributing the heated air will easily accept the Air Conditioner's cooling coil, and one distribution system will serve for both heating and cooling.

When Electricity is the Fuel of Choice:

In the chapter on Cost of Fuels, it was clear that electricity, a wonderful servant in all other respects, is a very expensive source of heat.

However, in the southern United States, with milder temperatures and lower electric rates than in the northern states, electric heat can be practicable.

• Provided that: Each space has its own heater on its own thermostat, and each thermostat
  has the lowest possible setting for that room's usual function (including Off in unused rooms).
• And Provided that: The walls, ceilings, and floors between heated spaces and normally-unheated spaces are heavily insulated.

Heat Pumps:

This section deals with Air-source heat pumps, not to be confused with Ground-Source heat pumps, which are described in Solar Heat.

The heat pump is, in essence, an air conditioner that is run backwards during the heating season, taking heat from the outside air and pumping it inside. As you might expect, taking heat from really cold outside air, in order to heat inside space up to 70°F, is a challenge!  The heat pump can meet it, however, because the exchange coils on the outdoor unit are kept colder than the outside air by Phase Change, described elsewhere.

There is heat in everything that is above minus 273 degrees Fahrenheit, so BTUs will move from the cold outside air into the colder coils, just as it does inside a household refrigerator. 

As backup, the heat pump is installed with electric resistance heaters that have the capacity to heat the whole house. These resistance heaters kick in when the heat pump cannot get enough heat from the outside air.

For the builder, there is money to be saved by combining the heating unit and the cooling unit in one appliance.  For homes South of about 38°N, the heat pump will probably provide comfort at reasonable expense.

However, for occupants north of the Mason-Dixon line, the operating cost of a heat pump will soon wipe out any savings in first cost, because: as the outside temperature drops to 40°F and below, the moisture in the air condenses on the cold outdoor exchange coils that are supposed to be extracting heat from the air. The moisture freezes there, insulating the coils and preventing that exchange. Now, to knock the ice off of the coils, the control reverses the system for as long as five minutes out of every fifteen.
The system is now working as a conventional air conditioning system in winter, taking heat out of the house to melt the ice off of the outside coils.

To accentuate the negative, the back-up electrical heaters had turned on when the house temperature started to drop. Thus, the heat that the heat pump takes out of the house, to melt ice off of the outside heat exchanger is now provided by electricity, the most expensive of heat sources.

Distribution of Heat:

In laying out a hydronic system for distributing heat, or an air-handling system for distributing heating/cooling, the installer should consider some variables that most have, so far, overlooked.

These  include:

1.    In a hydronic system, baseboard radiators are usually placed against an outside wall. We have measured the temperature on the outside surface of residences with baseboard heating, and found that the narrow band immediately outside of the baseboard is usually 10° to 15° warmer than the rest of the outside wall. That means  heat loss!.

The simplest solution is to ease the steel backing of the baseboard unit  far enough off of the wall to squeeze a ½” sheet of polyurethane or extruded polystyrene behind it.

2.    In a hydronic system, the hot water goes into the loop at one end, through every radiator "in series", and back to the boiler. This produces extra heat at the start of the loop, and less toward the end, as the water that was heated by the boiler gives up its BTUs in transit.

To compensate for the lower temperature at the end of the loop, increase the length of the radiators slightly, toward the end of each hydronic loop.

If the system is already installed, slightly close the flaps across the top of the convectors near the start of the loop.

3.    In air-distribution systems, central trunks serve each room separately.  If the HVAC installer has not adjusted duct sizes to compensate for the distance from the furnace, the diffuser shutters in the rooms closest to the start of the supply duct can be partially shut manually.

For multi-level homes and other buildings, designers and installers usually treat each level of a building as one zone with its own thermostat. Thus, all of the spaces on one level are on the same heating loop. When that zone's thermostat calls for heat, all sides of that floor get heat, even though bright sunshine may be pouring in through the windows on the South side.
As a result, it is not unusual, on the coldest days of winter, to see room air conditioners working busily in rooms and apartments on the South side of multi-unit buildings.

Burning the candle at both ends?

When planning a new building, or renovating an old one, specify that the North side of the building have one zone with its own thermostat, separate from the zone that heats the South side.