Building Materials – Heating

The final topic of our Building Materials portion of Building A Home In New Zealand concerns the topic of Heating.  This topic is intended to be read with the other Building Materials topics: Windows, Thermal And Noise Wall Insulation, Solar Heating and Electrical, and Attic Venting and Internal Moisture Control, and ideally before or in conjunction with Choosing a Builder, and Designing a House.

 

Every house is going to need some source(s) of heat. How you choose to heat your home can have a large impact on how comfortable your home is and how much you will have to spend on heating throughout your ownership.

One thing that you will need to also consider is how much heat will be required to heat your home. If you are building to minimum NZ building code insulation specifications, you will require larger heating sources than if you built to US or UK insulation codes because lower NZ insulation standards mean more heat will be lost. Therefore, getting a recommended 25 kW wood burner to heat your 200 square home will be overkill if you insulated properly. With proper insulation, smaller heat sources may provide enough heat and may have to be used less frequently.

Different heating types provide different amounts of heat at different costs, which will be covered in the individual heating sections below.  Each type of heating unit will have a specified heat output in kW.  Some of these are designed for heating individual rooms, and others put out enough heat to heat the entire house if distributed correctly. These aspects should be considered when choosing the main sources of heat.  Historically, houses that use natural gas (includes reticulated gas and 45 kg LPG cylinders) or solid fuel tend to be significantly warmer than electric and LPG-heated homes because of their higher heat output.

You have a few choices to discuss with your builder, and some of the most common ones are listed below:

  • Electric Heating Panels / Space Heaters
  • Heat Pumps
  • Wood Burner
  • Under Floor Heating
  • Central Air

 

Electric Heating Panels / Space Heaters

Electric space heaters and heating panels use electricity to heat small areas. Typically with small maximum output less than 2.4 kW. These are designed for individual rooms and may not provide enough heat for rooms or houses that aren’t well insulated.  The initial cost is relatively low. Ongoing costs include energy cost at 15.9 c/kWh with line charges of 92.9c per day (not additional to other electricity). Efficiency is normalized to 100%.  Houses with electric space heaters tend to have the lowest overall temperatures in winter either due to their small output or lack of use.

Space heaters can also be powered by natural gas.  These can be temporary or flued permanent structures.  If not flued, combustion byproducts are harmful and can also cause humidity problems (see ventilation). Flued natural gas delivers heat at about 8.6 c/kWh with daily ‘pipe’ charges of 112.3 c/day with an efficiency of 80%. Rooms heated with natural gas tend to have the second highest overall average temperatures, likely due to the larger heat output.

 

Heat pumps

Heat pumps are the most efficient electrical heating devices. These can be small and provide enough heat for a single room or can be large enough to provide heat through the entire house by a ducted central air system. Delivers warmth at 5.1 c/kWh with line charges of 92.9c per day (not additional to other electricity). Efficiency is calculated at 300% of heat kW output / kW input. Typical size is 4.8 kWh with the largest single room units being in the 8-9 kWh range.

These units also have the advantage that they can also be used to cool, with lower efficiency, in the warmer months.

 

Wood Burner

There are two types of wood burners, a standalone enclosed fire, and an open fire (fireplace). The enclosed fire is much more effective for heating compared to a fireplace. An enclosed wood burner delivers heat at about 10 c/kWh based upon 71% efficiency (71% of total heat transferred into the room). Open solid burners are much less efficient at ~15-20% efficiency with 85% of the heat being lost through the flue.  Fireplaces also induce a draft that pulls outside air into the house resulting in a cooler house outside of the room being heated.

Enclosed solid fuel burners to be the warmest homes at an average of 18.8°C because of the size of the heat output where open solid fuel burners are the coolest at an average of 16.0°C. Maximum output of an enclosed solid fuel burner is around 25 kW but most operate at 3-5 kW for a majority of the time regardless of their maximum.

Because of the large possible heat output, it is best to have an efficient heat transfer system to transmit the heat throughout the house. If choosing a ducted heat transfer kit, keep in mind that the capacity of the transfer kit. Effectively transferring heat from the fire may require high enough flows that more than one fan is needed.  This is because you are transferring marginally warmer air at ~30°C instead of much warmer air generated using a heat transfer pump.  The insulation on factory heat transfer ducts is also horrible. We recommend getting extra wall insulation to wind around the factory ducting to the air in the farthest rooms isn’t too cold.  It may be common to lose as much as 5-10°C from the air in factory ducts from experience. With the wall insulation, we lose <1°C / 18m of ducting.

Some install wetbacks on the fireplaces to also use the fire to heat their hot water. A wetback will transfer small amounts of heat to water that is transferred to the hot water cylinder, typically in a closed circuit.  However, if using the fire often, the wetback can overheat the water resulting in expulsion of the overheated water.

One other consideration for a wood fire is the fuel. Wood is a completely renewable resource and may be relatively inexpensive if a supply is readily and locally available.  In addition to the supply, storage of enough wood for a season or two should also be considered.

If choosing an enclosed fire, there are a couple of different types of flues. Both flues consist of two concentric tubes. The inner tube carries the hot exhaust from the fire out of the house. The second larger tube blankets the inner tube with cooler air to insulate and cool the exhaust. The least expensive standard flue version uses indoor air to insulate the inner tube. This is the same inside air that you are using the fire to heat. You are therefore losing a portion of your heat. The second type of flue uses either outside or attic air as the blanket air. This keeps the warm indoor air indoors and is more efficient than standard flues.

 

Under Floor Heating

Underfloor heating consists of two types, electric heating elements or heated water transferred through tubing. For both, the entire floor area is covered to provide a more even heating that space heaters.  The electric heating uses elements installed under the flooring material. Systems that move heated water through pipes in the floor (hydronic) utilize a ‘boiler’ to heat the water. Boilers can be run from solid fuel, gas, LPG, or heat pump technology. Solar heat is rarely recommended for this as solar is weakest in the winter months when it will be most needed.  In some instances, underfloor heating can also be hooked to a wetback on a enclosed solid fuel burner, but consider matching the size of the wetback/fire/heated area because traditional wetbacks for water heaters are typically too small for hydronic use.

If heating inside of a slab, remember that the slab holds a lot of heat and it will take time to heat up the slab. An advantage of this is that the slab will also hold that heat and slowly release it. If you are using a fire, you may not need to keep the fire going continuously to consistently heat your home.  You will need to ensure that your slab is fully properly insulated with no thermal bridging or you will be heating the ground under your house too.  In many areas, the under floor heating is laid on top of insulation on top of the slab to minimize thermal bridging with the slab and ground.  Additional detailed information can be found here.

 

Central Air

The trend of having central air is growing in NZ, likely because of international influence.  As of 2006, only 5% of NZ homes had central heat (including central air and under floor heating).  Since it may be new to some, central heating / cooling has one heating and cooling source for the entire house with ducts transferring heated or cooled air to individual rooms. This has an advantage that the entire house can be comfortable, and not just rooms that have heat pumps. Each room has a register that can limit or stop the flow of central air to that room.  The larger units also tend to be more efficient than having a separate heat pump in every room.

Central air houses, regardless of heating source, were the warmest behind enclosed solid fuel houses.

 

Next Steps:

This brings to a conclusion our Building Materials discussions for Building A Home In New Zealand.  You now have a starting point to Choosing A Builder, Designing A House, and discussions of multiple new Building Materials.  Your next step is Breaking Ground.

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