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Is the higher cost of heat pump compared to gas boiler worth it?

10 Sep
Is the higher cost of heat pump compared to gas boiler worth it?

The capital cost of a hydronic heat pump solution for heating and cooling, using water as the heating and cooling medium, will always be more expensive than a gas boiler because of the greater complexity of the heat pump. However, it does give you reverse cycle cooling also in summer that is very effective in a well-designed home in a southern or inland climate where humidity is naturally lower. 

Most people look to cover the higher capital cost of a hydronic Heat Pump through operational savings particularly if they have or are planning to have solar power. This, combined with the thermal mass of the house and batteries as they become more cost effective, can provide heating and cooling at night. 

Heat Load:

For a well-designed house (e.g. 6 Star in Victoria), an approximate winter heat load can be calculated based on 60W per m2, at least as a starting point for design. On this basis, a 220m2 home has an approximate heat load of 13.2kW. Therefore, a heat pump model such as the Stiebel Eltron WPL25ACS with a heat output of 14kW would match this heat load. In practice, there are many factors to consider such as opening windows, zoning, comfort levels, insulation levels, house design, local climate, and ancillary heating from sun radiation through northern windows, fireplaces and air conditioners.

An air conditioner in reverse cycle is a heat pump but it acts directly with the air and is the most cost-effective heat pump solution compared to a gas boiler but you need to be happy with heat being blown through a duct or from an indoor unit on the wall. 

Hydronic Heat Pump and Air Conditioning:

A popular compromise solution is to have a hydronic Heat pump for the living areas and a reverse cycle air conditioner (i.e. heat pump in winter) for the bedrooms. This is a particularly good solution in a two-story house where the bedrooms are on the first floor and the living areas are on the ground floor.

The air conditioner is best at cooling, where air flow is desirable in summer and the need for cooling on the first floor is greater. The cool air will fall down to the ground floor and work in conjunction with the passive cooling there from the reverse cycle hydronic heat pump. In winter, the superior from of radiant heating will rise up to the first floor and the air conditioner can provide a boost function in reverse cycle.

Operating Costs:

In regard to comparing operating costs of a heat pump with a gas boiler, it depends on many factors including the way the house is used. As a guide, let’s assume a heat pump runs in heating mode for 6 hours per day over 4 months of the year and in cooling mode for 3 hours per day over 4 months of the year to provide hydronic heating and cooling for the year. Let’s also assume the house has an area of 220m2 and heat load of 13.2kW. At $0.25 per kWh, this works out to around $800 per annum. However, if you are using your own solar power, the opportunity cost of this (i.e. the price you would otherwise receive from feeding it into the grid) is only around $0.06 per kWh. This works out to around $190 per annum.

You can construct your own operating cost comparison by comparing your gas heating bills with your estimated heat pump run time in heating and cooling mode over a year, then use a power cost factor relevant for your own solar power (opportunity cost of grid feed-in tariff) or grid power supply (your retail power price). Use a factor of 4.5 as the heating/cooling kW output from a single kW of power input.

Passive Cooling:

Hydronic heat pumps provide passive cooling by sending cool water through the hydronic circuit above the dew point, which is monitored by the controller. For underfloor applications, this is typically at around 18 deg C and a bit lower for radiators. The cool floor or absorbs heat from the air into the water and takes it away to the heat pump to be cooled in reverse cycle. Heat pumps used for underfloor hydronic applications are particularly good at absorbing radiant heat from sun radiating through windows in summer so you don’t get a heat bank effect from your house floor slab or masonry walls that causes discomfort in summer as the thermal mass releases its heat into the evening when you are trying to stay cool.

Air circulation is the other factor that aids passive cooling, as air moves across the floor or radiators and keeps you cooler too. The effect of air flow alone, across your skin, feels about 4 degrees cooler which is why fans in summer are so effective. Combining air flow from fans with passive cooling from a hydronic heat pump is an ideal combination. In southern and inland climates, this method of cooling will also maintain the humidity in the air which can be a good thing in summer. As we know from air travel, very dry air is not desirable and very humid air from a tropical climate can be uncomfortable too. The best quality air has humidity in the Goldilocks zone of not too much and not too little.

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