02 July 2001
I Heating and cooling demand in residential buildings
The heating demand of new houses has significantly decreased over the period 1990 – 2000, in some countries, for instance Germany and the Netherlands, by a factor of more than two. This has been achieved by new building codes which result in tighter buildings with respect to ventilation losses, and improved thermal insulation of the building envelope. Ventilation heat loss is currently about 50% of the overall heat loss of a house, compared to 20% for the existing building stock. Heat recovery from ventilation air is becoming increasingly important.
The heating demand of low-energy houses is lower than current building regulations require. The annual heating cost of such houses is low. This means that the payback period of the additional investment cost for an energy-efficient heating system is longer. This is even more true of so-called passive houses, which have an energy use for space heating of under 15 kWh/m2 per year.
The cooling demand of residential buildings is not regulated in Heat Pump Programme countries. The residential cooling demand is increasing as a result of architecture: larger windows, lighter construction and better thermal insulation lead to temperature increases in summer. The building may not be able to remove the heat fast enough.
II Distribution/ventilation systems
Different climates and different building traditions result in a variety of distribution system choices:
– In the USA and Canada, ducted air systems have a strong market share in large parts of the country. In regions without a cooling demand, direct electric heating and hydronic systems are used as well;
– In northern, western and central Europe, hydronic heat distribution systems dominate, except in Norway, where direct electric heating still has a strong market share. Residential cooling is usually not required;
– In Japan, ductless split room air conditioners are popular, because of their modest space requirements and their ability to supply both heating and cooling. Heating or cooling is supplied directly to the room where they are installed.
The selection of a heating and cooling distribution and ventilation system is a multi-variable problem. Decision-makers can be architects, home-owners, property developers, etc. Energy efficiency, indoor air quality, comfort and cost are among the most important factors to be considered:
– The system should be able to meet the required heating, cooling and dehumidification load in any situation. The cooling and dehumidification load can exclude hydronic systems, in particular radiators. The cooling capacity of a ceiling is about 60 W/m2 and of a floor about 30 W/m2, which is sufficient for many applications. However, when the outdoor humidity is high, these systems would require an additional system for dehumidification of the ventilation air, to avoid discomfort and condensation on the cooling surfaces.
– The energy efficiency of heat pump systems is closely linked to the heating and cooling distribution temperature. For a high performance, heat distribution temperatures should be as low as possible, and cold distribution temperatures as high as possible. For actual heating applications and systems, the seasonal performance factor (SPF) of a heat pump will increase by 15 – 17% when the distribution temperature decreases by 10ºC , with distribution temperatures ranging between 70ºC and 40ºC and source temperatures between -5ºC and +10ºC. Heat pumps using ducted air systems and hydronic distribution systems with a large surface area, such as floor heating, compare favourably to hydronic systems with a distribution temperature higher than 50ºC.
– The energy efficiency of hydronic systems is generally higher than for ducted air systems when parasitic losses by fans and pumps, and distribution losses by leakage and transmission are considered.
– The parasitic losses of ducted air systems are in the range of 10 – 20% of the electric energy required for space conditioning with a heat pump; in hydronic systems this is between 6% and 9%. Fan-coil units have parasitic losses of about 20% of the overall electricity consumption for space conditioning.
– Distribution losses in air ducts are from 20 – 55% of the total heating demand. For hydronic systems these losses are 3 – 7%. Air duct losses can be reduced by installing the ducts in the conditioned space.
– The economy of the distribution and ventilation systems depends on several factors. Especially in low-energy houses, where the annual heating cost is low, it is difficult to justify the investment in expensive energy-efficient heating systems.
– In most of the USA and parts of Canada, space heating, cooling and ventilation of buildings is required. An all-air system, which can combine these functions, tends to be cheaper than separate systems.
– In many European countries, space cooling of residential buildings is not required. Hydronic radiators and exhaust-only ventilation constitute the most economic option for most new houses.
– For low-energy houses, different solutions are required. Balanced ventilation with heat recovery is often a requirement for low-energy use. Economic comparisons show that integrated systems, which include production of domestic hot water (DHW) and air heating must be preferred.
– The heat distribution systems considered in this report are able to provide adequate indoor comfort, certainly in new, well-insulated houses. Floor and wall heating systems have the advantage of a high share of radiative heat transfer, low air speed, and little turbulence compared to other hydronic systems or ducted air systems. They can also provide limited cooling.
– An adequate ventilation system is the first prerequisite for satisfactory indoor air quality. Ducted air systems enable handling of the recirculating air. Floor systems have the slight advantage that the required room air temperature is up to 1ºC lower than for other distribution systems, because the radiative temperature of the building elements is higher in such systems than in systems which are mainly convective. Surprisingly, it was found that a low room air temperature is the primary indicator for a positive assessment of the indoor air quality.
III Trends by country
Four trends in distribution system choice can be observed. These trends depend on climatic zone and building tradition:
– Increased use of air systems, in the US and parts of Canada, as a result of higher comfort cooling demands.
– Increased use of hydronic systems in Norway and the UK. In Norway, the government promotes hydronic systems, which allow the use of various energy sources as alternatives to direct electric heating. In the UK, hydronic systems satisfy the higher comfort demands better than the traditional separate heaters.
– Increased use of low temperature hydronic systems in several countries in western and central Europe, in particular floor heating systems, as a result of higher comfort demands and government policies.
– Continued large-scale use of split-type room air conditioners, mainly in Japan and other East Asian countries.
These developments may work in favour of the heat pump market, probably except in the UK. There, the market is conservative, and the extensive gas grid does not make electric heat pumps very attractive. The choice of a distribution system is primarily governed by comfort, indoor air quality and other concerns such as space requirements. Energy efficiency considerations only play a minor role.
Ventilation is growing in importance as building envelopes become tighter. Mechanical ventilation guarantees sufficient, but not redundant, airflow and offers opportunities for heat recovery. Both exhaust-only and balanced ventilation with heat recovery save energy by avoiding redundant ventilation air flows. Exhaust-air-source heat pump water heaters (EAHPWH) are popular in several European countries, e.g. Denmark, the Netherlands and Sweden.
In countries such as Canada, the Netherlands, Norway and Sweden there is growing interest in balanced ventilation with heat recovery, but it is still a very small market. The economics are problematic but improving, as the performance of the equipment improves, and the price decreases as the market grows.
Selection of a heating and cooling distribution and ventilation system is a multi-variable problem, with first cost and comfort being key factors. Energy efficiency, indoor air quality, comfort and economy should be considered in a balanced way. As this study shows, each type of distribution system has specific advantages and disadvantages as regards the above factors. In fact, the quality of the individual building which needs a heating, cooling and ventilation system is the most important factor when choosing a suitable system.
Any distribution system should be designed for operating temperatures which are as close to the indoor setpoint temperature as possible. Differences between various options as regards efficiency, economy and comfort are fairly marginal.