01 August 2001

Retrofitting with heat pumps in buildings

Executive Summary

I Introduction

The potential market for heat pumps for retrofitting in existing heating systems is substantially larger than for application in new buildings. However, in most countries, this potential is far from being realised. This report provides an overview of what is being done to overcome the technical market barriers, in particular to find solutions for the high distribution temperatures which are often required for existing hydronic heat distribution systems.

Retrofitting is a means of rectifying existing building deficiencies, improving the standard of a building and, sometimes, making it suitable for alternative uses. It reflects the need to maintain and preserve the building for a longer period of time than the life of its technical equipment. Building retrofit often includes replacing the heating system. Heating systems may also be retrofitted without changing the building itself.

Retrofitting of heating systems with heat pumps has its own specific problems, of which the high distribution temperature used in existing hydronic distribution systems is the most prominent. This report provides possible solutions to these problems. Cooling is not considered in detail. The geographical focus is on northern, western and central Europe, as the challenges with high-temperature heat distribution are typical for these regions.

II The market

The potential market for retrofit heat pumps is about three times larger than the market for heat pumps in new buildings. The new building construction rate is about 2% of the existing building stock annually in most countries. If a heating (and cooling) system needs to be replaced after 15 – 20 years of use, then 5 – 6% of the existing building stock is confronted with the need for a retrofit every year.

However, in most countries the real market for retrofit heat pumps is substantially smaller than the market for their applications in new buildings. Even in the USA, only about 50% of unitary heat pump shipments replace existing heating and cooling systems (see Section 3.1.2). In the USA, heat distribution is by ducted air systems in most existing buildings, and consequently the barrier for incorporating the heat pump in the existing distribution system is expected to be smaller than in most northern and central European countries. In Switzerland, only about 20% of the heat pump market (<20 kW) is for retrofit. This corresponds to 3% of the market for replacing/retrofitting existing heating systems. In Germany, retrofit heat pumps are estimated to account for around 25% of the market.In Norway and Japan, the markets for heat pumps for retrofit are better developed. In Norway, approximately 50% of the heat pump installations are ductless air-to-air split units. Most of them are installed for replacing room heaters in existing single-family houses, which previously were heated with electric resistance heaters. In Japan, the replacement market is dominant, and heating and cooling heat pumps are replaced every 10 - 11 years. In 1999, the replacement market for heat-pump-type room air conditioners (RACs) was 5.4 million units, which is 88% of the total heat-pump-type RAC market. The 1999 replacement market for heat-pump-type packaged air conditioners (PACs) was 440 thousand units, which was 70% of total heat-pump-type PAC shipments. In the UK, the replacement market is strong in commercial buildings, estimated at 67% in 1996.An IEA Heat Pump Programme analysis in 1999 revealed that heat pumps can cut global CO2 emissions by more than 6% [1, 2]. This potential will be realised when 30% of the buildings are heated with state-of-the-art heat pumps, which save at least 30% CO2 emissions compared to conventional heating methods. This message is still valid today, but to realise the potential, the retrofit heat pump market must be increased.III Market barriersThere are several barriers obstructing the market for retrofit heat pumps, including both technical restrictions and economical factors. The main barriers are:- High distribution temperatures in existing systems. The traditional hydronic heating systems require high supply temperatures in the range of 90 – 70ºC. This is much higher than the high-temperature limit of most heat pumps, which is around 55ºC, depending on the type of working fluid.- As a result of the high distribution temperatures, the heat pump system must operate with high temperature lifts between the heat source and the heat sink. This means a lower COP for the heat pump system, and a higher compressor discharge gas temperature. This may result in a shorter working life for the heat pump, and a greater risk of compressor failure. The low COP is aggravated by the fact that outside air is the most obvious heat source, as it may be difficult to install a ground-source heat exchanger or well system near an existing house or building.- Heat pumps for retrofit tend to be less cost-effective than heat pump installations in new buildings because the higher temperature lifts result in lower COPs. The retrofit heat pump may also require extra investment for a machinery room, piping, automation and electric installations compared with installations in new buildings.- Heat pumps for retrofit often meet restrictions in the space available for the new installation. Heat storage tanks are often impossible, and the installation of a ground-source heat exchanger or a ground well may be difficult.IV Possible solutionsIva Reducing the required supply temperatureThere are several methods for reducing the distribution temperature of an existing heat distribution system. Old hydronic systems are often designed for higher heating capacities than the actual design heat demands. In that case, the distribution temperatures may be reduced below the original design values. The distribution temperatures may be reduced further by reducing the specific heat demand (W/m2) by improved insulation of the walls, installation of new windows with lower u-values, etc. Changing the strategy of night set-backs will lead to reduced heat demand in the morning, and lower distribution temperatures will be sufficient to maintain the required room temperature. However, it should be ensured that the heat pump in combination with the new control method really saves energy.Replacing the existing heat distribution system with a new low-temperature system is another measure to improve the operating conditions. A good technical solution is to replace the existing high-temperature hydronic system with e.g. a new floor heating system, but this may be expensive. Another approach is to install additional heat transfer surface in specific rooms by means of larger radiators or more efficient heat exchangers.Another possibility is bivalent operation, in which the heat pump covers the base load and an auxiliary heating system the peak load. This reduces the overall system efficiency, but makes it possible to operate with a supply temperature of e.g. 70ºC at design conditions.Ivb Ductless air-to-air systemsDuctless air-to-air heat pumps are attractive when there is no heat distribution system available. In Japan, the market is dominated by these systems, which are successful for retrofit as well. Their performance is similar in retrofit applications and in new buildings. However, most units on the market are not optimised for heating-only in cold climates.Ivc Working fluids for higher temperaturesThe highest recommended temperature in single-stage, small- and medium-size heat pump systems that use HFC refrigerants is 60 - 65C. With the use of larger centrifugal R-134a compressors in two-stage heat pump installations, the temperature limit may be increased. For small- and medium-size heat pump systems, however, higher temperature limits may be achieved by using natural working fluids such as hydrocarbons, ammonia and carbon dioxide.The hydrocarbons butane and iso-butane, and mixtures of hydrocarbons may be used up to 100C. They are therefore interesting for retrofit applications. The rather high pressures on the condenser side of ammonia systems limit their application in heat pumps for higher temperatures when normal 25 bar equipment is used. Carbon dioxide (CO2) can be used as a working fluid in a trans-critical process.Ivd Cycles and controlsThere are some technical possibilities to improve the chances for retrofitting with heat pumps, e.g. by using two-stage compression, compression with saturated vapour injection, and trans-critical cycles with hydrofluoroethers. Studies of such systems have been carried out in Germany and Switzerland.Ive Absorption systemsThermally-activated heat pumps have the advantage of a high-temperature driving energy. Gas-fired absorption heat pumps are therefore particularly suited for the retrofit market. The Dutch company Nefit Buderus has developed a 4 kW heating diffusion absorption heat pump for the retrofit market. Field tests have been completed in the Netherlands and Germany.Another gas-fired absorption heat pump for the retrofit market was developed by Heliotherm in Austria. The 18 kW heating heat pump can be used with various heat sources. A prototype has been tested, and a field test is planned in 2001.V Other factorsA high quality of system design and installation, and a focus on cost-effective solutions are at least as crucial in the retrofit market as in the market for new building construction. Concerning economy of the installation, for retrofit long operating hours at base load are very important. From that viewpoint, integrated systems for space heating and hot water production are potential retrofit applications. Health and care institutions with a day-round, high demand for space heating are also applications where successful retrofitting with heat pumps can be expected.VI ConclusionsThe market for retrofit heat pumps could be three times the market for heat pumps in new buildings if there were no barriers for retrofitting with heat pumps. These barriers are mainly technical, and especially related to high distribution temperatures in hydronic heat distribution systems, which reduce the efficiency and the economy of the heat pump installation. These barriers may be removed by reducing the distribution temperatures, and by introducing specially-designed heat pumps.