There are arguments in the industry about whether buffer tanks are needed for heating systems, particularly with respect to wet heat pump systems. The current default is to fit a buffer on the basis that it saves the installer having to properly balance the system, although that is unlikely to be the explanation given to the customer.
The following research was undertaken that refutes the need for a buffer in most cases.
Recommendations for Installers
Buffer tanks are unlikely to be required when the heat pump can modulate (i.e. if the heat pump is not fixed speed).
Buffer tanks should not be installed in unheated spaces.
Central heating to high temperature radiators should be weather compensated with a single internal over temperature limiter to avoid overheating or a self learning algorithm controller should be used. Underfloor or low temperature radiators will not see much advantage from this.
Design for a minimum run time of greater than 4 minutes for a GSHP and 10 minutes for ASHP.
The volume of water always open to the heat pump i.e. without reference to water potentially closed off by a TRV is as per Table 23 and Figure 51.
If the above volumes are not available (for example if TRVs are fitted) the system volume should be supplemented by a two pipe buffer tank placed in the return pipe to the heat pump with the appropriate volume (i.e. above the minimum system volume shown in Table 23).
If TRVs are felt necessary they should only be fitted in the bedrooms (or in 50% of the property).