Heat Pumps Today

www.acrjournal.uk/heat-pumps 15 T E C H N I C A L Figure 3 GSHPs on resilient pads Figure 4 GSHPs on inertia bases. The springs provide vibration isolation, but the concrete is unnecessary Figure 5 GSHPs on isolated frames (fitted whilst the plant remained operational) observation in a ected apartments simultaneously with controlled plant operation to evaluate the level and character of problematic sound, identify the relative significance of di erent items of plant and assess likely sound transmission paths; ° indicative airborne sound insulation testing between the plantroom and adjacent apartments; and ° indicative impact energy transmission tests to the pipework, using a mallet, to assess the relative strength of some structure-borne energy transmission paths. This initial research established the following: ° the predominant acoustic energy transmission path between the plant and apartments was structure-borne, with the building providing adequate airborne sound insulation, probably without the need for the false ceilings that had been fitted in some cases; ° the most significant sources of plant noise within the apartments were the ground loop pumps not the GSHPs themselves; ° the use of inertia bases for the GSHPs was inappropriate. The sole purpose of an ‘inertia’ base (i.e. mass of concrete attached to the source) is to reduce the amplitude of vibration of the attached plant, which, in this case, is not particularly significant. However, the vibration isolators on which the inertia base was mounted were appropriate and a more suitable solution for the GSHPs would be to mount on the vibration isolators using support frames; ° it appeared that significant energy was being transmitted into the structure via pipework connections to the plantroom floor, walls and, in some cases, ceilings as well as directly from the plant supports into the floor; ° the energy in the pipework through the remainder of the buildings was generally relatively insignificant; ° the building construction was relatively live and provided an e… cient structural energy transmission path, exacerbated by the beam and block spanning over void ground floor construction, which did not allow the ground underneath to provide damping to vibration transmitted from the plant above; ° there were strong modal responses in some rooms, typically in one or more of the 100 Hz, 250 Hz and 315 Hz one third octave bands; ° on some occasions there were significant interactions between the two GSHP systems adding a beat frequency to the sound; ° e ective vibration isolation of the relevant parts of the GSHP system should be able to provide significant attenuation to the sound within the a ected apartments without the need for any additional airborne sound insulation; and ° the strut channel frames used to support the plant and fittings could provide a useful mounting system for appropriate vibration isolation in most cases. However, it was not su… ciently sti for such an application with the ground loop pumps without significant alteration or replacement with sti er structural support frames. In addition to several sites with existing noise problems, three new sites were under construction. This gave the opportunity to incorporate appropriate mitigation during the design and installation phase of the GSHP systems. However, this could not delay construction, so decisions had to be made based on the information available at that time. Remedies At the existing sites, the plant and pipework were decoupled from the building structure. The resilient pads that had been fitted were unsuitable both because they would be expected to provide little deflection (and associated vibration isolation) under the loads and in the locations where they had been applied and because the frames were bolted directly to the structure through the resilient pads, bridging any isolation they may otherwise provide. It was determined that the necessary isolation could best be achieved with steel spring isolators. Figure 6 Strut channel bolted to wall with resilient pads, and pump on resilient pad on cantilever bracket Figure 7 Strut Channel bolted to wall with resilient pads