Dearborn, Michigan

NOISE-CON 2008 2008 July 28-30

Noise and vibration control for a residential geothermal heat pump installation Jeffrey L Fullerton

a

Acentech 33 Moulton St. Cambridge, MA 02138

ABSTRACT The installation of a geothermal heat pump in the basement of a 1900 era home introduced

significant noise and vibration to a bedroom above. The primary cause of the noise and vibration

transmission related to the rigid connections between the heat pump piping and the wood

structure of the bedroom floor. Various measurements were conducted to assess the conditions,

followed by different attempts to isolate the noise and vibration from the structure. A secondary

airborne path was also studied with an attempt at mitigation. This paper will discuss the

conditions of the installation, various methods of reducing the noise and future research and

mitigation.

a Email address: jfullerton@acentech.com

1. EQUIPMENT DESCRIPTION AND INSTALLATION CONDITIONS Geothermal heat pumps are becoming a popular type of equipment for heating and cooling on

projects where energy efficiency is a goal. These systems utilize a fluid loop, which typically

consists of water, to transfer heat between the conditioned spaces and the ground. The thermal

energy is exchanged between the fluid and the occupied space by using a reverse cycle heat

pump. Depending on the season, the heat pump is used to transfer heat either (in cold weather)

from the ground to the occupied space, or (in warm weather) from the occupied spaces to the

ground. For this installation, both heat pumps are connected to the same open loop well that is

about 20 feet from the house.

The equipment discussed in this paper consists of two geothermal heat pumps that each have a

cooling capacity of approximately 3-tons, and which are designed for forced air heat transfer in a

residential application. Both of the heat pumps studied are ClimateMaster Genesis systems,

though one is a packaged unit (the heat pump and fan coil in one piece), while the second is a

split system, with the heat pump and fan coil components separated; the split system components

are connected via a refrigerant line of approximately 35 feet in length.

Both heat pumps are mounted in the basement of the 1900 era residence. The basement floor is

concrete. The first floor structure is a wooden joist construction, with 2”x8” joists (which are a

true 2 inches by 8 inches) spaced unevenly between 16 and 17 inches on center. The flooring

consists of a nominal 1-inch thick plank subfloor perpendicular to the direction of the joists,

topped with a finished floor of 1-inch thick pine flooring.

Mechanical and plumbing contractors, who are well experienced at their trades, installed the

systems. They used traditional mechanical and plumbing practices during the installation. These

Figures 1 & 2: Rigid clamps between copper ground water piping and wood joist structure of first floor residence.

Figure 3: Sound pressure levels of packaged and split heat pumps.

typical practices included rigid plumbing clamps that were used throughout the project to fasten

the piping to the structure (see Figures 1 and 2).

2. HEAT PUMP NOISE AND VIBRATION

The noise and vibration from the heat

pump systems was clearly evident from

the first time that they were operated.

Interestingly, the noise and vibration

generated by the heat pumps was

different for each system. The split

system generated a significant audible

tone and feelable vibration in the floor

above. The tone was associated with

the heat pump. The split system heat

pump also generated fluid flow noise

when it operated. The operation of the

packaged unit, which includes both the

heat pump and the fan coil components,

did not produce the tonal noise of the

heat pump, though fluid flow noise was

clearly audible when this system

operated.

Figure 3 shows a graph of the sound

levels from these two systems when

they were operating. The sound levels

of the split system heat pump were NC-

55 and RC-53(HF), while the sound

levels of the packaged heat pump

system were approximately NC-55 and RC-49(MF). The tone at 63 Hz generated by the split

system was extremely prominent and very annoying.

The most concerning aspect of the noise and vibration was its significant impact to a bedroom

directly above the location where the split system heat pump was installed. The noise level was

clearly intrusive and would be bothersome to anyone who might attempt to sleep in this

bedroom. The packaged heat pump was located below an office space; while the fluid flow noise

from this system was clearly audible and intrusive, it would not contribute to the issue of sleep

disturbance and was tolerable for an office space. Regardless, mitigation of the noise and

vibration from both systems was necessary.

3. HEAT PUMP NOISE AND VIBRATION SOURCES AND PATHS

A. Sources One of the primary components of the heat pump system is the compressor. The compressor

introduces energy into the system to facilitate the heat transfer process between the heat source

and the heat sink. The compressor is also the main component within the heat pump assembly

that can produce noise and vibration that might be a concern to the building’s occupants. The

compressor is connected to the casing of the equipment at its base, often through vibration

isolation, such as spring or neoprene isolators. In addition to the connection at the compressor’s

base, there are numerous rigid connections between compressor and the fluid and refrigerant

piping, which are connected to the equipment casing and the structure. Interestingly, a standard

practice during the heat pump installation is to mount the casings on 50-mm (2-inch) thick foam

insulation, which is purported to provide vibration isolation between the heat pump and the

flooring.

The heat pump is connected to the ground piping loop via 2 pipes, which carry the supply and

return fluid. The fluid noise passing through these pipes is another potential noise and vibration

source, depending on the fluid velocity. In these systems, the fluid (water) flowrate was limited

to approximately 0.57 l/sec (9 US gal/min). When the systems operated, this fluid velocity

produced fluid borne noise, presumably due to turbulence generated at the fittings and elbows as

the fluid passes through the system. This fluid borne noise included airborne and structureborne

transmission to the adjacent space and structure.

Some designers of geothermal systems also incorporate automatic balance valves to ensure that

the water flow through the system optimizes the rate of heat transfer between the fluid and the

heat exchanger. In this case, it was the automatic balance valve that restricted the flowrate to

0.57 l/s (9 US gal/min). These balance valves are another potential noise and vibration source,

since as they restrict the water flow through the device, the valves also produce turbulent fluid

flow. Similar to the fluid borne noise in the piping, the balance valves transmitted the noise via

both airborne and structureborne transmission.

B. Transmission Paths Though there are several transmission paths, the piping connections between the heat pump and

the structure were the most significant on this project. This is because there are a large number of

piping connections between the geothermal systems, the associated plumbing components and

the structure. Each of these piping connections represents a potential path for noise and vibration

transmission to the building structure. These piping connections are described below.

Geothermal systems connect to a well or external closed loop via supply and return pipes. These

two pipes are typical of every geothermal system installation. Some manufacturers and

Figure 4: Piping wrapped with foam

insulation and supported with pipe

strapping.

contractors allow this piping to be PVC, while others prefer to use traditional copper piping. On

these systems, the piping between the well equipment and the heat pumps consists of copper.

These copper pipes were supported from the wood joist structure of the first floor using

traditional rigid piping clamps, as shown in Figures 1 and 2.

Another feature that some geothermal systems have is the ability to pre-heat domestic hot water.

This feature involves routing the domestic water service for the building to the inlet of a

“desuperheater”, which transfers excess heat from the heat pump to the incoming domestic hot

water supply. The domestic water lines on this project are copper. These domestic water lines

are routed to the hot water tanks, which are approximately 25 feet away from the heat pumps.

These domestic water lines, to and from the hot water tanks, are also supported from the wood

joist structure of the first floor.

Another rigid connection between the heat pump and the building structure is the supply and

return refrigerant lines. These are used between the split system heat pump and the fan coil unit.

Both of these lines consist of bendable copper tubing. The supply refrigerant line is typically

insulated. These lines were clamped to the wood joists.

Due to their connections with the compressor and equipment casing, these pipes often are very

effective transmission paths for the compressor vibrations.

In this project, there is also a concern for airborne sound transmission. The flooring consists of

merely two layers of nominal 1-inch thick wood planks that are not well sealed. There is also no

ceiling in the basement at this time, though a gypsum board ceiling is planned in the future.

3. NOISE AND VIBRATION CONTROL

A. Piping Isolation – Homemade The rigid piping connections to the wood joist structure were observed to be the most significant

transmission path between the heat pump and the first floor residence above. Modifications of

the rigid piping connections were needed to improve the isolation of the heat pump and fluid

noise and vibration. Ideally, the connections between the

piping and the structure would be removed all together;

however, this condition would leave the piping without

any support, which over time might cause the soldered

joints between the copper pipe fittings to fail.

To introduce a resilient connection between the piping and

the wood structure, the rigid clamps were individually

removed and replaced with a homemade resilient pipe

clamp detail. This detail consisted of foam insulation,

often used for thermal insulation, fitted around the copper

pipe with bendable steel pipe strapping loosely wrapped

around the insulation to secure the piping to the wood joist

structure. This arrangement allowed the pipe to be

separated from the wood joist with the foam insulation,

while being held into place with the steel pipe strapping

(see Figure 4).

The sound levels with this option were noticeably lower

and more acceptable. The ratings for the new sound

levels were NC-38 and RC-29(N), which represent

significant reductions in the audible sound levels. The

Figure 5: Compressed foam insulation.

Figure 6: Piping clamped in the

engineered pipe isolation products.

noise reduction occurred primarily in the mid and high

frequencies, above 125 Hz. There was also a significant

reduction at the primary tonal frequency of 63 Hz, where a

reduction of nearly 15 dB was achieved. The significant

decrease at 63 Hz was offset by a nearly 10 dB increase in

a tonal sound level at the 125 Hz third octave band

frequency. The final sound level was lower in sound level,

but almost as annoying as the original sound levels when

the rigid clamps were in place.

Over the course of about 1 year, one problem with this

homemade resilient pipe clamp detail became apparent.

During that time, the foam insulation slowly compressed

under the weight of the piping, so much so that the pipe

appeared to be resting on the pipe strapping with only a thin layer of compressed foam separating

them. A piece of foam insulation was removed and photographed, as shown in Figure 5. A

repeat of the sound level measurements indicated that the sound levels had increased several

decibels in across the spectrum. The latest sound levels were NC-42 or RC-29(MF), which

represented an increase in the NC rating, and a change in the Quality Index of the RC rating.

It was thought that this extreme compression resulted from the concentration of the piping

weight on a narrow strip of the pipe strapping. In an attempt to alleviate this concentrated

loading on the foam, the overcompressed section of foam insulation was removed and replaced

with a new section. This new section of foam was support using a thin strip of steel inserted

between the steel pipe strapping and the foam insulation. This strip of steel extended for

approximately 100 mm along the length of the pipe. The thought was that this new piece of steel

would distribute the support of the pipe strap across a larger area of the foam and therefore it

might reduce the compression of the foam. Several months later, it was apparent that even this

strip of steel was not alleviating the excessive compression that the foam insulation was

experiencing. For this reason, the foam insulation method for pipe isolation was deemed

unsuccessful.

B. Piping Isolation – Engineered Product During the time of this work, a new series of piping

isolation products were introduced to the market. They are

described as “engineered piping isolation products”, which

are designed to provide the noise and vibration isolation that

designers, contractors and owners are seeking for quieting

plumbing systems. The products come in a variety of

mounting options and styles to fit with any number of

construction types (wood or metal joists/studs), with two

sizes to handle ¼ to 1-inch diameter pipes (model 250 or

255) and 1 to 2-inch diameter pipes (model 280 or 285).

The pipe is held within the isolator by two rubber “cradles”

that isolate the rigid pipe from the rigid plastic of the

isolator structure.

The foam insulation and piping straps that supported the

piping were replaced with the engineered pipe isolators

(models 250 and 255). The installation of the new piping

isolators was straightforward and relatively easy to perform.

Figure 7: Sound pressure levels in bedroom with rigid and

isolated piping conditions.

Figure 8: 1-inch diameter pipe in (left) smaller acoustical pipe clamp

(model 250) and (right) larger acoustical pipe clamp (model 280). The red

circle highlights the thin neoprene contact point with the copper pipe.

(see Figure 6).

The audible sound levels with these new

piping isolators were significantly

different than when the previous isolation

was installed. A repeat of the sound level

measurements with these new piping

isolators resulted in sound levels of NC-

45 and RC-27(MF). For comparison, the

measured sound levels with the rigid and

isolated conditions are graphed in Figure

7.

Interestingly, while these metrics indicate

somewhat higher sound levels, the

spectrum was dramatically different with

a reduction of more than 10 dB at the 63

Hz tone and a 3 dB increase at the 125

Hz tone compared with the foam

insulation. However, subjectively, the

resulting spectrum was considerably less

annoying to hear in the bedroom.

While these products provided a

significant improvement in the

perceptibility of the sound level, the

overall sound level is still rather high.

Improvements to the effectiveness of the

isolation were pondered. Specifically, it

was noted during the installation that the

1-inch diameter pipes did not seem to be

as effectively isolated as smaller diameter pipes were in the chosen isolator size (model 250 and

255), which were intended for pipes up to 1-inch diameter. The concern was that the isolation

provided by the rubber cradle was diminished with the 1-inch diameter pipe, since the rubber

was only a small fraction of an inch when this large diameter pipe was used, as opposed to the

true cradling action that occurred when a smaller diameter pipe was used. The next larger pipe

isolation clamp (model 280 or 285), which is sized for 1 to 2-inch diameter pipes, provided the

desired cradling of the 1-inch

diameter pipe. See Figure 8

for a comparison of the two

clamps installed on a 1-inch

diameter pipe.

To test this theory, the next

larger pipe isolator size was

used in the installation for all

of the 1-inch diameter pipe

clamps. The larger isolators

were somewhat more difficult

to install, due to their larger

size and the tight constraints

between the piping and

structure. Once complete, the bedroom

sound levels with the larger isolators

were measured. The results indicated

that the larger isolators resulted in

sound levels of NC-40 and RC-30(LF).

For comparison, the third octave band

sound levels are plotted in Figure 9.

The loudness of the audible tonal

components of the larger isolators

varied, such that the tone at 125 Hz was

reduced by about 6 dB, while the tone at

63 Hz increased by more than 15 dB.

The subjective impression of the sound

was that the larger isolators provided

little to no sound level reduction,

despite the somewhat reduced NC and

dBA sound levels.

Further research is planned to install

one other variation of pipe clamp to

determine whether it provides any

further sound level reduction.

C. Airborne Noise Control – Loaded Vinyl Blanket

Another concern with the split system

heat pump installation was the airborne

noise that may be contributing to the

sound transmission to the bedroom

above. The wood plank floor construction between the basement and bedroom is not airtight and

it also lacks substantial mass, consisting of only about 1¾-inches of wood.

In an attempt to control the heat pump sound within the basement, a mass-loaded vinyl blanket

was placed overtop of the heat pump. The blanket consisted of a 1/8-inch thick mass-loaded

vinyl with a quilted fiberglass layer on one side, which was placed directly onto the casing of the

heat pump. The blanket was large enough to completely cover the heat pump with overlapping

the layers on the sides.

Sound levels were measured in the basement with and without this mass-loaded vinyl blanket

cover. Both sound levels in the basement were NC-61, though the uncovered heat pump sound

levels were RC-56(HF B), while the sound levels with the heat pump covered were RC-57(HF).

The difference in the Room Criteria levels was primarily due to a 6 dB reduction at 63 Hz, which

was indicated the “B” quality index when the heat pump was uncovered. The measured third-

octave band sound levels with and without the mass-loaded vinyl blanket cover are graphed in

Figure 10.

Subjectively, there was little change in audible sound levels upstairs as a result of the loaded

vinyl blanket. This result indicates that the application of the mass-loaded vinyl blanket, which

is laid directly onto the heat pump, is ineffective for significant noise control of the tonal

compressor noise. Various upgrades to the basement ceiling, such as suspended GWB ceilings

on resilient clips, are being considered to improve the airborne noise reduction further for the

bedroom above.

Figure 9: Sound levels in the bedroom with small and large

acoustical pipe clamps.

F. Conclusions Geothermal heat pumps have the

potential of introducing noise and

vibration into wood framed

constructions. The primary sources of

noise from these systems include the

heat pump compressor, fluid flow in

the piping, and automatic balance

valves. The noise and vibration can be

transferred to the structure via the

piping that carries fluids for either the

geothermal heat transfer or the

domestic hot water connected to the

heat pumps. On this project, the split

system heat pump generated

significant low frequency tones

generated by the compressor, while

packaged heat pump system did not.

A typical installation of rigid copper

piping effectively transmitted the noise

and vibration of these heat pumps to

the structure. Homemade pipe

isolation, consisting of pipe strapping

over typical thermal closed cell foam

insulation, provided significant noise

reduction of the fluid flow noise and

over 10 dB of noise reduction at the 63

Hz compressor tone compared with the

installation using the rigid pipe clamps. However, over time, the foam insulation compressed

due to the weight of the piping, and the noise and vibration isolation, which the foam insulation

provided, decreased. A series of engineered pipe isolation products were substituted for the

foam insulation and pipe strapping. These new isolators provided another 10 dB noise reduction

at 63 Hz, with comparable isolation at the other frequencies. The resulting sound level was

perceived to be considerably more pleasing compared to the previous piping conditions. The use

of loaded vinyl blankets placed over the heat pump did not produce significant reductions to the

airborne noise of the split system heat pump. Further improvements, such as alternate piping

isolators and a resiliently suspended GWB ceiling, are still being researched for reducing the

sound levels in the bedroom to within industry guidelines.

ACKNOWLEDGEMENTS I would like to acknowledge Holdrite for providing the numerous engineered piping isolators to

test with this installation.

Figure 10: Sound levels in the basement with and without a

loaded vinyl blanket covering the split heat pump.