Gaskets for heat pump
connections —
what vibration
and glycol require
The service conditions are different — compressor vibration that slowly loosens bolt load, glycol antifreeze in outdoor circuits, wide thermal cycling from below zero to heating flow temperature, and outdoor exposure for connections at the unit itself. Each of these affects which gasket grade is appropriate, and they act in combination.
Scope note: This article is an application-selection reference. Final gasket choice depends on the exact medium, oil or glycol chemistry, concentration, pressure, temperature, flange condition and any site or regulatory requirement.
What is different about heat pump hydraulic connections
A heat pump installation contains several distinct hydraulic circuits, each with different service conditions. The connections at or close to the outdoor unit are exposed to weather, vibration from the compressor, and in some configurations carry glycol-based antifreeze fluid. The indoor distribution circuit carries heating system water at temperatures similar to a conventional heating system. The domestic hot water cylinder connections carry potable or heated water.
Specifying the same gasket grade for all of these positions — because they are "all heating" — overlooks the real differences in service conditions between them. The outdoor unit connections and the indoor cylinder connections are not the same joint in different locations. They are different joints.
The four service conditions that matter
This effect is not typically severe enough to produce an immediate failure, but in a joint where the initial bolt load was marginal or where the gasket grade has limited resilience, vibration-induced relaxation can bring a borderline joint below minimum seating stress. Where anti-vibration mountings or flexible connections have not been fitted between the unit and the rigid pipework, this is a more significant factor.
Propylene glycol at concentrations typically used for frost protection (30–50%) is generally less aggressive than ethylene glycol, but compatibility still depends on the specific rubber binder or elastomer in the gasket grade. Check the gasket material datasheet for the specific fluid rather than relying on general material class assumptions.
Gasket grades that perform adequately through the thermal cycling range of a typical indoor heating system may show different long-term behaviour when the cold-end temperature is regularly below zero and the warm-end temperature is standard heating flow temperature.
Connection positions and what each requires
Outdoor unit — glycol circuit connections
Ground source or glycol-carrying air source connections. Service: glycol fluid, vibration, wide temperature range, outdoor exposure. Requires: glycol-compatible grade, good resilience under cyclic loading, rated for the temperature range.
Indoor heating distribution circuit
Connections to radiators, underfloor circuits, buffer tank. Service: inhibited heating system water, standard temperatures (35–65°C), indoor, stable. Requires: a grade compatible with inhibited heating water; WRAS matters only if the connection is on potable water.
Domestic hot water cylinder
Connections to the coil or direct connections. Service: potable or heated water, standard temperatures, regulatory compliance. Requires: WRAS-listed grade for potable water contact, checked against scope, listed conditions and maximum water temperature.
Buffer tank connections
Connections between heat pump and buffer. Service: inhibited heating water, temperature varies by system design. Requires: grade appropriate for sealed heating system water at the specific operating temperature.
Not all connections in a heat pump installation are the same service. A 1/2 inch BSP connection at the outdoor unit glycol manifold and a 1/2 inch BSP connection at the indoor cylinder are nominally the same fitting size — but the gasket specification for each may be different because the medium, the temperature range, the vibration environment and the regulatory requirements differ between positions. Selecting a single grade for all positions based on fitting size alone risks under-specifying some positions and over-specifying others.
Grade selection for heat pump hydraulic connections
| Kinetics Line grade | Glycol circuit | Indoor heating | DHW / potable | Notes |
|---|---|---|---|---|
| RUBBERSEAL PRO 110 (EPDM) | CHECK FLUID COMPATIBILITY | CHECK EXACT COMPOUND / LISTING | CHECK EXACT COMPOUND / LISTING | EPDM generally suitable for propylene glycol — confirm specific fluid and concentration. Good resilience for vibration service. 110°C continuous rating covers standard heat pump flow temperatures |
| GREENSEAL PRO 180 | CHECK FLUID COMPATIBILITY | CHECK EXACT COMPOUND / LISTING | SUPPLIER FDA / LEGACY KTW REF. | Supplier-sheet FDA 21 CFR 175.300 and legacy KTW references; no WRAS in the mapped source and no claim of current German product conformity. For UK WRAS-required potable water, specify REDSEAL PRO 110 or BLUESEAL ULTRA 350 within their listed scope. For sealed indoor heating circuits, check temperature, medium chemistry and face condition. Confirm glycol compatibility before outdoor glycol positions. |
| BLUESEAL ULTRA 350 | CHECK FLUID COMPATIBILITY | CHECK EXACT COMPOUND / LISTING | WRAS LISTED — VERIFY SCOPE | DVGW and WRAS documented for listed water-service scopes. For potable-water use, check WRAS listing scope, listed conditions and maximum water temperature. Higher temperature margin than GREENSEAL. For sealed heating circuits confirm medium and temperature; for glycol positions confirm fluid compatibility from datasheet. |
| FLEXSEAL PRO 350 | CHECK FLUID COMPATIBILITY | CHECK EXACT COMPOUND / LISTING | CHECK APPLICATION | High compressibility (25%) — useful where face condition or bolt load is limited. DVGW and WRAS documented; check the specific WRAS scope, listed conditions and maximum water temperature for potable applications. Confirm glycol compatibility separately. |
Suitability above is based on Kinetics Line technical data and material compatibility guidance. Glycol compatibility depends on fluid type, inhibitor package and concentration — confirm from the gasket material datasheet for the specific fluid before specifying. WRAS listing applies only to the stated scope, listed conditions and maximum water temperature; confirm current listing status before potable-water use.
Vibration and bolt load — a practical note
The vibration effect on bolt load is gradual and cumulative. A heat pump that has been operating for two or three years may show signs of slow-developing weeps at connections close to the outdoor unit, even where the original installation was correctly executed. This is not necessarily a failure of the gasket grade — it may reflect gradual vibration-induced relaxation of the bolt load over time.
The practical response when this occurs is to check the bolt load at the affected connection before concluding the gasket grade was wrong. If the connection is simply slightly loose — bolt tension has reduced from the original assembly value — controlled reloading in accordance with the joint procedure may sometimes resolve the issue where the gasket has not been damaged. If the gasket is degraded or has been damaged by the movement, replacement is needed.
Anti-vibration measures at the unit connection reduce the gasket selection problem. Flexible hydraulic connections between the rigid pipework and the outdoor unit — anti-vibration hoses or flexible connectors — limit the transmission of compressor vibration into the threaded joints. Where these have been correctly installed, the vibration-related bolt load relaxation is significantly reduced. Where they have not been fitted, this is a more significant factor in the long-term performance of the connections. The gasket grade is one variable — the installation design is another.
Select by position, not by fitting size.
The glycol circuit connections, the indoor heating connections and the domestic hot water connections in a heat pump installation each have different requirements. Glycol compatibility must be checked for any connection carrying antifreeze fluid — not assumed from the general material class. EPDM grades with good resilience are commonly used at vibration-exposed positions where the temperature rating is adequate. WRAS-listed grades are appropriate for potable water connections only within their approval scope, listed conditions and maximum water temperature. The correct specification reads from the service conditions at each position, not from a single choice applied uniformly across the installation.
Practical FAQ
What gasket material is best for heat pump connections?
The appropriate gasket material for a heat pump hydraulic connection depends on the position — whether the connection is in the outdoor glycol circuit, the indoor heating distribution circuit, or at the domestic hot water cylinder. For outdoor glycol circuits and connections close to the heat pump unit where vibration is present, an EPDM grade with good glycol compatibility and resilience under cyclic loading is commonly specified. For sealed indoor heating distribution circuits, a compressed fibre grade with appropriate temperature rating and compatibility with the system medium is also used. The key factors are glycol compatibility (for antifreeze-protected circuits), vibration resistance, and the temperature range the connection will experience — not all of which are the same for every position in the installation.
Does heat pump vibration affect gasket performance?
Vibration from the heat pump compressor can contribute to gradual loosening or loss of clamp load in connections close to the unit, particularly where support and anti-vibration measures are limited. Over time, this can reduce the seating stress on the gasket below the minimum needed to resist the operating pressure — producing a slow-developing weep rather than an immediate leak. Grades with some resilience may sometimes tolerate minor cyclic movement better than very stiff sealing materials in marginal installations — though the overall joint design, support and anti-vibration measures have a larger influence on this outcome. Where anti-vibration measures have not been fitted in the hydraulic pipework, a gasket grade with adequate resilience for the service conditions is one factor in reducing the rate of vibration-induced relaxation.
Do heat pump outdoor circuits use glycol and does this affect gasket selection?
In ground source heat pump installations and in some air source configurations where the outdoor pipework may be exposed to freezing temperatures, a glycol-based antifreeze solution is used in the outdoor hydraulic circuit. This is typically propylene glycol. Air source heat pump units where the refrigerant circuit is self-contained and only the indoor hydraulic connections carry heating system water do not require glycol in the hydraulic circuit, unless the outdoor pipework is frost-exposed. Where glycol is used, gasket selection should account for compatibility with the specific fluid type and concentration. Not all rubber and fibre binder systems are equally compatible with glycol-based fluids — check the gasket material datasheet for the specific fluid rather than assuming general material family compatibility.