Building and Piping a Bypass for the Heat Pump Correctly
A bypass is the small pipe loop that routes around a consumer – typically the heat pump – leading past it. It determines whether the system can be cleanly regulated, serviced and kept frost-safe. This guide shows the proven 3-valve arrangement, the correct position in the circuit, how to set the flow rate, and the order in which several consumers (heat pump, solar, dosing and measurement technology) should be piped up – with a complete parts list from our range.
Why a bypass is needed
A heat pump requires a limited, steady flow rate. If it hangs directly in the main line, too much or too little water passes through the heat exchanger depending on pump output – costing efficiency and, in the worst case, the warranty. The bypass solves three tasks at once:
- Set the flow rate: using the control valve in the main line you set exactly the water volume specified by the manufacturer.
- Service and winterise without downtime: isolate the heat pump while the pool keeps running – ideal for cleaning the heat exchanger or draining it frost-safe.
- Protect the consumer: a defined flow instead of a pressure surge; a check valve prevents back-flow when the pump is switched off.
The right position in the circuit: after the filter, before disinfection
The bypass for the heat pump belongs downstream of the filter system (the heat pump should see clean water) and upstream of disinfection – that is, before salt electrolysis, dosing and measurement technology. The logical order in the plant room is:
Skimmer/main drain → pump → filter → heat pump (bypass) → dosing/measurement technology → inlet jets
This way the water is heated first and its quality only measured and re-dosed afterwards – the measuring cell then sees exactly the water that goes into the pool.
The 3-valve arrangement explained
Between two tees in the main line sits a ball valve acting as the control valve (V3). On the branch to the heat pump there is a ball valve in each of the flow (V1) and return (V2) lines:
- V1 + V2 open, V3 fully open: most of the water takes the short route through the main line – hardly any flows through the heat pump (summer operation without heating, or the shoulder season).
- V1 + V2 open, V3 throttled: the further you close V3, the more water is forced through the heat pump – this is how you set the target flow rate.
- V1 + V2 closed, V3 fully open: heat pump completely isolated, pool keeps running normally – for servicing, cleaning or winterising.
Setting the flow rate correctly
Regulate solely via V3 in the main line, not via the branch valves. A guide value for many pool heat pumps: the temperature difference between flow and return should be around 1–2 K. A flow meter in the branch line makes the setting reproducible. Never throttle so hard that the pump works against excessive resistance.
Multiple consumers: each stage in its own bypass
If solar absorbers and dosing technology are added, each consumer gets its own bypass using the 3-valve logic described – in series, in the order heat pump → solar → dosing/measurement technology. This allows each stage to be isolated, serviced and adjusted individually, without affecting the others.
Choosing the pipe size – d50 or d63?
The decisive figure is the pipe outside diameter d (the inch/DN designation is secondary). For private pools, d50 is usually suitable; from larger circulation volumes upward, d63:
| Pipe d | DN | typical circulation |
|---|---|---|
| d50 | DN40 | up to approx. 14 m³/h |
| d63 | DN50 | up to approx. 22 m³/h |
Plan frost protection in from the outset: pipe up the heat pump and its bypass so that they can be drained in autumn – provide a fall and fit detachable unions at the heat pump so that no residual water can freeze.
Step-by-step installation
- Dry-fit: push the tees, valves and pipe sections together without adhesive and check the dimensions (clearances to the heat pump, accessibility of the handles).
- Fit the screw-on parts: mount the detachable unions on the flow and return of the heat pump.
- Prepare the bonded joints: degrease the bonding surfaces with cleaner, roughen the pipe and socket.
- Solvent-weld (cold welding): apply PVC-U adhesive thinly and evenly, push the part swiftly home to the stop, hold briefly, align.
- Fit the check valve: in the return line of the heat pump, observing the flow direction (arrow).
- Curing & leak test: allow the adhesive’s curing time to elapse, then fill slowly and check for leaks.
- Adjust: open V1/V2, throttle V3 step by step until the target flow rate is reached.
Parts list for this guide (d50 example)
An example item list for a heat-pump bypass in d50, PVC-U, solvent socket – all parts from the Praher/Peraplas range. Quantities depending on the installation situation.
| Item | Component | Size | Art. no. | KLN no. | Qty |
|---|---|---|---|---|---|
| 1 | PVC-U pressure pipe with socket | d50 × 2.4 mm (PN10) | 76928 | 51515822 | ~2 m |
| 2 | Tee 90° solvent socket, equal | d50 | 70022 | 51512912 | 2 |
| 3 | 2-way ball valve M1, solvent socket | d50 (DN40) | 128814A | 51511559 | 3 |
| 4 | Cone check valve S4, solvent socket | d50 | 121041 | 51510312 | 1 |
| 5 | Ocean® union solvent socket/solvent socket | d50 | 170397 | 51512461 | 2 |
| 6 | Elbow 90° solvent socket | d50 | 70506 | 51513082 | as required |
| 7 | Coupler, solvent socket | d50 | 70739 | 51513256 | as required |
| 8 | Griffon® UNI-100 adhesive | 250 ml | 20023 | 51512566 | 1 |
| 9 | Griffon® hand cleaner / degreaser | – | – | – | 1 |
Already know the main article number? Using the spare-parts finder you can find the matching seals and wear parts for every valve.
Frequently asked questions
Do I really need three valves?
For full convenience, yes: two valves isolate the heat pump and the third in the main line regulates the flow. A pure isolating bypass without a control valve is possible, but lacks precise flow adjustment.
d50 or d63 for my pool?
The decisive factor is the circulation volume: up to approx. 14 m³/h, d50 is sufficient; above that, d63 makes sense. When in doubt, adopt the nominal size of the existing pipework system.
Where exactly does the check valve go?
In the return line of the heat pump, with the flow direction towards the pool. This way, when the pump is stationary, no water can run backwards through the heat exchanger.
Does the bypass need to be frost-safe?
Yes. Plan in a fall and detachable unions so that the heat pump and bypass can be fully drained at the end of the season.
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