Can I join the PRV and condensate pipes? The PRV and condensate pipes are designed to do two very different jobs. So the initial answer is no, except for certain conditions.
What does the PRV pipe do?
The PRV pipework is a safety vent for scalding water/steam if the heating system develops a fault.
PRV pipework is designed to withstand the temperature of very hot water. It must also deal with any sudden pressure increase when the safety valve opens.
Plastic overflow pipework is not designed to work at high temperatures. It will sag if it gets too hot and the joints will fail.
What does the condensate pipe do?
The condensate pipe is used to drain mildly acidic water produced during normal operation. The plume of steam that you see when the boiler is running condenses inside the boiler. This then flows down the condensate pipe to the drain.
Condensate pipework is designed to deal with a constant trickle of low temperature water. It is normally plastic as the condensate is mildly acidic which would corrode metal pipe.
Why should I not join a PRV and condensate pipes?
The two pipes have fundamentally different jobs. PRV is hot water possibly at pressure. The condensate has low temperature low pressure water that is mildly acidic.
Each type of pipework is suited to its role but not to the other role.
How can I join a PRV to a condensate pipe?
It is possible to combine a PRV with a condensate pipe. Each manufacturer gives different advice:
Worcester Bosch
Viessmann
Viessmann allow a combined PRV and condensate as long as it is installed to their specification.
- Boiler
- Visible air break
- 75 mm trap
- Visible air break and trap not required if there is a trap with a minimum condensate seal of 75 mm incorporated into the boiler
- Sink, basin, bath or shower
- Open end of condensate drainage pipe direct into gully 25 mm min below grating but above water level; end cut at 45 degrees
- Sink lip
- Minimum internal diameter 19 mm
- Pipe size transition
- Minimum internal diameter 30 mm
- Water/weather proof insulation
This relies on the plastic overflow pipe being strong enough to withstand higher temperatures for long periods of time. For a normal sealed system in most circumstances the pressure will run out before the pipe melts.
Alpha
Alpha have combined the PRV and condensate inside their boilers with a piece of clear tubing.
Summary
The best option is to run the PRV and condensate in separate pipes.
Each pipe has a different role and they are not normally suited to the others role.
Some manufacturers state it is possible to combine the PRV and condensate. If they do then you must follow the instructions they provide. Instructions from one manufacturer may not apply to other makes of boiler.
Personally I think a condensate should run separately to a drain and a PRV should terminate somewhere visible.
When water flows from a PRV it is a sign that there is a fault. A combined condensate and PRV can hide this fault, causing long-term damage.
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