View Full Version : Curious French heat pump flow layout - advice please

05-11-2016, 02:08 PM
Good afternoon experts !

My large house in France has a 65 kW De Dietrich oil-fired boiler feeding 16 cast-iron rads. As you can imagine we use it as little as possible. The principal flow/return iron pipework, 60 mm diam (no, not 63 or 65) ascends from the cellar to the roof space some 15 m (50 ft) above, from where the rads are tee'd off in copper from the main distribution ironwork back down to the ground and first floors. It is sealed. All well and good apart from the running costs.

In addition there is an air to water heat pump dating from about 1983 - a pair of Saunier Duval MSDX 151 monobloc exterior pumps, 4.9 kW output each. It has never worked since before 2004, because the previous owner's plumber or electrician messed around with the control unit (satellite SP 22) wiring; also the internal pump was defective. So I replaced the pump, and was able to temporarily get each unit going again by bypassing some of the wiring. I even found a manual on the internet. They produce hot water for a few minutes until something cuts in to switch them off. So the refrigerant circuit seems to be intact - fortunate, because its R22 gas is now illegal to replace or replenish in France.

I could spend a lot more time understanding and re-installing the wiring - I hate the idea of throwing away kit that apparently still works - but the pumps are rather dated; for example, they don't function at an air temperature of below +7C. So I am thinking of replacing them with a 14 kW Mitsubishi monobloc pump (PUHZ-HW140VHA2) and control box (EHPX VM2C). Naturally I want to re-use the extensive 20mm copper piping from the pump/control location to the boiler room.

My main question is this: does the existing flow layout at the boiler as shown in the labelled photo below make sense (external pipe diameters in mm; Fe = iron, Cu = copper)?
As you can see the heat pump flow and return are BOTH on the main return, which seems counter-intuitive to me. But in fact this corresponds correctly to the Saunier Duval diagram below:
The heat pump connections to the existing circuit shown in the diagram above are enclosed in a dotted rectangle, which is also labelled as where to put an optional buffer mixing tank (which I don't have). The French instructions also specify that the main pump must be on all the time.

The three-way valve shown above the boiler is a hand-adjustable mixing valve. The main pump is a standard Grundfos UPS 25-60, which might seem to be rather feeble given the size of the installation, but seems to work fine at its high setting.

Can I just use this existing pipework, but exchanging the mixing valve for a three-way valve to isolate the boiler when the heat pump is running alone? Should I change the heat pump flow to connect to the main boiler flow (which would be annoying, because I have never done iron plumbing before)?

Your advice and any other comments would be gratefully received.

05-11-2016, 07:11 PM
Connection to the return is fine, it creates a form of low-loss header so that heat pumps pump is not doing the main circuit pumping.

09-12-2016, 05:08 PM
This set up allows your heatpump to run independently from the boiler and the boiler supply does not flow through the heatpump. Is the three-way thermostatic mixing valve set up to keep the temperature up in the boiler as shown in the picture?
The heat pump circulating pump must be on when the heat pump is on and will short circuit the flow if the main circulating pump is off. A drop in replacement refrigerant is available to replace the R22 in the system.

09-12-2016, 08:46 PM
Hi I have loads of [banned, illegal to use refrigerant] I am based on Portugal and I stocked up many years ago may save you money

Edited, this forum does not support illegal activities.

09-12-2016, 10:32 PM
Hi eurocooling.
Given how long you have posted.
I am surprised and disappointed that you are trying to deal in a a European wide Banned Refrigerant!
And are trying to do so on this Forum.

I have only issued a warning so far, Other moderators may feel differently!

Jeff Winsor
10-12-2016, 08:41 AM
Hi David
Not sure about France but certainly in the UK tube and pipe dimensions are measured differently.
Iron pipe would be internal dimension and copper tube would be external dimension.
Also in the UK there are at least 3 different pipe wall thicknesses so would affect outside diameter

10-12-2016, 10:52 PM
Hello David.
before forking out too much money on up grading heating system have the existing pipework analyzed for internal corrosion, dissimilar metals temperature change etc., take a sample of the water and have it tested for PH and iron content. If water is black and brackish looking it contains iron. Could mean a complete re-pipe to avoid multiple leaks everywhere down the track.
Just a thought.