We bought a house with floor heating, and replaced the old boiler. We installed a Saunier Duval Isofast condens 21 gas burning boiler and an external Zilmet zb190 heat exchanger between the boiler and the pipes in the floors (and an external pump to drive the secondary circuit).

After a few weeks everything was switched on I noticed that the secondary circuit is connected in reverse to the heat exchanger.

As far as I know, on this exchanger, the primary in (hot) should be near the secondary out (the secondary out which is going into the pipes in the floor) in order to catch as much heat energy as possible from the primary circuit.

I checked the manual of the zb 190 and it says too that the primary in should be near the secondary out. This is the manual: https://www.manualslib.com/manual/1947132/Zilmet-Z2.html?page=5#manual

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However in my installation the secondary out is near the primary out.

As a consequence (I guess) the temperature of the water going into the floor is quite low, similar to the temperature returning to the boiler from the exchanger.

I think that it would be better (less fuel consumption) if the exchanger would be connected correctly, but I don't know how much would it be better?

Is it worth to make the installer to correct the piping, or is it nitpicking (I think it is a few hours of work for him to re-route a few pipes) ?

1 Answer 1


You want to get that corrected.

Plate heat exchangers like the one you have can be used in two arrangements:

  • parallel flow (co-current): both inlets on one end, outlets on the other, which is what you have at present,
  • counterflow (counter-current): one inlet on each end (what you are supposed to have)

Comparison of con- and counter-current flow exchange Cruithne9, CC BY-SA 4.0, via Wikimedia Commons

The rate of heat transfer depends on the log mean temperature difference between the two streams. In a parallel arrangement, the temperature difference is very high near the inlets and drops to zero near the outlet end. This is useful when your primary goal is to cool one stream down as quickly as possible (say, to stop a chemical reaction), but otherwise it's horribly inefficient as the outlet half of the exchanger isn't doing much due to the small temperature difference there. Additionally, the outlet temperature of the stream getting heated will never exceed the outlet temperature of the other stream, so you can transfer at most half of the heat (assuming the same liquid and flow rate).

Counterflow is much more efficient, because the temperature difference stays high along the entire length of the exchanger. It's also the only way to transfer more than half of the heat in the feed.

Now, look at your yearly heating bill and think: If the suboptimal arrangement makes you lose even just 10% of efficiency (assuming that the floor heating isn't your only or main source of heat; if it is, the losses are definitely much higher), can you afford not to call the guy in ASAP?

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