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Sorry I can't find the article, what I meant was the return temp needs to be much lower than supply temp.
Discuss 15M pump required, one large Vs 2 small in series? in the Plumbing Jobs | The Job-board area at PlumbersForums.net
Most cylinder thermostats have a differential of about 10C.
If the hot water is stored at 60C and incoming cold is at 20C, the cylinder temperature will drop to 50C when 25% of the hot water has been used.
That's equivalent to two ten-minute showers for a 400 litre cylinder.
Honeywell L641A = approx 10C3°C is typical for a domestic mechanical cylinder thermostat.
Grundfos advised 1st pump should be configured with autoAdapt, with second pump fixed rate. This way AutoAdapt will adjust accordingly.
But Grundfos said 2nd pump would always be running at 100% which would reduce life and ties in with your comment.
I've got some more realistic prices, difference is about £200
I'm still leaning towards 2 in series.
Interesting discussion about 2 pumps in series. All the literature I've read says that 2 in series increases the head and 2 in parallel increases the flow. What I couldn't quite understand is how 2 pumps in series with different specs work in tandem? Eg. I have a 25-80 with a 15-60 in series - what exactly happens in that scenario, does anyone know?
It's nothing to do with higher temperature, e.g. 80C compared to 70C, but with the greater difference between flow and return temperatures, i.e 20C compared to 11C. The larger difference is achieved by a lower flow rate - 14.35 litres/min @ 20C compared to 26.1 lpm @ 11C for a 20kW boiler.Apparently modern condensing boilers are able to carry a higher temperature than older boilers
Oh I see.
In all honesty, I have only ever sized pumps correctly and fitted the right one. I can't say I have ever experimented with pumps in tandem. I have seen it done but only in a situation where the pump was undersized and someone added another to the circuit further along at the point where the pressure and flow from the original was exhausted. I can see how this may work but it is not something I personally would do.
There is no reason why pipes and pumps cannot be correctly sized for the job they are intended to do.
Reply to 15M pump required, one large Vs 2 small in series? in the Plumbing Jobs | The Job-board area at PlumbersForums.net
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