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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.
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Discuss 15M pump required, one large Vs 2 small in series? in the Plumbing Jobs | The Job-board area at Plumbers Forums
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.
It's a bit of a mish mash to be honest. The 25-80 should be more than sufficient for that zone so the second circulator needs to come out at some point. Probably when the Low Loss Header goes in
I was just intrigued as to how it worked as logically it doesnt make sense to me that a second slower pump can work in tandem with one which is capable of much higher flow rates. But it does work, so not sure.
How do you design a straight sided cylinder to prevent incoming cold disrupting the stratification?
I wasn't suggesting that showers were normally taken at 50C. The two 10 minute showers assumed that the shower temperature was 40C; i.e. 50% cold @20C and 50% hot at 60C.
I think we are talking about different things.No, you asserted that a 400 litre tank could only supply water for two 10-minute showers at 40°C.
I was just intrigued as to how it worked as logically it doesnt make sense to me that a second slower pump can work in tandem with one which is capable of much higher flow rates. But it does work, so not sure.
The second pump ADDS to the first pump's flow rate.
Much like running downhill, the hill being the first pump.
There's a bit more to it than that. For pumps connected in series the flow rate through each must be equal and the total pressure drop across the pair, Ptot, will be determined by the resistance of the rest of the heating system. One must then determine the pressure across each pump by using the 'pump curves' (pressure vs flow charts) to a combination of flow rate for which the sum of the pressure drops matches Ptot.
Such arrangements may or may not have stable solutions.
As the pumps are in series, the flow must be the same through both pumps. You just add together the head for each pump, at the same flow rate, to get the head of the pair of pumps.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?
Whats the setup though, unless your using a LLH you need to add the resistance of the heat exchanger into your pressure loss calcs. In modern boilers this can be a big amount due to the small waterways inside for effeciency.
Ive always worked out pressure loss of index circuit and work out your flow rate then cross refeence those two figures with the pump curves.