15M pump required, one large Vs 2 small in series?

[Last Plumber]

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.

It sounds like Ballcocks to me but like I say, it's not an experiment I would get involved in. What's the point?

 

[Chuck]

How do you design a straight sided cylinder to prevent incoming cold disrupting the stratification?

You get a reasonable degree of stratification without doing anything special, but good quality cylinders, e.g. a Megaflow, have a carefully designed diffuser on the inlet to minimise mixing.

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.

No, you asserted that a 400 litre tank could only supply water for two 10-minute showers at 40°C. Since the V40 capacity of a 400 litre tank, i.e. the amount of water it can provide at 40°C, is typicially 450-500 litre you must have assumed that the showers were using 20-25 litres/minute.

 

[doitmyself]

No, you asserted that a 400 litre tank could only supply water for two 10-minute showers at 40°C.

I think we are talking about different things.

I was just saying that, after two 10 minute, 10litre/min, 40C showers (50% @ 20C, 50% @ 60C), the mean temperature of a 400 Litre cylinder will have dropped to 50C, at which temperature a typical cylinder stat will turn the boiler on.

Of course you can get more than two showers out of a 400 litre cylinder.

 

[snowhead]

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.

 

[Chuck]

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.

 

[fezster]

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.

I was looking at the pump curves for the 2 pumps earlier and determined that even the 15-60 pump can handle the flow rates my boiler expects, just at a much reduced head. This is actually OK in my scenario, though, as the 25-80 is providing adequate (but maybe marginal) head for my index circuit, so the slight extra head the 15-60 gives (roughly 1m at the flow rate Im looking at) is probably all that's needed. What you say corroborates this I think - and more importantly, the setup is working (and has been for ~5 years).

I'm still convinced the second pump can be taken out, but I'm loathed to call someone in just to do that, so will wait until I have other work carried out.

 

[tolly]

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.

 

[doitmyself]

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?

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.

See Pumps in Parallel and Series.

 

[tolly]

I should probably read the thread first. Most of what i wrote has been said already!

The domestic heating design guide has a good bit on sizing pumps and working out your flow rates etc. I think its published by the CIBSE...the orange book. Worth a read if youve ever got a largish heating system to work out calcs on.

 

[fezster]

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.

Yes, the Vaillant 438 has a 4.05m head loss at 38KW/20 degree differential (minimum flow rate 0.45 l/s)! This is far higher than my CH index circuit which is around 2.5m. So total head loss of ~6.55m.

The grundfos 25-80 provides ~6.7m head at this flow rate (speed setting 3) and the 15/60 around 3.5m (speed 3 - ive reduced to speed 1, which is around 1.4m).

So (in theory) the second pump is now unnecessary. I'm hoping once I have a LLH fitted, I can use the 15/60 to pump my heating circuits, and use a 25-55 (I have spare) for the primary circuit. The 25-80 would probably be overkill looking at the pump charts, and a 15/60 too small to overcome the hex resistance.