Mains booster pump to increase shower flow rate from combi boiler

[JezW]

Hi all

I'm in a first floor flat and the shower pressure/flow is pretty poor. At a temp of 40c, the shower flow rate is 7.5 l/min. The cold mains is 8.4 l/min.

I'm thinking about fitting a mains booster pump like this: https://www.salamanderpumps.co.uk/products/mainsbooster/homeboost/ They're not cheap, so I'm trying to figure out if it's going to be worth it, in terms of the increase in flow rate.

My boiler is a 25kw combi, with the following specs:

https://professional.vaillant.co.uk/downloads/product-manuals/ecofit-pure-1/combi-5/ecofit-pure-combi-installation-and-maintenance-instructions-1943272.pdf

I have a few questions...

1. The boiler is a glow worm energy 25c (same as vaillant ecofit pure 825). What's this "maximum water flow rate" of 8 l/min listed in the specs?

2. These pumps can supposedly boost the mains supply to 12 l/min. The mains is now 8.4 l /min. That would be a 43% increase.

Currently the cold mains is 7 degrees. Hot water from the boiler is 55 degrees. I'm not a heating engineer, so please forgive any stupid assumptions here, but to get 40c at the shower, that's approx (70% x 55c) + (30% x 7c) = 40.6c. So am I right in saying 70% of the mains flow is going through the boiler?

3. If 70% of the mains flow is going through the boiler, and the mains is boosted to 12 l/min, 70% x 12 = 8.4 l/min being heated by the boiler and going to the shower (before inefficiencies etc so a further 10% may be lost). The ΔT @ 35k is 10.4 l/min for my boiler (eg. 7c --> 42c). So it appears that the boiler has plenty of headroom to heat this extra water?

Following on from that is the main question...if the boiler has the capability to heat the extra water, can I assume this increase of flow through the mains will result in a similar increase in flow to the shower, ie. around 43%?

4. My final concern is that my flat is on the first floor, so water pressure will be lost going up one floor. Will this affect the above figures? That pump says it will boost the pressure to 1.6 bar, but the recommended boiler specs (above) are 2 bar. Will ΔT @ 35k still be 10.4 l/min if the supply pressure is lower than 2 bar?

Thanks

Jez

 

[muggles]

Your boiler has an internal flow restrictor limiting it to 8.0l/min. They aren't massively accurate though so it could be that 7.5 is all you can get through it

 

[JezW]

Hi Muggles

What's the point of vaillant stating a flow rate of 10.4 l/min with a temp increase of 35c, if the max flow is limited to 8 l/min?

However, if I can get 8 l/min through the boiler (instead of 8.4l/min in my third point above), and that's 70% of the flow rate going to the shower...I could potentially get something like 11.4 l/min to the shower. That's a huge increase. Are my numbers wrong somewhere?

 

[Johntheo5]

Hi all

I'm in a first floor flat and the shower pressure/flow is pretty poor. At a temp of 40c, the shower flow rate is 7.5 l/min. The cold mains is 8.4 l/min.

I'm thinking about fitting a mains booster pump like this: https://www.salamanderpumps.co.uk/products/mainsbooster/homeboost/ They're not cheap, so I'm trying to figure out if it's going to be worth it, in terms of the increase in flow rate.

My boiler is a 25kw combi, with the following specs:


View attachment 372183

https://professional.vaillant.co.uk/downloads/product-manuals/ecofit-pure-1/combi-5/ecofit-pure-combi-installation-and-maintenance-instructions-1943272.pdf

I have a few questions...

1. The boiler is a glow worm energy 25c (same as vaillant ecofit pure 825). What's this "maximum water flow rate" of 8 l/min listed in the specs?

2. These pumps can supposedly boost the mains supply to 12 l/min. The mains is now 8.4 l /min. That would be a 43% increase.

Currently the cold mains is 7 degrees. Hot water from the boiler is 55 degrees. I'm not a heating engineer, so please forgive any stupid assumptions here, but to get 40c at the shower, that's approx (70% x 55c) + (30% x 7c) = 40.6c. So am I right in saying 70% of the mains flow is going through the boiler?

3. If 70% of the mains flow is going through the boiler, and the mains is boosted to 12 l/min, 70% x 12 = 8.4 l/min being heated by the boiler and going to the shower (before inefficiencies etc so a further 10% may be lost). The ΔT @ 35k is 10.4 l/min for my boiler (eg. 7c --> 42c). So it appears that the boiler has plenty of headroom to heat this extra water?

Following on from that is the main question...if the boiler has the capability to heat the extra water, can I assume this increase of flow through the mains will result in a similar increase in flow to the shower, ie. around 43%?

4. My final concern is that my flat is on the first floor, so water pressure will be lost going up one floor. Will this affect the above figures? That pump says it will boost the pressure to 1.6 bar, but the recommended boiler specs (above) are 2 bar. Will ΔT @ 35k still be 10.4 l/min if the supply pressure is lower than 2 bar?

Thanks

Jez

Your calculations are fairly spot on, these calcs might be of further assistance.

"Thermally", the max flowrate through any combi is........
LPM = kW*860/60/(mixedtemp-mainstemp), in your case, 25*860/60/(40.0-7.0), 10.86LPM. (required mixed temp (shower temp) is 40C).
It doesn't matter, thermally, whether you get this without any mixing or you heat the water to say 55C and mix it with mains water to give the required shower (mixed) temperature. If the combi HW temp is 55C then the boiler flowrate, LPM, is, 25*860/60/(55-7), 7.47LPM, and will require, 10.86-7.47, 3.39LPM of mains water to give a shower temp of 40C, HW percentage, 7.47/10.86, 68.8%, mains %, 3.39/10.86, 31.2%.

I'm not sure what your shower flowrate is just now but assuming its 8.0LPM (at 40C) then the max increase possible to maintain the flowtemp at 40C, is, 10.86-8.0, 2.86LPM, 2.86/8.0, 35.75%.

 

[JezW]

Hi John, thanks for your reply and those calcs!

Gotcha - so the max flow rate through my 25kw combi, with desired temp of 40c and cold mains of 7c, is 25*860/60/(40.0-7.0) = 10.86LPM

And if we do this calc for a ΔT of 35c: 25*860/60/(35) = 10.23LPM, which interestingly is slightly below the manufacturers specs of 10.4LPM.

That sounds great, but I don't suppose you know what the maximum water flow rate of 8LPM is, which is detailed in my boiler's specs? Although it doesn't look like this is going to be a limiting factor anyway, because according to your calcs, the boiler can output 7.47LPM with a ΔT of 46c anyway.

That's what I thought - as long as you have sufficient flow from the mains, and you're not maxing out your boiler's flow rate, it doesn't matter whether the boiler is heating the water to the desired temp, or if it's heating it to its max temp and then it's being mixed with cold water at the tap/shower.

The max flow rate at my shower now, around 40c is 7.5 l/min. So the max increase (at a ΔT of 33) could be up to 10.86-7.5, 3.36LPM, or 44.8%.

That would be amazing...but then this is theoretical and not accounting for the fact that my cold mains is 8.4LPM...but I'm only getting 7.5LPM out of the shower, around 40c. The boiler is very close to the shower and it's not a low flow shower head or mixer valve. Where do you think roughly 10% being lost?

 

[Johntheo5]

10% flow? could be lost any where, partially choked filters etc, you could turn the shower to max temp and the combi DHW temp to minimum to force all the water to the shower and see if the flowrate increases or decreases.

Specsheet says boiler output to HW is 25.2kW
The max flowrate specified at 8.0LPM, dT, 25.2*860/60/8.0, 45.2C, don't know why this 8.0LPM is stated, the mains water would almost have to be at 0C to give a showering temp of 45.2C, no mixing, too hot for me at any rate.

 

[JezW]

Hi John

Just did that - dropped combi temp to minimum (35c), and shower temp to maximum.

Flow rate through the shower is 6.5LPM.

What does that tell us?

I would've expected the flow rate to increase...not decrease!

 

[Johntheo5]

The shower mixing valve (TMV) will now only be allowing water flow from one supply (hot) to one side the the shuttle so that will probably be the reason, especially if any restriction in the check valve & filter, might be worth checking these if accessible, but I would think that generally, the max flow through any shower will be when the shuttle is in mid position, not sure if you can have a cold (cold) shower with your one, if so, switch off the boiler and turn the shower to fully cold, measure the flow, whether or which, turn the shower to its minimum hot setting, maybe 35C?, if you let DHW temp set to 55C then the HW%, will be, 1/((55-7)/(35-7)), 1/1.714, 58.34%, and cold, 41.66%, if the HW temp is increased to max then the HW% is 1/1.89, 52.91% and cold, 47.09%, see what one, or both those settings give.

 

[JezW]

Haha I'm getting a bit lost John! The tests that you are suggesting - what will they tell us?

Are we just trying to work out the maximum possible flow rate through my shower?

I'm pretty sure this is my shower mixer and these mixers should allow high flow rates - https://www.showerspares.com/aquali...d-mixer-shower-with-adjustable-head-drm001ca/

I just re-tested my cold mains.

From the basin tap, I'm getting 7.4LPM
Out of the shower, (but with the shower head on the shower tray), I'm getting 9.5LPM

I suppose that's expected? Lower outlet = higher pressure = higher flow?

If I fit a booster pump (I'm now looking at these, they look much better than the salamander ones- https://www.stuart-turner.co.uk/product/stormboost-47708) and the pump outputs 12LPM, also at a higher pressure (up to 3.2 bar), can I expect 12LPM at all outlets around my flat on the cold mains?

 

[Johntheo5]

So, the shower flow increases from ~7.4LPM to 9.5LPM by lowering the spray head by say 2M, 0.2bar??, you are getting a 28.38% increase in flow rate by just doing this?, head or Pressure is proportional to the square of flow so the head must have increased by a factor of the sq of (9.5/7.4), 1.648, are you sure that its not some sort of hybrid shower you have, ie hot mains and cold gravity??, if mains fed then to give that 28.38% increase in flow means that the pressure/head at 7.4LPM is only 0.3086bar and it increases to 0.5086bar with the shower head on the ground, the flow then rises to 7.4*sqroot(0.5086/0.3086), 7.4*1.283, 9.5LPM. IF gravity fed then the CW cistern water level would have to be nearly 3.1M above the shower head when in its holder.

 

[trojanhawrs]

Do you have a fill loop at the boiler you could disconnect and check the flow rate from there? This might be helpful if your basin tap is more restrictive than the fill loop valve

 

[JezW]

Hi John and Trojan

I think ignore those slightly dubious figures. I've just done some more tests...

From the basin tap: 7.5LPM
From the shower, hose attached, no head attached: 7.5LPM
From the shower outlet in the wall, no hose or head attached (bottom right of below photo): 12.6LPM !

So the tap may have a restrictor in it, but the shower hose surely doesn't.

So I'm losing 5LPM when the shower hose is connected to the shower outlet.

I assume this indicates that my water pressure is simply too low to maintain a higher flow rate?

So if that booster pump adds 2.5 bar, I might get close to 12.6LPM out of the shower..?

 

[Johntheo5]

Its quit possible that there is a restrictor in the shower hose, difficult to see you losing 5 LPM through the hose itself except partially collapsed. What is the length and ID of the hose? look for a restricting washer on the end of the hose where its attached to the shower, you can get a good idea of the hose ID if you have a set of drill bits to fit into the hose end.

 

[JezW]

Just double checked, no restrictor in either end of the hose.

Bore is about 7mm, tested with a drill bit, good idea thank you

Just ordered a large 11mm bore shower hose.

Maybe that sorts it!

 

[Johntheo5]

A lot cheaper than a pump, if it works!.

1.5M of 7mm pipe has a loss of 2.63M @ 7.4LPM, 11mm, loss of 0.29M
1.5m of 7mm pipe has a loss of 4.17M @ 9.5LPM, 11mm loss is 0.45M

1.5M of 7mm pipe has a loss of 6.94M @ 12.5LPM, 11mm, loss of 0.767M

Friction Loss Calculator

Calculate the friction loss in pipes for different materials using the Hazen-Williams equation.

www.omnicalculator.com