pump on combination cylinder

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leaky luke

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Hi all,

So, basically I have a h/w combi cylinder that isn't giving me quite enough flow in my flat and I want to put a pump in. Bath takes an hour to fill (electric shower is used instead) even the kitchen sink and the basin give 2 l/m which is just useless. I've been thinking about pumps and obviously I know 'you can't pump a combi cylinder' as it runs dry.

However, I had a thought yesterday. On our boat and also in my converted van I use 12v pumps that pump around 6 l/m. I thought could I apply this in my flat? My combi tank has a 15 litre cold tank and recovers at around 7/8 l/m with the kitchen cold tap on full and closer to 10 with no taps open, therefore in theory tank will never run dry..

Next issue is going from 12v to 240 mains.. I have some 12v transformers knocking around from installing 12v extractors in bathrooms so I figured I could wire it through this. I've searched lots of forums to find any info on this. What I'm basically hoping someone can help me with is a, has anyone tried this and did it work? and b, in terms of regulations am I in serious breach electrically or with water regs? Or is this something that would be acceptable just very uncommon?

I've attached a picture of the pump and also the transformer I would like to use. Any advice or knowledge would be greatly appreciated
 

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Surely the water pressure and flow should have been checked and found sufficient before fitting a combi boiler? Assuming it was, why don’t you look at finding out why it now isn’t flowing enough?
 
Oops. Sorry. Just trying to be helpful and didn’t read it properly. Anyway, I don't even know what one of those are so I’d best STFU! :D
 
leaky luke said:
obviously I know 'you can't pump a combi cylinder' as it runs dry.
Click to expand...
They can be pumped, but only at a very low flow rate.

The 12V pump will not work with a fan transformer, the pump in the picture is 100W, so would need at least 8 amps. Fans are typically around 20W, or less than 2 amps.
This transformer: https://www.tlc-direct.co.uk/Products/LTWP100.html may just about do.

However with the cost of that and the pump probably better to just fit a cheapo mains voltage shower pump with flow restrictors on the outlet to limit the flow to less than the refill rate. Obviously any warranty won't apply, but the same applies to the 12V one.
 
As above, you can install a standard mains shower pump, use it to pump both the hot and cold feeds from the combi cylinder. BES (BES.co.uk) do a range of full bore 15mm isolators that take flow restrictors, so if your refill rate for your combi tank is 8 litre/minute then you can safely install 4 or 6 litre/minute flow limiters to hot and cold low pressure supplies (i.e. the outlets from the pump) with little risk of the header tank running dry.

Can you accommodate a pump running when someone flushes a toilet at 1am? That's the downside of pumping the hot and cold.

MM
 
An hour to fill a bath? It looks like the cold feed from the tank to the cylinder is blocked. Try ramming somthing down it. Scale can accumulate in the pipe.
 
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MeldrewsMate said:
so if your refill rate for your combi tank is 8 litre/minute then you can safely install 4 or 6 litre/minute flow limiters to hot and cold
Click to expand...

Bearing in mind you'd have to halve the 8 liters/minute figure as that's refilling the hot water cylinder beneath too.
6lpm restrictors would result in you running the cistern dry in no time.
 
I'd be getting someone in to survey and see if there are any alternatives. Get your mains survey'd and see what else may be achievable.
 
copea said:
Bearing in mind you'd have to halve the 8 liters/minute figure as that's refilling the hot water cylinder beneath too.
6lpm restrictors would result in you running the cistern dry in no time.
Click to expand...
I think you misunderstood. The mains I refered to is mains electricity, the power for the pump. The reason for the restrictors, on the pump outlets, both hot and cold, are to prevent the header tank (which refills at 8 litre/minute) running dry if mixing the hot and cold.
For instance, if using 6 l/min restrictors (12 l/min total) there would be a shortfall of 4 litre/minute in the refilling rate. A typical 30 litre header tank would run dry in about 7 minutes, during which time over 80 litres of blended bathing water would have been delivered, enough for a single bath, and plenty for two or three showers.
 
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I don't know what relevance the word 'mains' has here, or indeed the bold text.

You said two 6lpm restrictors.
2 x 6 = 12.

The refill rate is 8lpm according to the OP.

8 - 12 = -4

So there's a 4lpm deficit, meaning the tank will run dry very quickly.
 
copea said:
I don't know what relevance the word 'mains' has here, or indeed the bold text.

You said two 6lpm restrictors.
2 x 6 = 12.

The refill rate is 8lpm according to the OP.

8 - 12 = -4

So there's a 4lpm deficit, meaning the tank will run dry very quickly.
Click to expand...

I agree, and the bold was put in to allow the important points to be ingested while skim reading.
I also re-read the OP post, and he stated the header tank is 15 litres, not the 30 I stated (busted there!), so the tank would dry up in about 4 minutes, after delivering 40 litres.
Consider, then, the use of 4 litre/min restrictors where the net loss to the header tank would be zero, or 4 lpm on the cold and 6 lpm on the hot. In that case a 2 lpm deficit, taking 8 minutes to drain the header, would allow 8 x (4+6) = 80 litres....enough to fill a bath, etc.
Hope this clarifies my point.
 
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