CWB with break tank on a combi?

26 Jan 2013
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United Kingdom
Hi, I have an old heating system that consists of an Ideal Mexico Super 480FF boiler which has gravity hot water to a regular insulated copper cylinder. 11 radiators with one grundfoss pump. Honeywell Evohome TRV's on all radiators and a control valve connected to the return line of the hot water loop to prevent cycling the gravity hot water when the heating is on.

The mains water pressure is rubbish and cannot be increased as I've checked this out at great length with my water authority. So I have a DAB cold water booster with 250l break tank in an outbuilding which has enough output to send continous 3 bar at my taps.

My hot water pressure is also rubbish as the cylinder is vented and it's a flat roofed house so the cold water tank sits on a shelf directly above the cylinder. I've got a 3bar shower pump on the cylinder output to boost the hot water to taps.

There are two bathrooms and a WC, one bath and two electric showers (installed before we owned the house and added Honeywell control and cold/hot boosting).

At the moment it works and gets us by. But we would really like to get rid of the electric showers and fit mixers instead. Ideally I'd like to go with a high flow combi boiler as I'd like to get rid of the cylinder, cold water and f&e tanks to install a shower cubicle in that cupboard. As at the moment the shower is over the bath but thats getting replaced with a Japanese soaking bath (don't ask...OH's idea and is a deal breaker if not installed).

So finally, my questions.

Can I run a combi off my cold water booster? It has a loop with non return fitted that will allow the rubbish mains pressure/flow to continue to flow to the house should the pump/power fail or break tank run dry. I think its about 7bar max from the pump, currently set at 3bar with variable flow to maintain 3bar with more than one outlet in use.

If I put in mixer showers and combi, will I run the break tank dry in a matter of minutes?

I'd guess even a high flow combi won't output the same flow as I get from my cold booster, so I guess I could turn the CWB down till the flows balance?

I've tried googling a lot online, but I haven't managed to find answers to my specific issues.
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A 35kw combi will give the equivalent (when/if mixed with cold water) of.........
14.75LPM winter from cold at 6C to 40C Hot
25.09LPM summer from cold at 20C to 40C Hot
17.92LPM average from cold at 12C to 40C Hot
How much water would I use from my break tank then for reasonable flow rated mixer shower for 10min shower? I'm trying to work out now if my 250l break tank is big enough to allow 2 people to shower for 10 mins. I think my electric shower is about 3.5lpm, so I'd assume about 8-10lpm from the shower head would be a reasonable upgrade?
Assuming all water supplied from break tank then one shower each per day at 10LPM then usage 2X10X10, 200 Litres, so 250L break tank adequate, even if mains not refilling break tank, but during this time it should be filling it as well for 10 minutes, it can surely fill it at a minimum of 10LPM so 100L to break tank in 10 minutes so worst case, even if two showers used (if you had two) would mean 250+100-200, 150L remaining in break tank. If only one shower, then same total usage but over 20 minutes, so mains will have supplied 200L, break tank remains, 250+200-200, still full?
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Have you looked into a mains boost pump? Will deliver 12L/Min @ ~ 1.5bar dynamic, on a typical low flowing mains supply. Are the mains suppliers delivering at least the Min required flow and pressure? Typically around 9L/Min @ 1 bar.

Draining the tank all depends on usage and flow from the tank - if you are running hot and cold supplies from the break tank and you want to supply more than 1 hot outlet at a time though then a combi may not be the best option.

You can easily test the scenario where you set a flow rate of 10L/Min and see how the break tank copes - of course the shower will also then need to be carefully throttle to ensure they don't exceed a specific output.
I have seen 6LPM throttling orifices in alot of hotels abroad, think you can also get 9LPM ones which should be OK.
Ok, sounds like it should be fine. I've got the worst case option of increasing the break tank size in my outbuilding as long as its same footprint, just means I'd loose a couple of storage shelfs above, not a major deal breaker.

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