Combi Boiler + Tank + Water Pump?

23 Jan 2014
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United Kingdom
Hello - I'd like to switch from a system boiler + tank + water pump to a Combi-boiler, maintaining a high flow rate in the showers (2 bathroom apartment on 4th floor, water tanks on roof).

Clearly I'm not an expert - my objective is to a) add storage by eliminating the cylinder and b) maintain high flow in the shower. I read you cant put a water pump after a Combi Boiler so I was hoping there is a way to leverage from the water tanks on the roof. Any idea?
I've been suggested the following: mains flow is about 9-10 l/m so quite low for a good shower. Instead of connecting the boiler to the mains, I can connect the boiler to the roof tanks, add a water pump in between to create higher pressure/flow. The Combi Boiler (36kw) output is 14 l/m so that can then be pushed to the showers and I believe it will be a decent flow rate. Would this work?
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Not my area of expertise, you may want to post in the plumbing section to get a more detailed technical answer....but

I have an old fashioned gravity fed system with storage tanks and an unrated (lower than G) rated boiler (never broken down). When I replace it, I will be getting a system boiler and a tank because I have more than one bathroom and want hot water without any loss in pressure.

I have been looking into all the options and pros and cons, so here are my thoughts:

As you say you cannot attach a pump to a combi boiler. You are limited by your incoming mains pressure.

Basically you need a good plumber who will (if you choose to go with a combi) properly install a combi boiler. Usually, combi boilers with bad pressure can be a result of bad installation more than anything else. As you have low mains pressure, you may want to look at replacing your old, (possibly lead) mains supply pipe. This can dramatically improve mains pressure and result in a combi boiler with much better pressure. A good plumber should consider this when installing a combi in an old property. They should do tests on the mains pressure before it comes in to your property to assess where the restriction may be and if anything can be done to improve it.

However, if you are used to good water pressure. Loss of pressure in a combi is something you will have to get used to. I have yet been in a house with a combi boiler without crappy pressure loss. It would do my head in...

However, there is a third option. A combi-boiler with a mini storage tank built in. These are larger than a combi but may give you a good half way house. It should allow you to get much higher water pressure for short periods of time.

However, as I said, this all depends on how good the installer is. I would still think that your mains flow rate would be a problem and would need to look at replacing you mains supply pipe (if it is an old lead one) to see if it improves. The limitation of the flow could also be elsewhere in the street, so it is not always simple, which is why a combi boiler is not always the best solution, and why a good plumber is...

You may find that solution will give you enough mains pressure to avoid the more complicated solution you propose.....

from what I have (your proposed solution) can/has been done but is not recommended.....I also wonder if warrantee and guarantees will be honoured if you install a combi contrary to manufacturer’s specs or recommendations...

here is an post on the subject:


I also found this article online, it is selling a product so cannot vouch for how good it is or if it works:

Like I said, if you can test the mains pressure before it reaches your house, you may find a much easier solution to the problem

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