Combi boiler fed by pump from tank

Comedydave · 2 Apr 2022

Hi
TL;DR - I want to get a combi boiler to save space but want to increase the pressure of my water by pumping water from my tank through the combi

I currently have a 2bar shower pump pumping cold from my tank and hot from my cylinder to my shower. Now we're getting the house renovated, and one thing I'd like is to consider a combi boiler to save on storage.

I know I cannot use the shower pump with the combi boiler. However I'd like to maintain a decent head of water in the shower. Furthermore, the mains pressure is not ideal - I'm getting about 8 litres per minute from the mains to the kitchen sink (ground floor two storey house).

It was suggested to me that I use a mains pump on my attic tank (instead of on the mains) to feed the combi boiler. This would in theory give enough pressure to result in flow to the shower similar to what I have already. However, there are questions I have and am not sure how to answer them so looking for help here.

1. The tank in the attic is 50 gallon. Will putting a mains pump on the outlet of this run the risk of pumping it dry in no time? 50 gallons is quite a lot of water.

2. Would this result in too much pressure on the other hot water outlets (kitchen sink, bathroom sinks, bath tap) and result in drippy leaky taps (and a constant on-off of the pump as the pressure drops and it recognises that)?

3. How to choose what size mains pump? I see a 4bar pump - but 4bar seems quite a lot especially when the original shower pump was only 2bar

Any help appreciated!

DP · 2 Apr 2022

Forget the combi, keep the hot water cylinder.
Cannot understand people being obsessed by combi boilers.

Madrab · 2 Apr 2022

Comedydave said:
I see a 4bar pump

A mains pump won't push 4 bar, a whole house pump will but they are quite different beasts.

A mains pump will only pump to a max of 12L/Min @ ~1.5bar.

Pumping the 50gal cistern is again a different prospect, if you can pump the water out faster than the mains can replace it, it can eventually run dry, that can be overcome by using a limiting switch that turns the pump off once the water level drops to a certain point in the cistern but that's then getting complicated and messy.

Combi boilers are only really designed to be good at specific things in specific situations. Their main drawback, use more than 1 HW outlet at a time and they falter. Only go to a combi if you and the occupants can mange that expectation otherwise, as suggested, stick with what you have.

Comedydave · 3 Apr 2022

Thanks for the responses chaps.
The storage space would be the only advantage of the combi it seems - and so the question is do I really need it.
@Madrab thanks for the info re whole house pump and limiting switch.
Perhaps there is a third option - better placing the hot water cylinder to maximise space usage..

MeldrewsMate · 3 Apr 2022

I echo the advice to avoid a combi. If you must, then you can use the shower pump to boost the tank cold supply INTO the combi's inlet, and the otherwise 'hot' side of the shower pump to boost an extra feed of cold from the tank to the shower, thereby better balancing the hot and cold supplies to the shower. Be aware of the warnings from others re: using more than one outlet at once, and that the shower pump will run every time a hot tap is opened elsewhere in the house.

Consider also what may happen if gas supplies are limited by future government eco policy. Any ASHP will require a hot water cylinder, and you'll also need one to get the best out of solar energy (PV or thermal).

Comedydave · 3 Apr 2022

Cheers @MeldrewsMate
These are all very good points. Thanks for the idea on using the shower pump to rig up a boost input to the combi on the cold, then a balancing boost to the shower cold in.... I think it's probably a bit of a "hack" though, and liable to end up costing more to rip out.

I think I'll stay with the cylinder so - your points re future-proofing it are important.

Bit off topic but incidentally, I had another thought today - and am looking at the space under the stairs where I could fit a 140l tank instead of my current 117l one, but horizontally instead of vertically... this would still allow me to get my storage space AND have a cylinder.. I think this is what I'll do.

Thanks everyone for your input!

Madrab · 3 Apr 2022

Comedydave said:
but horizontally instead of vertically

Horizontal cylinders are actually a bit of a strange breed, due to the nature of the design they aren't very efficient at heating the whole cylinder and given it has a single top outlet then you don't actually get access to the hot water at the other end when the cylinder is being used and refilling.

Cylinder design works well vertically as the coil is at the bottom and the hot water rises to the top of the cylinder, where the draw off is and striation keeps the cold entering into the bottom from mixing with the hot at the top. With the draw off on the side and the coil in the middle and laid horizontally then they just don't work as well. Avoid if at all possible, they should really only be used if there's no other option IMO

Comedydave · 4 Apr 2022

Madrab said:
Horizontal cylinders are actually a bit of a strange breed, due to the nature of the design they aren't very efficient at heating the whole cylinder and given it has a single top outlet then you don't actually get access to the hot water at the other end when the cylinder is being used and refilling.

Cylinder design works well vertically as the coil is at the bottom and the hot water rises to the top of the cylinder, where the draw off is and striation keeps the cold entering into the bottom from mixing with the hot at the top. With the draw off on the side and the coil in the middle and laid horizontally then they just don't work as well. Avoid if at all possible, they should really only be used if there's no other option IMO

Hi Madrab - thanks for this, appreciate you coming back.
One question though: your point to "avoid if at all possible" - did you mean "avoid laying a vertical cylinder on its side" because (a) the drawoff would be 1/2 way down and (b) the heating water in the coil would now have to go in an "up down" motion (i.e., vertical cylinders just don't work well at all laid on their side) which would generally result in not great efficiency OR did you mean "avoid any horizontal cylinder including those designed to be installed horizontally" (eg like this or like this) because they are just totally inefficient in comparison to a vertical cylinder?
Thank you once again,
David

Madrab · 4 Apr 2022

No ... horizontal cylinders are constructed differently to vertical cylinders but use the same process to heat and draw the water.

Vertical cylinders work well due to the position of the components and the fact that it's a narrow, tall cylinder that bells at the top and allows the hot water to be heated from the bottom, rise to the top and then drawn consistently and efficiently from the top. Vertical cylinders cannot be used on their side.

Horizontal cylinders are designed specifically to be horizontal, they are usually double ended using a similar cylinder shape but being sat on its side with the draw off in the top of that 'side'. This means the hot water is held all along the length of that 'side', with the outlet being just at one point means it's a lot harder at getting all that hot water out of one small outlet, so the amount of usable hot water that they actually deliver is a lot less than the volume held, compared to vertical. Invariably they aren't held horizontal either, rather they are fitted with a slope towards the HW outlet to allow for venting (with an open vented system) and to allow the gathering of more HW towards the outlet.