Boiler advice

330d

I have just bought a 3 bedroom house with one bathroom. I am doing an extension to create 2 new bedrooms and along with an ensuite. 

The house has a system boiler with a unvented hot water cylinder.

My original plan was to remove the cylinder and replace the boiler with a high power combi, around 38kw. This was so I could get instant hot water and to save space my removing the cylinder.

But i am a little skeptical about this now as I dont think a combi will give me sufficient flow and pressure. Especially when two sources of hot water are required.

So I think my options are;

- Keep the current system boiler and move hot water cylinder in loft? 

- Fit a combi (i dont think this will be sufficient unless told otherwise?)

- 3rd option is fit a combi along with a hot water cylinder. Will this option help in terms of multiple hot water demand at once?

Thank you

grumbler

"two sources of hot water " is too vague, isn't it?

The common view is that (almost?) any domestic boiler isn't powerful enough for two simultaneous  showers, let alone for a shower + filling a bath.  Typical advice is one shower to be electric.

330d

grumbler said:

"two sources of hot water " is too vague, isn't it?

The common view is that (almost?) any domestic boiler isn't powerful enough for two simultaneous  showers, let alone for a shower + filling a bath.  Typical advice is one shower to be electric.

A mixer shower and a tap or a bath, etc.

tacpot12

Your best option is to relocate the cylinder to the loft (to gain the space). A system boiler and unvented cylinder will give you higher flow rates of hotter water than a large combi boiler. You can always add pumps to the showers if the flow rates from the cylinder are not adequate, but with the correct size pipes, you shouldn't need them.

A large combi might output 50kW, which if you are trying to run two hot taps means that each one is only getting the equivalent of 25kW. In my experience you need more than this to get adequate flow rates. 

330d

Thank you, great advice.

I did think about this and forgot to write it in my original post. This option does work for me as due to the building work happening at the house I can get the hot water cylinder into the loft. 

But with this option I am concerned about getting instant hot water. If the cylinder runs out, I believe it can take around 30-40mins to refill and geat up again. This is the reason I was thinking of a combi.

fezster

What are your shower flow rates? And can your incoming main provide that? Regardless of combi or unvented, this will limit what you can achieve (without getting into accumulators or booster tanks). 

A large combi will provide around 18l/min (possibly more for a limited burst if its a storage combi), but remember this is at a 30 degree rise. This means in winter, when the incoming water temperature is a lot lower, your combi flow rate will drop to provide the same temp or the output temp will have to be lower. Therefore, the ratio of hot to cold in your thermostatic shower will alter too.
An unvented does not have these limitations, as the water is heated and stored at 60 degrees. Yes, you are limited by the size of the cylinder, but it should be sized to cope with your requirements. 

Also worth noting that combi boilers are more complex and therefore more expensive to fix.

If you have the room, and with multiple showers, an unvented is the preferred solution if your water main is adequate. 

330d

I need the measure the flow rate.

What about a combi boiler along with a unvented hot water cylinder? Will this not provide instant hot water but yet have a good flow rate if two showers are both running? As the showers will be getting hot water from the cylinder? Is this an option?

Jeepers_Creepers

I agree with the above - the safest, most reliable bet would be to stick with what you have.

You shouldn't really have any issue with a cylinder running out of hot since the boiler will fire up to keep it heated as soon as it detects the temp drop to below the set stored - what? - 67oC? I think you'd have to have two greedy simultaneous shower users for the tank to run cold. And then they frankly deserve what they get.

You could also flick (or set on a timer) the tanks's immersion heater to work alongside the boiler. If, for example, folk all get up at 7am and expect hot showers, then have your DHW come on at 6.30 at the latest and your immersion click in for an hour at 7 itself to help out.

If a member of your family does take too much advantage of the gushing showers provided, then tweak down the HandC shower mixer isolating valves so that it reduces the max flow available to each shower. This doesn't have to cripple the quality of shower, just stop folk taking the piddle. :-)

I understand that unvented cylinders are essentially 'powered' by the incoming mains pressure and flow (although they also have expansion vessels in them), so for this to work well you'd need to ensure you do have an adequate mains supply; 3bar min and in excess of 20lpm flow? (that's minimum...). If your incoming supply is a bit pants and this cannot be sorted by improving the incoming supply pipe, then you can fit booster tanks such as made by Grundfos. These WILL give you all the water you need, but you'll need room for it - best if you can install this in your garage.

(AfaIk, you cannot add shower pumps to the outlet of unvented cylinders as you would be effectively trying to 'suck' the mains. But I don't know for sure.)

If your current system gives a good flow from two fully-open taps and a flushed toilet, you should be fine :-)

.

Jeepers_Creepers

If you really want to try and go 'combi' for 2 showers, then you will almost certainly also require a mains booster tank* as well; combis NEED that reliable powerful water source - that is all they have to 'power' them.

*Grundfos, Challis, etc

grumbler

Jeepers_Creepers said:

If you really want to try and go 'combi' for 2 showers, then you will almost certainly also require a mains booster tank as well; combis NEED that reliable powerful water source - that is all they have to 'power' them.

What difference does 'combi' make in this respect compared to the existing unvented system?

Jeepers_Creepers

grumbler said:

Jeepers_Creepers said:

If you really want to try and go 'combi' for 2 showers, then you will almost certainly also require a mains booster tank as well; combis NEED that reliable powerful water source - that is all they have to 'power' them.

What difference does 'combi' make in this respect compared to the existing unvented system?

My understanding (tho' I'm not a plumber) is that unvented cylinders have expansion vessels in them which 'charge' with pressure and so assist in pushing the water out when taps are opened. Ie it doesn't just rely on the incoming mains flow to power the water supply.

Combis, however, are fully reliant on the mains flow. And this is likely to be reduced at peak times.