Boiler advice

Jeepers_Creepers

330, you currently do have a system boiler - it was converted into one (thanks Fezter for that info).

If you wanted a boiler mounted in the loft, it would be system type or combi as these two types are sealed and pressurised and don't require the small header tank above them.

As for the hot water side, you'd either need to retain the existing type of hot cylinder you have and also keep it below loft level (along with that existing CWS in the loft), or else - if you wanted the hot cylinder up in the loft alongside the boiler - it would be an unvented type, driven by mains pressure (you'd then lose the CWS tank).

I don't know how tall your loft space is, but it might be possible to have a vented cylinder up there with the CWS mounted higher above it, but I suspect it wouldn't be successful; the CWS would struggle to replenish the hot cylinder quickly enough.

So, your options are;

1) Stick with what you have; system boiler, vented cylinder (where it is) and shower pumps.

2) Fit unvented cylinder in loft side-by-side with system boiler. This will be as good as your cold mains supply - however good that is. You can always add a mains accumulator if needed.

3) Fit combi in loft, and accept it won't run two showers at the same time.

330d

Jeepers_Creepers said:

330, you currently do have a system boiler - it was converted into one (thanks Fezter for that info).

If you wanted a boiler mounted in the loft, it would be system type or combi as these two types are sealed and pressurised and don't require the small header tank above them.

As for the hot water side, you'd either need to retain the existing type of hot cylinder you have and also keep it below loft level (along with that existing CWS in the loft), or else - if you wanted the hot cylinder up in the loft alongside the boiler - it would be an unvented type, driven by mains pressure (you'd then lose the CWS tank).

I don't know how tall your loft space is, but it might be possible to have a vented cylinder up there with the CWS mounted higher above it, but I suspect it wouldn't be successful; the CWS would struggle to replenish the hot cylinder quickly enough.

So, your options are;

1) Stick with what you have; system boiler, vented cylinder (where it is) and shower pumps.

2) Fit unvented cylinder in loft side-by-side with system boiler. This will be as good as your cold mains supply - however good that is. You can always add a mains accumulator if needed.

3) Fit combi in loft, and accept it won't run two showers at the same time.

Thanks. Its strange that the previous owner has fitted a regular boiler but then converted it to system! Why not just fit a system! Anyway!

I think option 2 is the best option. This way I loose the CWS and can move the boiler and hot water cylinder into the loft. (obviously I need to buy a system boiler)

Ideally what is the minimum cold mains supply pressure required for this option?

Jeepers_Creepers

On-line consensus seems to suggest a min of 1.5bar and 20lpm flow.

Almost certainly that pressure - 1.5bar - is 'dynamic' and not 'static'.

Ie, if you measure the pressure sitting in the cold mains pipe coming into your house with all the taps shut off, then that will be 'static' pressure and much higher. Once you open a tap/shower/toilet, one end of that pipe is effectively opened so the pressure will usually drop significantly, although it is being replenished all the time by the mains flow - that's 'dynamic' pressure. I'd imagine that a healthy 3+bar static pressure could easily drop to 1.5bar once a tap is opened fully.

Dynamic pressure is important as it's what drives the water flow and maintains it; it's what you actually use and feel.

You will also lose around 0.1bar of pressure for each metre height you go up.

If the figures you have measured are found to be borderline, before going to the expense of a new boiler + unvented cylinder in the loft, I think it would be worth having worked out in the back of your mind the option of adding that mains booster accumulator if it is needed. One of these would certainly solve any flow/pressure deficiencies. Ie, I wouldn't just go ahead with fingers crossed and then moan if it didn't deliver the two amazing showers I wanted, but instead I'd do so knowing that I'd thought through the solution should it turn out that way.

I mean, if the costly new boiler + unvented tank don't provide the gushing two simultaneous showers you hoped for (it should, but they might not be as impressive as you hoped), it would be a shame if you ended up asking yourself why you didn't go for the much cheaper and more energy-efficient combi instead.

fezster

Not sure if you missed my previous post @330d :
> A system boiler will give you nothing on top of the boiler you already have, set up for a sealed system. 
There's many benefits to your system having been sealed. I prefer a separate expansion vessel, for one, as replacing one in a system boiler requires a GSR engineer and is usually more expensive to buy and fit. Same for the pump. Yes, you need additional space, so if that is the concern, a system boiler would be more compact. Otherwise, there is no difference in performance or output.

@Jeepers_Creepers is correct about the minimum dynamic pressure and flow rates. However, these are sometimes difficult to measure if you do not already have 2 outlets on your mains - one to measure the pressure, and the other (usually an outside tap) to measure the flow. Also, the minimums may not be what you are hoping to achieve with 2 showers. So quite a number of things to think about and discuss with your installer (make sure he or she is G3 certified to work on unvented cylinders).

Jeepers_Creepers

Absolutely, Fezster.

You are at the mercy of the G3 with their judgement that "it'll be fine...". If ever there was a need to have 3 quotes, it would be for something like this and not just to compare prices.

If 330d has a contingency - ie the planned space for a Gundfos or Challis, ideally in the garage - then it should give confidence that an unvented system WILL absolutely work BRILLIANTLY even if it needs this extra expense (factor in £1k to £1.5k).

I don't wish to add a cloud over these plans, but the new boiler and unvented tank fitted in the loft is going to cost - what? - £4k? If at the end of this it gives one GREAT shower but two only 'ok-ish' ones, will he be happy?

Anyhoo, see what the G3s say. Please keep us updated, 330d - these sorts of threads are really useful for folk in your position.

Oh, and a Happy New System :-)

330d

fezster said:

Not sure if you missed my previous post @330d :
> A system boiler will give you nothing on top of the boiler you already have, set up for a sealed system. 
There's many benefits to your system having been sealed. I prefer a separate expansion vessel, for one, as replacing one in a system boiler requires a GSR engineer and is usually more expensive to buy and fit. Same for the pump. Yes, you need additional space, so if that is the concern, a system boiler would be more compact. Otherwise, there is no difference in performance or output.

@Jeepers_Creepers is correct about the minimum dynamic pressure and flow rates. However, these are sometimes difficult to measure if you do not already have 2 outlets on your mains - one to measure the pressure, and the other (usually an outside tap) to measure the flow. Also, the minimums may not be what you are hoping to achieve with 2 showers. So quite a number of things to think about and discuss with your installer (make sure he or she is G3 certified to work on unvented cylinders).

Jeepers_Creepers said:

Absolutely, Fezster.

You are at the mercy of the G3 with their judgement that "it'll be fine...". If ever there was a need to have 3 quotes, it would be for something like this and not just to compare prices.

If 330d has a contingency - ie the planned space for a Gundfos or Challis, ideally in the garage - then it should give confidence that an unvented system WILL absolutely work BRILLIANTLY even if it needs this extra expense (factor in £1k to £1.5k).

I don't wish to add a cloud over these plans, but the new boiler and unvented tank fitted in the loft is going to cost - what? - £4k? If at the end of this it gives one GREAT shower but two only 'ok-ish' ones, will he be happy?

Anyhoo, see what the G3s say. Please keep us updated, 330d - these sorts of threads are really useful for folk in your position.

Oh, and a Happy New System :-)

Thank you for the replies. 

So abit of background to the reasoning I am changing the boiler. I bought a 3 bedroom house which has the Worcester Bosch Greenstar 15i System boiler. I thought the boiler was a little undersized for the property but then again there was only one lady living there so maybe OK.

But we are doing an extension to create 2 extra bedrooms, ensuite, additional radiators and upsizing some of the current radiators. Therefore i WILL require a higher capacity boiler. So I am not just replacing the boiler for the sake of it, it needs to be upsized to around 30kW. Plus the hot water cylinder capacity is 117l which is a small for the 4 of us. I believe I require 200l+.

Secondly the current position of the boiler and cylinder gets in the way of then extension so it has to be moved. I originally thought it can go into the garage but this will involve to rerouting pipes half way through the house which will be expensive.

On the other hand, moving it to the loft will be easier as its currently on the 1st floor, therefore it just needs moving 'up' into the loft. 

EDIT: forgot to add, I would love a combi boiler due to the ease of them but everyone is telling me it would struggle to supply the CH and DHW simultaneously. 

southcoastrgi

I guarantee you won't need anywhere near a 30kw boiler 

330d

southcoastrgi said:

I guarantee you won't need anywhere near a 30kw boiler 

The 30kw value was taken from the Worcester website after entering my house details.

southcoastrgi

how how many rads do you think you will end up with ?

330d

southcoastrgi said:

how how many rads do you think you will end up with ?

Approximately 11