Which boiler system?

MX5huggy

I’d go back to the start do you need to up grade the boiler at all?
You’re having an extension built which will be significantly more thermally efficient than the external envelope you are covering over. (Maybe solid walls in to cavity, old DG windows to new? Etc) so the heating demand won’t change drastically. Then you are going from 4 people living in a 1 bathroom house to them living in a 2 bathroom house overall water demand should not increase unless the existing shower is pathetic (like my childhood one that you had to put in the bottom of the bath to make it flow, before putting back on the bracket). Do you ever run out of hot water now? 

I’d change the cylinder for an unvented one 200 litre and see how you get on. You can always change the boiler if I’m wrong. Or do some more maths to check it. 

Jeepers_Creepers

You might find that your plumber is simply not familiar with accumulators, so will be wary of them. However, in situations where the mains flow is not quite good enough (or complete pants), and this cannot easily be addressed by, say, increasing the incoming mains pipe size, then pumped accumulators work. They totally do. My bro had one fitted to a trickling cold mains 6 floors up in a flat in London. The shower pressure is now painful (he had to have a PRV fitted to its outlet   )

fezster

Just remember that the combi is working on the 35 degree temperature rise! So in winter when the incoming cold water is 5 degrees, you'll only have the 16 litres/min to get to 40 degrees (i.e. no cold water being blended in). Even in Summer, when the incoming water is 15-20 degrees, the combi will give max 50-55 degrees, whereas a cylinder will store the HW at 65 degrees.

330d

fezster said:

Just remember that the combi is working on the 35 degree temperature rise! So in winter when the incoming cold water is 5 degrees, you'll only have the 16 litres/min to get to 40 degrees (i.e. no cold water being blended in). Even in Summer, when the incoming water is 15-20 degrees, the combi will give max 50-55 degrees, whereas a cylinder will store the HW at 65 degrees.

This is a very good point, thank you. 

But if someone has a tap running when a shower is on, that shower will no longer be benefitting from the 16l/min flow rate as the tap is taking it.

Isn't this where the accumulator comes in? It can continue to provide the boiler with a good amount of flow (I assume it will drop slightly from 16l/min) and also provide flow to that tap.

fezster

Yes, if its a cold tap. No, if its a hot tap.

Your internal plumbing also needs to be considered. If you have 15mm pipe running throughout the house, including to showers, it'll be more susceptible to loss of flow rate and pressure drops than larger diameter pipe.

Jeepers_Creepers

fezster said:

Yes, if its a cold tap. No, if its a hot tap.
Your internal plumbing also needs to be considered. If you have 15mm pipe running throughout the house, including to showers, it'll be more susceptible to loss of flow rate and pressure drops than larger diameter pipe.

330's current system is 'low pressure', being fed from a CWS in the loft. The H&C pipes supplying the bathrooms will almost certainly therefore be 22mm - the larger size allows for an adequate flow with the relatively low pressure driving the water (typically a fraction of 1 bar, 0.2bar, that sort of figure).

Mains-driven systems (unvented tanks and combis) run at much higher pressures - typically 2 bar and above, tho' it varies a lot - these don't require 22mm pipes since this additional 'mains' pressure more than overcomes the resistance of smaller, 15mm, pipes (which is what will likely supply your extension).

Ain't as simple as that, tho'! How well your new system will actually work will be affected not only by the mains supply rate (~20lpm for you) but also the pressure it's delivered at (which we don't know). If this mains pressure is 'only', say, 2 bar or below, then - yes - expect flow rates to drop off noticeably when additional taps are opened. Having a potential 20lpm available is great, but if this isn't backed up with 'force' - pressure - then that 20lpm ain't going to climb hills, and isn't going to successfully supply other taps once others are opened. In essence, with a low supplying pressure, that 20lpm is the absolute best you are going to get in total, and that'll just be out of your kitchen or garden tap. Ask it to supply a tap upstairs, and it could suddenly become 17lpm. Ask it to go through a combi and then go upstairs and it could become 16lpm (so the max rate of the 40kW combi of this figure for just hot becomes a bit moot...). Opening a downstairs tap with either scenario will do obvious detrimental stuff to this already compromised flow rate.

This, again, is where an accumulator will work; it provides both flow and pressure. Therefore opening additional taps will have much less of an affect. I'm not saying these bad things will happen, but it could - depending on your mains pressure.

330d, I can't tell you which system to go for, unvented or combi - that has to be your call.

I think some things that I can suggest will most likely be the case, tho', are:

1) Both systems will 'work' successfully as they are for most purposes. I suspect, with both, opening a second tap will 'affect' the shower, but not to anything like a critical point. It might even just be a minor inconvenience - ie you notice that the shower flow has dropped by a third, something like that, and then it goes back to normal when the other source is shut off (the thermo should keep the temp constant). And if your shower isn't even being run 'full' on, the effect will be even less. So, good chance, both systems will be fine as they are, with only minor compromises as explained.

2) On your current mains supply (although I only know the 'flow' rate) the unvented cylinder will work a bit better than a combi; it's just more 'free-flowing', and isn't limited in its output like a combi is. I doubt it'll be a huge difference, tho'.

3) If you find, with either system, the compromise made by having 'only' 20lpm available to drive them is just too noticeable, too annoying, then adding a pumped accumulator will transform them both.

4) With an accumulator fitted (or with a very good mains supply - mid-20s+ flow, 3bar+ pressure), then the unvented will come into its own, and beat the combi. Why? Because the combi is still limited to ~16lpm of hot delivery, whereas the unvented will gush out its contents as much as you want (tho' it'll need to be kept heated up!).

So, the unvented+accumulator should easily supply two showers simultaneously, and not be bovvered by another tap being opened.

A 40kW combi+accumulator should also supply two showers simultaneously, and not be bovvered by another tap being opened. The combi's output will, tho', always be limited by its max output of 16lpm shared between two showers, although with ~40% of cold being blended in, this will still give two good showers. I wouldn't expect the showers to be affected by a cold tap/flushed loo/cold-feed W/M being used at the same time, but good chance the 'combi' shower will be affected by another hot tap being used.

Perhaps, then, decide on which system to go for based on other factors;

a) how much do you want the cupboard space that the unvented would take up? (Go combi, then).

b) Or, how much is absolute guaranteed top performance to two showers a 'must'? (Go unvented then).

c) How much does installation costs matter? (Combi should be less by ~£1k?)

d) What about running costs? Combi uses slightly less energy as it only heats the HW that's required. Combi slightly less annual servicing (the unvented will need a G3)

e) Reliability? Combis are more temperamental than system boilers.

And then plan for either system to be backed up with an accumulator only if you find it's wanted/needed to boost performance in future. You could well be quite happy with how either system performs as fitted. 

coffeehound

Jeepers_Creepers said:

What about running costs? Combi slightly more energy efficient.

We've been here before, haven't we, Jeepers.  

No, a combi is not more efficient than a properly set up unvented hot water storage cylinder.  

And this counts double with a high-power combi, which has been shown to run inefficiently when overspecified for the central heating system.

Jeepers_Creepers

coffeehound said:

Jeepers_Creepers said:

What about running costs? Combi slightly more energy efficient.

We've been here before, haven't we, Jeepers.  

No, a combi is not more efficient than a properly set up unvented hot water storage cylinder.  

And this counts double with a high-power combi, which has been shown to run inefficiently when overspecified for the central heating system.

Thanks - I amended my post to explain what I meant; it uses slightly less energy as it only heats the HW that's actually required.

Your point about a 40kW combi being overpowered for the CH system could well have value too, tho' I suspect it's not a significant issue - I have no figures, tho'.

coffeehound

Jeepers_Creepers said:

Thanks - I amended my post to explain what I meant; it uses slightly less energy as it only heats the HW that's actually required.

Again, that is not a given.  A combi heats tap water less efficiently in the first place, and a HW cylinder does not automatically 'lose' HW that has been heated.  Even large capacity modern cylinders lose well under two units of heat per 24-hour day, even when maintaining water continuously at 65℃.

Jeepers_Creepers said:

Your point about a 40kW combi being overpowered for the CH system could well have value too, tho' I suspect it's not a significant issue - I have no figures, tho'.

If it reduced C/H efficiency by 10% then that would represent a significant chunk of extra gas per year.

Jeepers_Creepers

coffeehound said:

Again, that is not a given.  A combi heats tap water less efficiently in the first place, and a HW cylinder does not automatically 'lose' HW that has been heated.  Even large capacity modern cylinders lose well under two units of heat per 24-hour day, even when maintaining water continuously at 65℃.

If it reduced C/H efficiency by 10% then that would represent a significant chunk of extra gas per year.

"...a hot cylinder does not automatically lose HW that's been heated". Well, like, yes, it does. Unless it's held in a perfect vacuum.

Ok, well-insulated cylinders don't lose that much, but 'lose' heat they do.

"If it reduced C/H efficiency by 10% then that would represent a significant chunk of extra gas per year..." No Shhhherlock; 'reduced efficiency' = 'extra gas'. Obviously, yes. But why would a 40kW combi in a 5-bedroomed, 2-bathroomed house suffer 'reduced efficiency'? Is it somehow too large for this house? And where did the 10% come from?

I have to hold my hands up; I do not know how the benefits of a combi in that it only heats up the DHW it actually requires compares with the benefits of a system boiler in how it can, often, be smaller compares in overall annual 'energy consumption'.

Do you?