Which boiler system?

330d

Hi, I am after some advise to what boiler type I should be upgrading to.

House is currently 3 bedroom and 1 bathroom. It is being extended to 5 bedroom, 1 ensuite and 1 bathroom.

House currently has a 18KW heat only boiler, 117l hot water cylinder and cold water storage tank in loft.

Due to the size increase of house, I need my boiler and hot water system to be upgraded.

I am thinking towards changing to a system boiler of around 30kw and larger 200l hot water cylinder.

An engineer came out today who advised change to a 40kw combi. I am a bit skeptical as I believe a combi suffers wit pressure loss with multiple taps or showers. He checked the current flow rate and was 20l/min and said this would be fine.

Is this correct? Would a combi be fine for my house? I am little concerned as I dont want to pay £1000's for a new system only then to find out it is inadequate. On the other hand, I am happy to go with a 40kw combi if it is suitable. 

Or any other solutions you can recommend?

bris

Nope a combi just makes it easy for him.
An unvented system with a tank around the 210 to 300 range would be better. You can get rid of the tank in the loft at the same time.

There is a but, we don't know what your demand for hot water will be, that's family size and how many people will be using it at the same time so I thought I should just point out the combi could work if the demand was low.

330d

bris said:

Nope a combi just makes it easy for him.
An unvented system with a tank around the 210 to 300 range would be better. You can get rid of the tank in the loft at the same time.

There is a but, we don't know what your demand for hot water will be, that's family size and how many people will be using it at the same time so I thought I should just point out the combi could work if the demand was low.

Yes this was my plan too. An unvented system boiler around 28kw along with a 200l cylinder. 

But he just confused me when he said a 40kw combi should be ok.

There are 4 of us in the property, 2 adults and 2 children. My main concern with a combi would be a pressure drop in the shower if a tap gets turned on.

I have also been doing some research and two other options may also work;

- Combi water storage boiler (Vailant 938)

 - Combi boiler (40kw) and an unvented water cylinder

Any views on this? Especially on the option regarding adding a unvented cylinder on a combi.

330d

Any views on this please?

koalakoala

if it's only 2 showers I don't see how a 40kw combi wouldn't work.  We have only a 35kw and it is superb for our needs, also 2 bathrooms.
We replaced a boiler and water tank , boiler was only 7 years old but was rubbish

fezster

A 40KW combi will deliver 16.4 litres/min at a 35 degree temperature rise. In Summer, this will mean a roughly 60/40 split of hot and cold for a 40 degree shower, but in winter the hot water ratio will be a lot higher. So you'll have approx 17 to 27 litres/min at 40 degrees.

Now consider the shower's you'll be having installed (thermostatic mixer, I presume) to calculate if that's enough.

Also consider that combis are generally more complicated and expensive to fix compared to system or heat only boilers. 

Finally, consider your incoming cold water main, as this will ultimately dictate your flow rate, regardless of which system you have installed.

Jeepers_Creepers

330d.

You have an incoming supply of 20lpm. That's pretty good, but that's yer lot. So (unless I'm mistaken), it isn't actually going to matter that much whether you go for a combi or an unvented cylinder - both are 'powered' by - and will be able to supply - 20lpm maximum, and that's shared between the hot and cold (which will be blended in a shower).

What's your incoming mains pressure like? That's another factor, just as important; if the pressure is just 'ok' (like your flow) - say around 2 bar - then opening a second tap will have a much greater detrimental effect. If the pressure is 3bar upwards, then opening a second tap will have little effect - the 20lpm will still be maintained.

Will running a hot or cold tap/flushing a loo/running a washing machine affect the flow in the shower? Yes. With both systems? I think so - after all, they are both supplied at 20lpm max. Will it matter? I suspect not - the thermostatic mixer will take care of 'oooohs!' and 'aaaaahs!', so all you should experience - with either system - is a slight drop in flow, and maybe not even that unless you like having your shower running at full chat? (Ie, if you typically run your shower at 3/4rs flow - ~15lpm (that's a good shower) - then the drop in flow from having another tap opened should be minimal.)

Again, unless I'm mistaken, I suspect there will be little difference in performance between these two systems, for the reason stated.

On a different note, will you at any time hope to have two showers running at the same time? If so, neither system will provide a 'full' shower to both, but I guess you know that. Could it work at all? Yes - if both shower users agree to run theirs at half-mast.

So, what's to choose between these two systems?

Combi: Pros - small space with no tank. Instant, endless hot water. No recovery time. Slightly better fuel running costs as it only heats what it actually uses. Cons - less reliable overall.

Unvented: Pros - a warm airing cupboard. Very good performance IF the mains supply is there. Generally reliable. Cons - higher installation cost. Highly annual servicing. Tank takes up space.

Ironically, your existing vented system can provide two showers standing on one leg, but it'd likely need modifying with pumps and stuff.

Any other systems to consider? A pumped mains accumulator - around £1k and can be fitted in your garage - can provide all the mains supply you could want (40+lpm), and this could then drive either the large combi or the unvented cylinder to perfection.

330d

Jeepers_Creepers said:

330d.

You have an incoming supply of 20lpm. That's pretty good, but that's yer lot. So (unless I'm mistaken), it isn't actually going to matter that much whether you go for a combi or an unvented cylinder - both are 'powered' by - and will be able to supply 20lpm - maximum, and that's shared between the hot and cold (which will be blended in a shower).

What's your incoming mains pressure like? That's another factor, just as important; if the pressure is just 'ok' (like your flow) - say around 2 bar - then opening a second tap will have a much greater detrimental effect. If the pressure is 3bar upwards, then opening a second tap will have little effect - the 20lpm will still be maintained.

Will running a hot or cold tap/flushing a loo/running a washing machine affect the flow in the shower? Yes. With both systems? I think so - after all, they are both supplied at 20lpm max. Will it matter? I suspect not - the thermostatic mixer will take care of 'oooohs!' and 'aaaaahs!', so all you should experience - with either system - is a slight drop in flow, and maybe not even that unless you like having your shower running at full chat? (Ie, if you typically run your shower at 3/4rs flow - ~15lpm (that's a good shower) - then the drop in flow from having another tap opened should be minimal.)

Again, unless I'm mistaken, I suspect there will be little difference in performance between these two systems, for the reason stated.

On a different note, will you at any time hope to have two showers running at the same time? If so, neither system will provide a 'full' shower to both, but I guess you know that. Could it work at all? Yes - if both shower users agree to run theirs at half-mast.

So, what's to choose between these two systems?

Combi: Pros - small space with no tank. Instant, endless hot water. No recovery time. Slightly better fuel running costs as it only heats what it actually uses. Cons - less reliable overall.

Unvented: Pros - a warm airing cupboard. Very good performance IF the mains supply is there. Generally reliable. Cons - higher installation cost. Highly annual servicing. Tank takes up space.

Ironically, your existing vented system can provide two showers standing on one leg, but it'd likely need modifying with pumps and stuff.

Any other systems to consider? A pumped mains accumulator - around £1k and can be fitted in your garage - can provide all the mains supply you could want (40+lpm), and this could then drive either the large combi or the unvented cylinder to perfection.

What a brilliant reply, thank you.

The 3rd option you mention, combi with an accumulator sounds like a viable option. This setup will have the pro's as you mentioned but with the added benefit of supplying good pressure and flow rate when multiple water sources are on.

Is this correct? If so, then this would help my biggest concern with a combi which is a drop in pressure when a shower and tap is on at the same time.

koalakoala

I think you're overthinking the drop in pressure here. Unless you need the showers going at full force? I certainly don't, and have never noticed any drop at all, with taps and washing on at the same time.

330d

koalakoala said:

I think you're overthinking the drop in pressure here. Unless you need the showers going at full force? I certainly don't, and have never noticed any drop at all, with taps and washing on at the same time.

Maybe. But i rather go the extra mile to ensure no problems.

In my old property it had a 37kw Vailant combi and there was a drop in flow when two taps were running at the same time. 

Jeepers_Creepers

330d said:

What a brilliant reply, thank you.
The 3rd option you mention, combi with an accumulator sounds like a viable option. This setup will have the pro's as you mentioned but with the added benefit of supplying good pressure and flow rate when multiple water sources are on.
Is this correct? If so, then this would help my biggest concern with a combi which is a drop in pressure when a shower and tap is on at the same time.

Yes.

Your current situation - incoming cold mains is 20 lpm. Ie the max available flow shared between hot & cold is 20 lpm. A 40kW combi should deliver around 16lpm of hot water, but with cold also needed in order to blend it down to around 40oC, you will likely only be asking it to deliver around 13lpm in order to have a good shower temp (ie 7lpm being blending cold). And this is with a shower going full blast.

With an unvented cylinder full of hot water, it - too - will only be asked to deliver around 13lpm of hot for a full shower, since it - too - will require around 7lpm of cold to blend it to the right temp (both combi and cylinder will be supplying water at around 65oC).

I reiterate this because some sites like https://www.boilerguide.co.uk/articles/best-40kw-combi-boilers state that 40kW combis 'can supply two bathrooms at the same time'. As far as I can see, they can not - UNLESS the cold mains is at least, ooh, mid-20s+ of flow, backed with a solid pressure of, ooh, approaching 3bar. In which case it can do ditto with an unvented cylinder.

I reiterate this for another reason - I'd love an actual plumber to confirm whether I've got this right or wrong, because it strikes me as being fundamental; neither system will give you fully what you want unless your cold mains supply is improved. As far as I can see, both systems will react in a similar way when a second tap is opened. Not disastrously, but it'll be noticed.

(Don't get me wrong, 20lpm is 'fine'. But just not enough for a shower to not be affected to some degree when a tap is turned on. I think my flow is more than that, but it's noticeable when a tap is run during a shower.)

Will a pumped accumulator solve this? Yes. It'll provide 40lpm+ at 3bar, and this will drive anything, unvented or combi. It'll even allow you to run two showers at the same time from a 40kW combi, and not be affected by an further opening tap. Bear in mind, tho', that each shower will still be limited by the combi's power - so each will deliver around 12lpm (this is combined H&C - half of 16lpm of hot + half as much again of cold to blend), which is actually a perfectly good shower.

This is where the unvented will win. This can deliver any amount of hot, waaaay over the 16lpm of the large combi. So, if the unvented is driven by the accumulator, then it can provide 15lpm of hot to each, and also 5lpm of cold = two gushing showers!

Ooookkkaaaaay, what to do?! If you want a guaranteed performance from two showers running at full blast, then the unvented + accumulator makes most sense. For this option, I think what I would actually do is to fit the unvented and leave the option of adding the accumulator afterwards if I deemed it necessary; you might find that what you currently have is actually quite liveable-with. Ie, I'd make sure that, in the plumbing design, I could slot in an accumulator without hassle if needed - eg in the garage, out of the way.

This also means that the 40kW combi is a goer. If I really preferred a combi over an unvented (say, I didn't want cupboard space taken up by the hot unvented cylinder), then I would go ahead with the 40kW combi and see how it goes. I would also have, in reserve, the option of adding the accumulator if I felt it was needed.

With the 40kW combi installed, I'd see just how much the shower was affected by other taps. It might not be that bad. If, for example, the shower you fitted was actually restricted down to, say, 15lpm flow max (they all vary), then turning on another tap shouldn't have that great an effect 'cos you have at least 5lpm in reserve. In fact, since you would almost certainly have isolating valves fitted on the H&C supplies to it, these could be tweaked down to halfish-way, so that the max flow from this main shower is limited to, say, 12lpm (still waaay more than an electric shower) and that should leave it relatively unaffected by other taps being used.

Like with the unvented, if you later decide 'You know, I really want the water blastin'...', the  fit the accumulator - an extra £1k-ish.

To sum up (!), an accumulator will give you ultimate performance (as and when it's required). With a 40kW combi, this should drive two showers at the same time and not be affected by other taps. Bear in mind that each shower in this scenario will be limited to some degree in performance due to it being supplied by a combi - say 12+lpm for each shower (still perfectly good). (If you run only one shower, it'll run faster because the thermo valve will use all the available hot and add the required cold = max flow. If you run both showers so that each only gets half the hot flow, then the thermo will automatically blend the correct amount of cold to give you two 'half' showers.)

An accumulator with the unvented cylinder should have no limits since it has the hot water stored, so can provide it in huge quants - until your hot water runs out!

Sooo, how much more would you prefer a combi over the unvented? And why?

Costs: unvented ~£1k, system boiler ~£800, accumulator ~£1k plus installation.

Combi ~£1.2k, accumulator ~£1k plus installation.