Boiler advice

[Russtcm]

Hi, I am hoping there are some friendly folk on here that can help with a bit of advice.

I have a house with 18 rads, 3 bathrooms/shower rooms (one electric so effectively 2) and I have a conventional boiler set up at the moment.

I am wanting to change to a combi and I am looking at the Worcester 8000 range. Worcester themselves have recommended I match the boiler to my flow rate and if I have a high enough flow rate and can get the 50kw one then I should, as that will give me the best outcome should two showers be turned on at the same time.

Sounds straight forward enough but I am wondering if anyone actually has first hand experience of how these boilers perform when two showers are switched on? Does the 50kw manage two no problem or could I drop to the 40 or 45 and have pretty much the same experience?

They are apparently designed specifically for running two bathrooms at once but what’s the reality?

Trying to make sure before I make the switch. Thanks.

 

[ShaunCorbs]

What’s your mains flow and pressure ?

combis don’t like to run more than one outlet even with a 50kw that’s around 13-14lpm so less than 7 per outlet most showers require around 10lpm to be effective

 

[SimonG]

You need to be checking your flow rate, static pressure and dynamic pressure.

I dont care what manufacturers say, if it's more than one bathroom then it's not a combi for me.

 

[Russtcm]

What’s your mains flow and pressure ?

combis don’t like to run more than one outlet even with a 50kw that’s around 13-14lpm so less than 7 per outlet most showers require around 10lpm to be effective

Thanks for the quick response. I have ordered a flow cup to test the flow but using a jug for a rough estimate, I reckon it’s over 20lpm. The flow for the 50 is rated at 17.9lpm at 40 degrees. Presumably when mixed in with cold water in a mixer shower this then gives suitable flow for 2 showers when required?

Sorry if I’m not quite understanding but that’s why I’m here. Thanks.

 

[John.g]

40C is the temperature rise through the boiler.
If you are happy with a 40C showering temp then a 50 kw combi should give 20.46LPM mixed with cold water at 5C, 23.9LPM mixed with water at 10C and 28.64LPM mixed with water at 15C

 

[gmartine]

50kw, assuming your current gas meter is up to it also?

Don't listen to WB...they will sell you any old turd. If you're insistent on not having any large HW water storage then you should consider a storage combi like the Viessmann 111w or push the boat out with a 222F

 

[ShaunCorbs]

Thanks for the quick response. I have ordered a flow cup to test the flow but using a jug for a rough estimate, I reckon it’s over 20lpm. The flow for the 50 is rated at 17.9lpm at 40 degrees. Presumably when mixed in with cold water in a mixer shower this then gives suitable flow for 2 showers when required?

Sorry if I’m not quite understanding but that’s why I’m here. Thanks.

that’s the flow rate in summer winter will be a bit less would estimated the water temp at around 5 dc (worst case) so 18lpm at 45dc max temp if you want higher temp your flow rate has to slow / drop to around 15lpm for 60dc

 

[John.g]

I was assuming that the actual boiler output is 50kw, if not then forget my figures as they are based on this, a 40 kw storage combi will theoretically give the same flowrates but only for a minute or two.

 

[ShaunCorbs]

I was assuming that the actual boiler output is 50kw, if not then forget my figures as they are based on this, a 40 kw storage combi will theoretically give the same flowrates but only for a minute or two.

 

[John.g]

If they make the storage cylinder big enough they will then be almost as good as a heat only boiler, imagine just a 15 kw combi with a 300 litre storage cylinder, you still won't believe its output!.

Oil fired combis usually give the hot water capacity equivalent which is more useful IMO.

 

[Gasmk1]

we fitted one several years ago all 3 showers running and kitchen tap no problem

 

[Russtcm]

I watched the video. Thanks for the advice.

So that I understand, would a normal combi boost the pressure but the 111 won’t? In the video he mentions that it will only have the pressure as supplied from the road.

 

[Gasmk1]

no a normal Combi doesn't it just uses incoming mains same as

 

[gmartine]

I watched the video. Thanks for the advice.

So that I understand, would a normal combi boost the pressure but the 111 won’t? In the video he mentions that it will as only have the pressure as supplied from the road.

Nope neither "boosts" pressure as such but should provide you with a steady 20l/m across more than one outlet but it's still dependent on your incoming water pressure like any combi. The reheat on this unit is very fast so you shouldn't run short of hot water even during high demand. Also although you want something as compact as a combi this isn't it's about twice as large and as heavy but it is wall mounted like a combi.

 

[John.g]

This is how I see it,
Your 50kw boiler above will as stated give a continuous flow rate of 20.48 LPM at 40C from a cold water temperature of 5C. A 35 kw boiler non storage combi will give a flow rate of 14.33 LPM under the same conditions. The 111 above is, I think, a 35kw boiler with a (primary ) storage of 46 litres, I don't know the temperature but if 75C is assumed, then that 46 litres will supply 92 litres of water at 40C from cold water at 5C, if you want the same flow rate of 20.48 LPM at 40C then this 92 litres will last for 4.5 minutes, after that you either reduce the flowrate to 14.33 LPM to maintain the 40C showering temp or you leave the flow rate at 20.48 LPM and accept a showering temp of 29.5C.

 

[EvilDrPorkChop]

IMO storage combis are pointless, for the size, complexity of them and the limited amount of stored HW they can supply you then if you're having one of them you may as well have an Unvented cylinder.

OP - in reality will you ever use two showers at once?

I have an Unvented system and we can use two at once. In the 4 years of living here I think we've done it twice....

Combis are great if you haven't got the room, I don't always agree with the if you've got two bathrooms you need a cylinder - whilst it's always ideal to have an Unvented cylinder costs, space and installation issues don't always allow - in these situations a big output non storage combi will give you decent HW.

 

[Russtcm]

IMO storage combis are pointless, for the size, complexity of them and the limited amount of stored HW they can supply you then if you're having one of them you may as well have an Unvented cylinder.

OP - in reality will you ever use two showers at once?

I have an Unvented system and we can use two at once. In the 4 years of living here I think we've done it twice.***

Combis are great if you haven't got the room, I don't always agree with the if you've got two bathrooms you need a cylinder - whilst it's always ideal to have an Unvented cylinder costs, space and installation issues don't always allow - in these situations a big output non storage combi will give you decent HW.

That you all for your advice. It’s has been very helpful.

I think this reply sums up my situation. In reality it is only going to be the odd occasion where I need two showers running at once. Therefore I think a high output combi like the 8000 45 or 50 is probably going to be sufficient. If there happens to be two showers in use at once then they will still be satisfactory.

Presumably I will save money on gas and I will also be able to remove the old tanks to create more storage, so I think those pros outweigh the cons.

Would you agree with that thinking?

 

[EvilDrPorkChop]

That you all for your advice. It’s has been very helpful.

I think this reply sums up my situation. In reality it is only going to be the odd occasion where I need two showers running at once. Therefore I think a high output combi like the 8000 45 or 50 is probably going to be sufficient. If there happens to be two showers in use at once then they will still be satisfactory.

Presumably I will save money on gas and I will also be able to remove the old tanks to create more storage, so I think those pros outweigh the cons.

Would you agree with that thinking?

Yeah sounds about right.

As other's have said you need to check your water pressure, static and dynamic pressures and flow rates. Providing they are OK then it'd be worth you having a large output combi, I've seen people run two showers on a 40kw before now so a 45 or 50 should be fine. IMO they're probably a bit overkill, as your modulation range will be reduced for the heating however i'm not familiar with Worcesters new 8000 range so they may have a large modulation range now.

In regards to saving Gas, depends how long you're in the shower for! At 50kw it'll be using a far bit 😂

 

[Russtcm]

Yeah sounds about right.

As other's have said you need to check your water pressure, static and dynamic pressures and flow rates. Providing they are OK then it'd be worth you having a large output combi, I've seen people run two showers on a 40kw before now so a 45 or 50 should be fine. IMO they're probably a bit overkill, as your modulation range will be reduced for the heating however i'm not familiar with Worcesters new 8000 range so they may have a large modulation range now.

In regards to saving Gas, depends how long you're in the shower for! At 50kw it'll be using a far bit 😂

The 8000 45 model has Max heating output of 35kw a modulation ratio of 1:10 and therefore a min heating output of 3.5kw so it should be ok on the heating side Id imagine.

 

[gmartine]

We all have our favorites, I think if you trawled these boards WB wouldn't come out as one though for a variety of reasons but basically for the UK domestic market it's been more a triumph of marketing than performance/maintenance for them.

Seeing as your HW requirements aren't quite as what we assumed initially you might as well make the jump in quality to the 200w for around the same unit cost of just under £2K. It still knocks the spots off the 8000 from what I can tell, similar HW output flowrate but far superior heating modulation rate, stainless hex. etc controls and fully integrated weather comp which'll save you a bucket if you allow it.

 

[Russtcm]

We all have our favorites, I think if you trawled these boards WB wouldn't come out as one though for a variety of reasons but basically for the UK domestic market it's been more a triumph of marketing than performance/maintenance for them.

Seeing as your HW requirements aren't quite as what we assumed initially you might as well make the jump in quality to the 200w for around the same unit cost of just under £2K. It still knocks the spots off the 8000 from what I can tell, similar HW output flowrate but far superior heating modulation rate, stainless hex. etc controls and fully integrated weather comp which'll save you a bucket if you allow it.

The only thing about that is that it doesn’t have the flow rate of the 8000. So if I do have an occasion where two showers are on it won’t be as good presumably.

 

[John.g]

32kw I think, so 13.1 LPM from 5c to 40C.

 

[Russtcm]

32kw I think, so 13.1 LPM from 5c to 40C.

32kw heating and 34kw dhw. I have 18 rads so is this not a tad light on the heating side?

This is what I thought I was just getting. So is 34kw dhw going to run two showers satisfactorily? Or do I need the extra 10 kw for when that situation arises?

 

[gmartine]

32kw I think, so 13.1 LPM from 5c to 40C.

More than that last time I checked _more like over 16lpm and doesn't require huge power requirements because of hex efficiency.

https://webapps.viessmann.com/vibooks/api-internal/file/resources/technical_documents/GB/en/VDP/6154113VDP00001_1.PDF?#pagemode=bookmarks&zoom=page-fit&view=Fit

 

[gmartine]

32kw heating and 34kw dhw. I have 18 rads so is this not a tad light on the heating side?

This is what I thought I was just getting. So is 34kw dhw going to run two showers satisfactorily? Or do I need the extra 10 kw for when that situation arises?

Do you have the gas supply/meter to even feed a 50kw combi otherwise you're wasting your time considering one?