Am I missing something? Why shouldn't I get a combi boiler?

Sorry for the essay but I want to make sure I'm not being stupid!

We're upgrading our heating/hot water systems soon and want to get rid of loft tanks etc so our options are either a combi or unvented system boiler. Getting around 20l/min from kitchen and garden tap running at same time. It's a soon to be 4 bed 2 bathroom semi as we're having a loft conversion.

Problem is space is still a little short where the unvented cylinder could be fitted so I was looking at large combi's. Seem to be a few around that will do 20l/min or even 25. If the benefit of an unvented cylinder was clear to me I would still go for it, but I don't get it.

People talk about about the flow of combi's being poor when two outlets are in use, but surely it can supply the same max 20l/min (ie mains flow rate) as an unvented cylinder could? They both run off mains pressure right? I don't see the difference. Surely they would both fluctuate the same?

Even if we want a more than 35C rise seems unlikely we would get less than 15l/min through the combi. Our current shower is giving 6l/min, and whilst it's nothing special whatsoever it's absolutely fine. So with the combi surely we could still run two showers when we wanted? And surely this is ultimately the same as an unvented cylinder could supply us with?

As they're both mains pressure they're both going to supply the loft bathroom the same right?

I get that if our mains flow was more than the combi could heat then we would see a benefit in the flow we could draw. But in our case, am I right in thinking that there would be no benefit in going unvented?

I was actually looking at the floorstanding Greenstar Highflow 440CDI and 550CDI, they spec 20l/min and 25l/min. Does anyone know if they will provide this continually, or does that include using internal storage tanks? Do they even have internal tanks? I can't seem to find out.

Any help appreciated!

ChrisZx said:

soon to be 4 bed 2 bathroom semi as we're having a loft conversion.

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ChrisZx said:

Problem is space is still a little short where the unvented cylinder could be fitted

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Make space. This is fundamental to the comfort of your life in the (likely) single biggest investment other than children in your life.

ChrisZx said:

so I was looking at large combi's. Seem to be a few around that will do 20l/min or even 25

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ChrisZx said:

I was actually looking at the floorstanding Greenstar Highflow 440CDI and 550CDI, they spec 20l/min and 25l/min.

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Read the specification more carefully.... If the document has a colour picture? Ignore it.

Dan Robinson said:

Make space. This is fundamental to the comfort of your life in the (likely) single biggest investment other than children in your life.

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We will do if need be!

Dan Robinson said:

Read the specification more carefully.... If the document has a colour picture? Ignore it.

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I couldn't find a decent spec sheet before but clearly didn't look hard enough https://www.google.co.uk/url?sa=t&r.../12532&usg=AFQjCNHLVPlTQqOd0Mnp6wJQhCNS98GT9Q

This says 20l/min for 10mins for the 440 and 14 after that so a bit weedy, but 25 for 10mins and 19.6 after that for the 550 which seems alright to me. What else have I missed?

They still have limited performance, and no backup. Better than some but poor compared to a cylinder with an immersion heater backup.

ChrisZx said:

What else have I missed?

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Consider when the combi goes tech and your family has no hot water until an engineer can repair it.

As the last two have said, back up! Secondary reasons would be the greater scope for multiple draw-offs and a faster response for DHW demand.

I grew up with a traditional system. Electric back up was useful for covering the odd boiler fault. At least you can wash!

In my own houses, they have both had the previous owner's swap tanks for combis. I appreciate the space saving, but I have had winter breakdowns, which are awful. If i could replace the lot, I would love to stick a cylinder somewhere, and heat it with a system boiler.

A lot of valid votes for backup, but that aside and concentrating on performance...

Dan Robinson said:

They still have limited performance

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richw said:

Secondary reasons would be the greater scope for multiple draw-offs

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How is the performance limited? How could an unvented cylinder supply more than the 20l/min that the combi boiler can provide considering our mains can only supply 20l/min? And thus how could it support more draw-offs? I'm not trying to be awkward or trying to get anyone's back up or anything, I genuinely want to find out any reasons, but it seems to me that no one really knows!! People just say combi's are worse without being able to give an explanation. But this is my point, I want to find out if in our situation this is a fair analysis. It doesn't seem so at the moment...

richw said:

and a faster response for DHW demand.

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I don't think the response would be any slower with the combi due to the internal storage tank.

richw said:

In my own houses, they have both had the previous owner's swap tanks for combis.

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And what is the performance like? A colleague of mine says his 5 bed Victorian detached is fed by a combi and that the performance is very good, had it for 5 years and no complaints. It's what got me really thinking about it.

Are there any other disadvantages? Would it work well with underfloor heating? Weather compensation?

It may sound like I actually have my mind made up and am looking for confirmation of it, but I really don't. I just don't like doing things without facts!

ChrisZx said:

A lot of valid votes for backup, but that aside and concentrating on performance...

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When its gone tech and your waiting for a specialist engineer to visit,hopefully with spares in van.Performance will be rather awful.

Its your choice what is installed,what ever is installed has to suit your needs and future needs just suggesting an unvented hw cylinder as its a tried and trusted system (with back up ).

ChrisZx said:

I was actually looking at the floorstanding Greenstar Highflow 440CDI and 550CDI,

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If you have floor space to fit one of them boilers then you have room for an unvented cylinder and a boiler above,Have you considered gas consumption with a highflow ?.

Its your choice and i wish you good luck

@ChrisZx A big disadvantage of the Highflow is that its minimum power output to heating is circa 10kW, which is likely to be much higher than your heating system will need. This will lead to considerable cycling, which is known to both reduce heating comfort and significantly increase running costs.

If you do want a storage combi, the Viessmann 222F 35kW is a much better option, as its advanced control options and lower modulation will make it considerably cheaper to run whilst giving you all the performance on hot water that your mains can handle

Other than that, the Vaillant Green iQ 843 will give you 18l/min all day long if you want a wall-hung one box solution, again at lower running costs with the right controls than the Woofter.

A system boiler/cylinder combination would give you the same performance at a lower price and with one mounted above the other would have the same footprint as either of the storage combi solutions

If this has been mentioned sorry, but the only time you are going to get 20 or 25 litres from a combi is if there is storage somewhere. A lot of combis have storage built in. Once the storage has been used you are back to a more typical flow and temperature rise.

Dan Robinson said:

Read the specification more carefully.... If the document has a colour picture? Ignore it.

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Also be wary of claims for spectacular efficiency which based on test results but then "adjusted" by the application of "proven" formulas.

This from the Building Research Establishment makes for informative reading.

https://www.bre.co.uk/filelibrary/.../STP10-B08_Lab_Tests_Gas_Oil_Boilers_2010.pdf
https://www.bre.co.uk/filelibrary/SAP/2012/STP10-B08_Lab_Tests_Gas_Oil_Boilers_2010.pdf <corrected by mod

Especially the vast differences in efficiency for the same boiler between the various ways that hot water is used

The smallest combi boiler can be just about passable in the summer when the water is coming in at around 15c, all(?) combis are rated at a 35c temp rise when flow rates quoted, so in winter when the incoming water is 5c and your combi is flat out the water will be 40c, then you need to lower rate to increase temp, combis also work very hard with lots of components moving from heating to hot water and back say 40 times a day ? if you have the space an unvented cylinder is a better choice between the two as the cylinder will give guaranteed flow rate all year round.

Some good information here, thanks.

I originally planned for the boiler to be above the cylinder, but the plumber we asked assured me it wouldn't fit with a 250 or even 210l tank. So then I thought horizontal which takes up a lot more room.

Might have to get a second opinion by the looks of it as a vertical cylinder under the boiler is definitely my preferred option! We need the boiler to have it's flue out the side as it were, plumber implied it would have to go out the top and then sideways which eats into the space available.

Can anyone say how much room is required above the cylinder/below the boiler?

What are the wall dimensions floor to ceiling ?

Is the installation area in an alcove ?

Old&Bold said:

What are the wall dimensions floor to ceiling ?

Is the installation area in an alcove ?

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2.4m, installation area is a bit funny. It's currently the corner of our kitchen but will become a utility room next year so we're not worried about short term awkwardness (want to do it now to make the loft conversion easier and so we can remove airing cupboard and do some more work there over the winter). But there's also a window on the wall perpendicular to the wall the boiler is on, about 400 in off the top of my head