Discuss Are storage combi boilers better than a system boiler? in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

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We are currently looking to replace our 40 year old vented system to a system boiler. We have had a suggestion of getting a storage combi boiler instead of a vented boiler with cylinder. Space is not a problem. I am more than happy to replace both boiler and water cylinder. We have many options of where they could be situated. My main question is whether a storage combi boiler is as "good" at providing adequate DHW as a system boiler.
We are extending so will have 5 bedrooms, 1 bath with shower and 2 further showers.
Mains water pressure is considered "excellent" - "off the scale". I'm afraid I don't know actual figures as I am merely a home owner, not a professional.
I would really appreciate any views on whether a system boiler would be considered the best fit solution, with the storage combi a second best option if you don't have the room for an extra water cyclinder.
Or, if indeed, the storage combi really is a good idea, and would ultimately save money, both in instalation and running costs.
In short, are storage combis as good as the adverts, or shall I keep with the system boiler idea.
 
They are not as good as a boiler and unvented hot water cylinder. Its a cheaper, space saving option
 
Very very popular in Italy. I hate the poxy things. BUT...

It's better than a bad combi in that you have a small reserve of hot water. As they do cycle between hot and warm, you can find that the water temperature varies. But the flow when say, running a bath is still limited to the boiler output. I have a thermostatic bath mixer and running a bath is a skill as you need to run the bath fast and hot enough to force the boiler to fire constantly instead of cycling between 45 and 60°C but not so much that the water temperature drops below 45 and the mixer then gives you lukewarm water. Mine doesn't modulate, so perhaps a more modern one would maintain a more stable temperature.

On the other hand, if all the taps were themostatic mixers and the bath were out of the equation, it does save heating 27 gallons+ of cylinder to then end up using less than 20 litres a day, so is probably more efficient with gas. Heating a cylinder seems to be very inefficient: you heat the boiler to transfer the water to a cylinder via a coil. The boiler has to run hotter than the desired DHW temperature and you lose heat in the connecting pipework. Then the cylinder loses some stored heat as however well lagged they may claim to be, it's not enough really.

With 3 showers, unless you never plan on using them together or have some of them electric (which isn't as stupid as it sounds), the standard answer is probably NO.

My hot bath tap in the UK (old-fashioned vented cylinder and gravity flow) runs 20 litres per minute. You won't get that out of a combi.
 
Incoming water supply pressure and size are the most important thing 25 mm mdpe minimum and 3 bar pressure at 20 + litres a minute flow rate for a good size family home, a unvented cylinder and a separate boiler is really the best combis have there place but not really in your situation. Kop
 
That's great, thanks all for your replies.
Another question then. We will have a 5 bed house with 3 bathrooms. The biggest storage boiler is 32kw, (unless you go to ridiculous mansion sizes). Will this mean the boiler is running at maximum capacity for most of the time? Or, as someone has recommended, would a 40kw combi boiler working in conjunction with a hot water cylinder put less pressure on the system and be more efficient.
Thank you for your time.
 
32kw boiler will conceivably do a 32 bed house with one rad in each. It will be ample for your requirement and will work best being made to work hard
 

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