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What boiler should I use for my house: Sytem Boiler or Combi Boiler?

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Hello experts,


We have recently bought a house and are working with a builder on a large scale renovation of the property. We've been discussing boilers and what options are best. I would personally like to opt for a combi boiler and don't want a water tank. 

Here's what what I think matters.


House size:

  • Downstairs: 110 sqm (hallway + large open living room / kitchen + utility + WC). All ceilings are 2.6m high.
  • First floor: 65 sqm (3 bedrooms + bathroom + hallway). All ceilings are 2.6m hig.
  • Loft: 35 sqm (1 bedroom  + bathroom). Ceiling is sloped one one side, but 2.2m high at it's highest.


Heating consists of 10 radiators:

  • 3 in open living room / kitchen (as extra to boost in case UFH isn't sufficient -- actually not sure if this is needed, would love input)
  • 4 upstairs (one in each bedroom + 1 in bathroom)
  • 3 upstairs (1 in bathroom + 2 in bedroom)


Full UFH coverage downstairs (110 sqm surface area / 2.6m high).

My rational is that with 2 bathrooms and 3 people living in the property it'll be very unlikely that we will have issues with our DHW needs. I don't know a lot about boilers, but I did some research and found that combi boilers go to ~43kW. As I was researching, I thought that maybe the Vaillant ecoTEC Plus 940 40kW Combi Boiler was a good choice.

When I spoke to the builder about this he said that a combi boiler would not be powerful enough, but that he'd check. Later when came back with his quote which he said he discussed with his boiler specialist, he suggested an "Valliant EcoTEC Plus 635" system boiler and a 250-300l Megaflo watertank. That boiler is only 35kW though. When I asked him why I can't use the combi boiler I suggested earlier he was getting a bit defensive, after which he said he'd have to go and discuss this with the boiler specialist again. 

I wanted to get some thoughts here from all you experts on here. Also, I appreciate all recommendations regarding specific boiler models if people feel there is a particular one they would recommend.


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Let's start from scratch

First you need to understand your heat loss this will determine boiler size. Chucking the biggest boiler you can find is foil hardy will lead to rubbish performance due to likely short cycling.


An unvented cylinder will give way better performance from DHW than a Combi. You can make them perform well, but most plumbers have zero idea.


Boiler control

Say a big fan NO to S or Y plan, you need priority hot water on demand and weather compensation for the heating. Ideally you will have an electronic mixer on the UFH so that is also controlled by the boiler.


Cylinder use a heat pump cylinder with a 3m2 coil.


Size the radiators for dT 25


The above will give a very efficient system and lowest gas usage. 

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Thanks @JohnMo for sharing your insights, tremendously helpful. 

Like I mentioned I know very little about boilers, so you are totally right I should not just have made assumptions that bigger is better. What I was wondering though was: "if a 35kW system boiler is enough to take care of the CH (radiators + UFH), does there exist a combi boiler that gets the job done as well?" -- I just used the "there exists 40kW+ combi boilers" as an example. It sounds that "yes, if a 35kW system boiler is good then there will exist a combi boiler that gets the job done as well", or did I misunderstand that?

Once again apologies for my ignorance, but everywhere I've lived for the past 15 years we've had a combi boiler and never really had any issues with DHW supply even in larger properties with multiple bathrooms. So I'm simultaneously trying to better understanding the pros / cons of both and energy efficiency is definitely a good one.

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Seriously, with a full renovation, I would be planning ahead for WHEN you won't be able to get a gas boiler.  Yes you can get one now, but how long will it last?  and what will you replace it with.


At that point starting with a broken, irreplaceable gas combi will be a bad starting point.


So I like the suggestion of a good modulating system boiler, with proper thought out controls (priority DHW) and a "heat pump" unvented hot water cylinder.  Then next time, you (or a future owner) can swap the gas boiler for an ASHP.


I have never owned a house with a combi but used plenty when staying with relatives and I just don't like them.  Every one I have used you turn the hot tap down to get a nice gentle flow to wash your hands and the water jumps up to scalding hot.  Taking a shower and someone else flushes the loo and your shower temperature changes.  Once you have tried hot water from a well designed Unvented cylinder system, you will not want to go back.  As long as you have good mains pressure water to support it.

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30 minutes ago, ZenMad said:

a 35kW system boiler is enough to take care of the CH (radiators + UFH),

No problem at all, most likely a small boiler would do it. But do the heat loss calculation, if you don't know where to start post another question on a new thread, post as much information as you can, house plan, wall, roof and floor make up/insulation levels etc.  Also any idea on airtightness etc.  Also how do you ventilate?

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Never I repeat ever allow a builder or a builders plumber anywhere near your heating system install.


The house needs a system boiler on PDHW and a proper mixing control for the UFH circuit


Or design the radiators and UFH to all run at the same flow temperature which would be the ideal.


I have the same size house as you with a 630 ecotec plus and thats more than doubly oversized. I range rated it down to 6kw and it still heats the house perfectly.


If you get a Viessmann 200W boiler this will mitigate the need for an accurate heat loss calculation since it has such a good modulation ratio.

Edited by Lofty718
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If you're a doing a large scale renovation then I presume you're upgrading the fabric of the building so that it becomes more energy efficient?


I also presume that you're replacing the existing radiators?


Your builder and their heating expert should have told you that your boiler needs to be specified on the basis of a room by room heat loss calculation and if it is going to be a combi, which should really be your last option, it is sized based upon DHW demand and required flow rates.


Then they should have told you that the new system (i.e. new radiators) should now be designed for a maximum flow temperature of 55C


Your DHW cylinder should also be sized according to usage and the figures you've given so far indicate that a cylinder of about 180l is all you would need, maybe 210 max.


These are now part of of new Building Regulations and an industry requirement.


For me all the warning bells are ringing. Try phoning around for another heating engineer and with the first question, ask them how they go about selecting the right boiler and radiators.

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