Unvented System vs Combi Boiler

Andrei_A · Oct 14, 2019

Hi folks, new member to the forum here.

Me and my wife are in a bit of a dilemma with our hot water/heating system we currently have. We are planning to do a kitchen extension at the back of the property (currently the kitchen is at the front, with the boiler in a cupboard), + add one more shower in the downstairs toilet. We have the unvented cylinder in the airing cupboard upstairs + 2 other tanks in the loft. We are 3 people currently living in the 2 bed house + another one on the way soon, so we will be 4 in the near future.
My question would be - what's best? our current system, or a combi boiler? We really want the space in the airing cupboard to free up - is there a possibility to move the cylinder in the loft next to the other 2 tanks? We would have the space there. Also, we have a small cupboard outside of the property, at the front (i guess it's not insulated) - would we be ok to put the new boiler there (either combi or the one we already have). Should we consider some insulation so that it's not too cold? Would we be allowed to make such a modification? We have requested a quote from a company who said we will be fine to move the boiler in the front of the house cupboard, but it can get really cold in London in Jan-March.

Cheers in advance!

Deleted member 32524 · Oct 14, 2019

Personally I would say the better system is the unvented. Combi is very limiting in terms of hot water performance. Poor performance when more than one outlet open, even on larger ones. Big plus you never run out of water and you don't heat a big cylinder every day and keep it warm.

Frank Nicklin · Oct 14, 2019

I have just done exactly the same upgrade. We dumped our copper cylinder in the bathroom and our boiler in favour of a combi boiler. I wouldn't recommend putting the cylinder in the loft. Moving the boiler will involve some serious pipework changes. Mine was an in-situ replacement with the existing pipework upgraded to suit the combi boiler. The pipework in the bathroom was re-worked after removing the cylinder and loop back to the boiler. A new shower was also installed on the circuit.

In the pictures attached, the copper cylinder was behind the blue doors in the bathroom. The second picture is as it looks now after the airing cupboard was removed.
[automerge]1571065234[/automerge]

DuncanM said:
Personally I would say the better system is the unvented. Combi is very limiting in terms of hot water performance. Poor performance when more than one outlet open, even on larger ones. Big plus you never run out of water and you don't heat a big cylinder every day and keep it warm.

Hi Duncan, isn't your comment a bit confusing. You say Unvented is best but then go on to say that you don't have to heat a cylinder of hot water every day to keep it warm. I upped the size of my combi boiler that was originally quoted to help combat the flow issues. Shower and hot water taps, flow well with both open. Yes there is a drop, but nothing worth worrying about. The space gained in the bathroom was amazing. We had a separate toilet and bathroom that I knocked in to one after removing the airing cupboard. Wish I'd done it years ago.

Andrei_A · Oct 14, 2019

DuncanM said:
Personally I would say the better system is the unvented. Combi is very limiting in terms of hot water performance. Poor performance when more than one outlet open, even on larger ones. Big plus you never run out of water and you don't heat a big cylinder every day and keep it warm.

Thanks for the reply Duncan. Would we be ok to move the cylinder in the loft to free up some space? Will that cause any problems on the system in terms of performance?

Deleted member 32524 · Oct 14, 2019

I am saying that the advantage of a combi is you don't run out of hot water, no matter how long the tap is on or you are in shower. Also don't have to heat a big cylinder every day and keep it warm as it looses heat. Water performance with multiple outlets operational does not work with a combi. Unvented you can have two showers on and both work.

centralheatking · Oct 14, 2019

Frank Nicklin said:
I have just done exactly the same upgrade. We dumped our copper cylinder in the bathroom and our boiler in favour of a combi boiler. I wouldn't recommend putting the cylinder in the loft. Moving the boiler will involve some serious pipework changes. Mine was an in-situ replacement with the existing pipework upgraded to suit the combi boiler. The pipework in the bathroom was re-worked after removing the cylinder and loop back to the boiler. A new shower was also installed on the circuit.

In the pictures attached, the copper cylinder was behind the blue doors in the bathroom. The second picture is as it looks now after the airing cupboard was removed.
[automerge]1571065234[/automerge]

Hi Duncan, isn't your comment a bit confusing. You say Unvented is best but then go on to say that you don't have to heat a cylinder of hot water every day to keep it warm. I upped the size of my combi boiler that was originally quoted to help combat the flow issues. Shower and hot water taps, flow well with both open. Yes there is a drop, but nothing worth worrying about. The space gained in the bathroom was amazing. We had a separate toilet and bathroom that I knocked in to one after removing the airing cupboard. Wish I'd done it years ago.

No combi for DuncanM in my opinion
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Look the way this works is
1. Combis were only ever intended to be installed in flats where space was limited and for
conventional heating where do you site the tanks...so they came from the continent in the late 70's
2. They are very simple to fit and save space they became popular especially amongst heating engineers whom could offer a cheaper heating system than their conventional colleagues.
3. There are still heating outfits that will fit a big combi anywhere any gaff ..big means
lots of kw just to get say 11 litres of hot water per min...If the water main will supply it at peak times when you whole neighbourhood all with combis want their hot water...result
poor performance ...unhappy people street by street
4. Answer if your gaff is more than two bedrooms more than kitchen and one bathroom
dont waste your money you will regret it
5. unvented is one way with a system set up , others on PF will have their input
That is mine....ask any questions thats what PF is for ....centralheatking

Andrei_A · Oct 14, 2019

centralheatking said:
No combi for DuncanM in my opinion
[automerge]1571067651[/automerge]
Look the way this works is
1. Combis were only ever intended to be installed in flats where space was limited and for
conventional heating where do you site the tanks...so they came from the continent in the late 70's
2. They are very simple to fit and save space they became popular especially amongst heating engineers whom could offer a cheaper heating system than their conventional colleagues.
3. There are still heating outfits that will fit a big combi anywhere any gaff ..big means
lots of kw just to get say 11 litres of hot water per min...If the water main will supply it at peak times when you whole neighbourhood all with combis want their hot water...result
poor performance ...unhappy people street by street
4. Answer if your gaff is more than two bedrooms more than kitchen and one bathroom
dont waste your money you will regret it
5. unvented is one way with a system set up , others on PF will have their input
That is mine....ask any questions thats what PF is for ..centralheatking

Well we will have 3 bedrooms after we make the extension at the back, + we want to add another shower downstairs so will be 3 bed house + 2 bathrooms (both with showers) & the kitchen. All for 4 people

Cheers for the detailed reply!

quality · Oct 14, 2019

unvented every time with your demands

EvilDrPorkChop · Oct 14, 2019

Andrei_A said:
Hi folks, new member to the forum here.

Me and my wife are in a bit of a dilemma with our hot water/heating system we currently have. We are planning to do a kitchen extension at the back of the property (currently the kitchen is at the front, with the boiler in a cupboard), + add one more shower in the downstairs toilet. We have the unvented cylinder in the airing cupboard upstairs + 2 other tanks in the loft. We are 3 people currently living in the 2 bed house + another one on the way soon, so we will be 4 in the near future.
My question would be - what's best? our current system, or a combi boiler? We really want the space in the airing cupboard to free up - is there a possibility to move the cylinder in the loft next to the other 2 tanks? We would have the space there. Also, we have a small cupboard outside of the property, at the front (i guess it's not insulated) - would we be ok to put the new boiler there (either combi or the one we already have). Should we consider some insulation so that it's not too cold? Would we be allowed to make such a modification? We have requested a quote from a company who said we will be fine to move the boiler in the front of the house cupboard, but it can get really cold in London in Jan-March.

Cheers in advance!

Are you sure you've got an Unvented system currently? As if you've got two tanks in the loft currently it would suggest you've got a Vented system. One tank for cold water storage (big tank) and one tank for F&E of the Central heating system (Small tnak).

If so you've got a vented system and not Unvented, which will change suggestions.

Stigster · Oct 14, 2019

I always tell people that if they have the room for an unvented cylinder, fit an unvented cylinder. Combis are great for smaller properties with few people and few hot outlets, or properties which lack space. If that isn't you, I would recommend an unvented cylinder because the performance will always be better when compared to a combi.

The limitations of combi boilers apply more or less equally whether oil or gas and I never much liked them as they have the aforementioned performance issues and there's more to go wrong and can be more awkward to work on when problems arise. Same goes for system boilers although I digress...

What is your incoming mains pressure and flow rate as those results can help steer the answer to your question one way or the other? It is fairly common to install cylinders in loft spaces so usually no great issues there.

gmartine · Oct 14, 2019

Andrei...as pork chop says you don't have an unvented system and if space is paramount and water pressure and flow rate allow a storage combi maybe a reasonable solution for you. They are more expensive and more hot water capable than a regular combi but not as expensive as a fully unvented cylinder based system.

SimonG · Oct 14, 2019

gmartine said:
Andrei...as pork chop says you don't have an unvented system and if space is paramount and water pressure and flow rate allow a storage combi maybe a reasonable solution for you. They are more expensive and more hot water capable than a regular combi but not as expensive as a fully unvented cylinder based system.

Glad I wasnt the only one who read it like that.

OP any chance of some pics of the tank in the airing cupboard to confirm.

Brambles · Oct 14, 2019

Andrei,

I have just undertaken a similar installation in North London (Acton). Boiler in an eternally accessed compartment with an unvented cylinder (210 litres) in the attic mounted on strengthened joists. All works well.

One thing that you should check with any proposed installer is the the water flow rate you have coming into the property. Large parts of London ( well North anyway) are on minimum flow rates which is a major problem with combi boilers and is becoming an issue with unvented systems.

Always best to ask the installer to guarentee the performance of the system they are quoting you

Andrei_A · Oct 15, 2019

Brambles said:
Andrei,

I have just undertaken a similar installation in North London (Acton). Boiler in an eternally accessed compartment with an unvented cylinder (210 litres) in the attic mounted on strengthened joists. All works well.

One thing that you should check with any proposed installer is the the water flow rate you have coming into the property. Large parts of London ( well North anyway) are on minimum flow rates which is a major problem with combi boilers and is becoming an issue with unvented systems.

Always best to ask the installer to guarentee the performance of the system they are quoting you

All - here are the pictures of the cylinder only - my wife can't get in the loft by herself lol. It does say unvented on it .. so i went with that.

Regarding the water flow .. i did discussed with Thames Water as British Gas have refused to undertake any works due to low water pressure. The flow at the meter outside the property is 25L/min; at the kitchen tap we had 7L/min. After some complaining and back and forth with Thames Water i have now close to 14L/min (measured it myself with the cup). Apparently the pressure reducing valve (seen at the back of the cylinder, left hand side) was not performing as it should and was obstructing the flow. Substantial improvement to be honest and i feel it when showering, and also if i turn any sink tap all the way open water jumps everywhere

So i assume 14L/min is a pretty good flow for both systems? or at least good enough to take the water all the way to the loft to the proposed new location of the cylinder?

With this Cylinder - is the expansion done inside it? As i can't see another smaller cylinder anywhere.

In the loft i do have 2 tanks, one small one big (not very big would say half meter square) - will take a pic tonight when i get home.

Cheers all!

gmartine · Oct 15, 2019

Well, that certainly is an unvented cylinder...question is what the eck are the two tanks still doing in the loft unless they've been disconnected and left there. Take a picture of the boiler too.

Andrei_A · Oct 15, 2019

gmartine said:
Well, that certainly is an unvented cylinder...question is what the eck are the two tanks still doing in the loft unless they've been disconnected and left there. Take a picture of the boiler too.

Pic of the boiler attached - Potterton Promax HE

Don't think the 2 tanks in the loft have been disconnected as the house was built in 2005 and previous owner hasn't changed anything as far as i know. Still runs on the system installed by the developers.

Will get pics of the loft installation tonight.

Cheers

Unvented System vs Combi Boiler

Andrei_A

Deleted member 32524

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Frank Nicklin

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