Combi/Unvented

[ir50]

Ok so I need a new boiler.

I'm not not sure if I will get away with a combi or if Ill need an unvented cylinder. given my property, I'm guessing most would jump in and say unvented straight away.

So you can get a rough idea, the property was once 2 x 3 bedroom semis but is now a 5 bed detached. So there are around 20 rads. Has 1 bathroom and 1 ensuite shower room. Shower is used more in the bathroom than the bath. Rarely are both showers used at same time. There a 5 people.

I was using a system boiler with a home made heatbank. This worked extremely well until the cork seals around the flanges all started to leak. I've had no success removing the flanges to replace seals.

So through half the winter until now, hot water has been supplied using just the plate heat exchanger taken from the heatbank. Takes a while before hot water reaches the taps but once it's there, its absolutely fine. So in effect I am using a combi setup just with the PHE outside the boiler casing.

My boiler is now on its last legs and I cant get parts so it has to go.

My question is: is there any disadvantage of using a combination boiler over a system boiler to feed an unvented cylinder? Reason being, combi maybe Ok for our needs. If not, I'm thinking I could add an unvented cylinder at a later date if necessary.

Any suggestions on boilers too? How about the Intergas ones?

Cheers

 

[Dan Robinson]

The main disadvantage of a combi boiler for you is the lack of a backup should it ever conk out.

 

[Terry lambert]

If as a potential customer you posed this question.

I would be working out the heat requirement for your property. I would also be measuring our flow and static pressure of your incoming main to see what kind of performance you could expect. Also the size of the incoming main.

I would recommend an unvented

The expansion vessel and pump in the boiler will not be sufficient and so you would need to fit additional of both. I would convert it to at least two heating zones if it isn't already.

You could convert the boiler to serve an unvented at a later date if you found your hot water performance inadequate, and then just run the hot from the combi to a single tap, ie kitchen.

I'm fitting my first Intergas next week and so have no direct experience of them yet, although some engineers on here that respect speak very highly of them. A 36/30 giving a good performance of 15 litres a min at a 40 degree temp rise. Not sure of they're range yet and you may need a bigger combi to satisfy your heating load

 

[Terry lambert]

Or Dans more succinct awnser

 

[Dan Robinson]

 

[ir50]

Dan, I take your point on board but an £800 cylinder just because it has back up doesn't win it. I have a back up electric shower and a kettle!!

Terry, calculations aside as the boiler output for the heating would need to be matched to the right boiler in either case. It's just the hot water that's the issue. I have a large external expansion vessel already so that's not a problem. Again I assume the pump is likely to be the same in a system boiler as it is in the combi. Original question was really is there any difference in heating an unvented with s combi over a system boiler? Then again should I maybe be looking at a heat only boiler instead?

 

[Terry lambert]

You can use a combi to heat an unvented the same as a system

I wouldnt pay £800 for a cylinder though.

Look at RM

 

[Agile]

If the supply is adequate then an unvented is best and I would be recommending 300 litre.

A heat only boiler is usually cheaper. Any combi could be used.

But with an Intergas the combi is only about £50 more so useful if its incorporated in a useful way.

Tony

 

[Dan Robinson]

Why 300 litres?

 

[Terry lambert]

The pump had to overcome the resistance in the boiler and some have higher resistances than others ( thanks Stealmason ) and so once the the pump has overcome the boiler there may not be enough for your system with a property of your size, or it could be slow to heat.

Hence a second pump

 

[Agile]

Why 300 litres?

50 li plus 50 li per person ( 5 ) = 300 li

That will give 60 li per person of preheated hot water which with some cold added will allow say 80 li each for immediate use.

What figure did your course teach? 300 li for two and a half ?

 

[Dan Robinson]

With one bathroom and one shower and a 20 minute reheat?

Will confess I missed the 5 people part of the OP... But i would be looking at a 210.

There is only so much water at 60° 2 showers can get through.

 

[ir50]

Hence a second pump[/quote]

When you say second pump, do you mean having this in addition to the one that would be part of the boiler? Didn't realise 2 could be run in line

 

[Dan Robinson]

Yep - that's the idea.

 

[ir50]

With one bathroom and one shower and a 20 minute reheat?

Will confess I missed the 5 people part of the OP... But i would be looking at a 210.

There is only so much water at 60° 2 showers can get through.

On any given day 2 people might want a shower straight after each other. The others will take theirs throughout the day. As long as the hot water lasts 10 mins for me, that's fine!