Combi or unvented, which is cheaper to run

[Jones82]

Interesting thought

I currently have a 28kw combi, 1 bathroom, 2 up 2 down semi.

I'm g3 and gas safe. I fit unvented and replacement boilers all the time.

I'm thinking about replacing my combi with a small 120 litre unvented cylinder in the loft.

An unvented cylinder usually draws about 3-5kw per hour, and heats up in about 30-45mins

I reckon the new cylinder will lower my gas bills?

Thoughts....

 

[ShaunCorbs]

Combi will be cheaper to run

 

[king of pipes]

Combi will be cheaper to run as Shaun said . Kop

 

[townfanjon]

I recently bought another house for Mrs TFJ and I , Only 1 bathroom so I have ditched the unvented idea and gone combi , time will tell if its been a good move , but pretty sure it is .

 

[Jones82]

I know conventional knowledge says combi as it only heats the water you use but think about it.

Shower 10 litres a min

5 min shower
2.3 kw on a 28 kw combi

Or 50 litres of a 120 litre cylinder maybe 1.5 kw?

The big plus on modern cylinders is the insulation, the tank will stay hot and it’s a sealed system so no open vent heat loss.

I reckon you could have the hot water on for 45mins twice a day and easy have 50 litres. The second 45 mins prob would only be on for a third of that

There’s also the wear and tear on the combi, parts etc and the plus of having a better flow rate to the shower.

28kw an hour over 3kw an hour, for 100 or even 200 litres of water.....

I’m starting to doubt combis being the most efficient option

 

[Chalked]

Obviously combi, but not the same flow rate.

 

[Jones82]

Another way of putting it using my rough maths

28kw 10 litres per min

In one hour and 28kw of gas I would have 600 litres

In one hour and 3-5 kw of gas I could have 120 litres or even more with a bigger cylinder

 

[hammers4spanner]

Unvented less water wasteage and over time you would be on second combi by time your cylinder goes

 

[gmartine]

I'd say it really depends on your pattern of use, if the combi is firing up say twenty times a day for a minute or two that's less efficient than heating up your cylinder for about the same amount of time.

 

[Rob Foster]

I know conventional knowledge says combi as it only heats the water you use but think about it.

Shower 10 litres a min

5 min shower
2.3 kw on a 28 kw combi

Or 50 litres of a 120 litre cylinder maybe 1.5 kw?

The big plus on modern cylinders is the insulation, the tank will stay hot and it’s a sealed system so no open vent heat loss.

I reckon you could have the hot water on for 45mins twice a day and easy have 50 litres. The second 45 mins prob would only be on for a third of that

There’s also the wear and tear on the combi, parts etc and the plus of having a better flow rate to the shower.

28kw an hour over 3kw an hour, for 100 or even 200 litres of water...

I’m starting to doubt combis being the most efficient option

Combi was only ever intended for small semis and flats and came from France in the early 80's. Then loads of mickey mouse outfits started wacking them into big places cause its basically a system in a box.
So the manus responded with silly sized output units which are only as good as the mains water supply. Its horses for courses in my opinion
centralheatking

 

[EvilDrPorkChop]

Unvented I reckon is more efficient in most situations, but you'd need a cylinder bigger than 120l. We have 170L (That I inherited with the house) and with our two power showers off it we run out pretty quickly. I usually fit no smaller than 200ls nows for familys.

But to put it simply, our showers are 17l/m and in summer our gas bill is only £25 actual usage (not estimated) and that's with hob usage for cooking.

Problem is with combis is theres so much wastage and wear & tear from people actually just running the tap for 10 seconds. They'll run the tap and switch it back off. Diverter goes through it's cycle, burner fires full load, it's just a waste.

 

[64degrees]

I’d go with the unvented if you got space. A modern well insulated cylinder is a thermal store, much more efficient - with the additional benefits high flow rates and immersion backup if the boiler goes down.
With a cylinder the boiler will be in condensing mode more often, the plume from a combi flue when there’s hot water demand says it all.

 

[Matthew Score]

If going with a cylinder make sure it has a high rated coil and wire it up for priority hot water. Means you can have a smaller cylinder.

Or go the Intergas way, combi boiler fitted with a 50l cylinder which mixes the cold feed with hot water meaning you can get 20lpm from a combi. Less parts to go wrong on a intergas as there is no plate hex, divertor or aav.