The only advantage of a combi is saving space

[mike_s]

If I moved into a house with a good stored hot water system I'd never rip it out for a combi unless I was desperate for space.

Combi's cost more to run, there's more to go wrong and generally you can only have one hot tap on at a time.

In a couple of weeks I'm doing a 300 litre unvented with 40kw heat only boiler in a large house, external expansion vessel on the heating.

It took them months to agree to the job as the customers dad had told them their Worcester highflow was great.

So I calculated that if you use a 40kw highflow for 2 hours a day that will cost around £1000 per year on hot water alone. The unvented cylinder would cost around £80 per year on the hot water if you have 2 baths or showers a day plus an hour washing up or about £125 if you have 3 baths/showers per day.

That sold the job as the extra grand in installation costs of the cylinder would be payed for in a year.

Even in small houses, combi's are often inefficient and inconvenient. All you need is a cylinder that is big enough so you arn't running out of hot water all the time.

Combi's sound like a good idea to many but I point out why they arn't. I've got 4 boilers to install in July, only one is a combi and that's a swap.

Builders are another culprit of the combi culture as they hardly build airing cupboards any more.

Another customer from America chose me for their job as I was the only one who didn't reccomend a combi, she had never seen a combi before coming to Europe and thought they sounded inefficient. They had a large house with a whole room just for the cylinder, perfect.

A customer today with an Ideal Mexico 2 said to me today "British Gas said I want a combi"
I said "Why? Do you run out of hot water all the time? Are your tanks not high enough, do you need the space?
The answer was "No, shower is great it works off the cylinder with good pressure as the tank is high, never ran out of hot water, my wife loves the airing cupboard, I just thought everyone gets combi's these days"

It winds me up, especially British Gas ripping out perfectly good S and Y plans for Worecester 24i Junior's in 4 bed houses.

Customers needs to be educated on what is best for them.

 

[johnnyplumb]

i agree with everything your saying, but it's usually easier to fit a combi, so that's why we all like them.
then there's the scrap from the old cylinder to think about,
i just love bg when they do all the spade work for you, they convince the customer that a combi is best, quote them £4000+, i go in and price at around £2000, get the job without even trying. [ well i'm not gonna talk them out of it].

i just love bg and combi's. lol

 

[Mike Jackson]

I'm not sure of the costings but 2 hours a day is a lot of hot water. I'd be surprised if mine was used for much more than half an hour on most days (2 ten minute showers and the odd bit of washing up). Occasionally it will see more use when it's time to wash the dogs or SWMBO has a soak in the bath.

Combis are great in smaller properties with low occupancy rates but as soon as you start talking larger properties with multiple bathroom unvented is definitely better. Mind you if the customer wants a combi they get a combi.

 

[SimonJohns]

How did you get to your costings.

I'm sure that no matter how you heat the water if the water needs X amount of kw to give a 1c rise then this applies to both a combi or any other system

I can see savings by having a lower flow temp and keep the boiler condensing.

I cannot see an £800 difference in cost

 

[diamondgas]

How did you get to your costings.

I'm sure that no matter how you heat the water if the water needs X amount of kw to give a 1c rise then this applies to both a combi or any other system

I can see savings by having a lower flow temp and keep the boiler condensing.

I cannot see an £800 difference in cost

I don't know the maths but I've often wondered how efficient the p-hex is on a combi? I'm a lover of stored water & am putting an unvented in my own home. I believe the combi market took off when cost became an issue and the market for living flame inset gas fires. Folk wanted shot of having to get on their knees to light pilots on fireside back boilers and the great big piece of furniture stuck on their livingroom wall!!!

Along came the combi ... With the addition in a lot of instances of giving the customer extra storage space/bathroom space with the removal of the cylinder! A good few new builds now seem to be steering back to stored hot water!

 

[Millsy 82]

Dont mean to sound funny but I would love to know how you have worked out the costing of it.

I have a vaillant with about 11.5 l/min dhw and a normal sized bath it takes around 7-8 mins to fill the bath 30 secs for a basin full of hot water and a minute for a sink.

With a highflow the flow rate is greater so all of these times will be less 5-6 minutes for a bath etc.

So if we say with a highflow per occupant of the house would be 1 bath 2 basins of water and 2 sinks full of water so lets say 8 mins of water a day add on an extra 5 for washing of hands so we would have 13mins of water per day. Add on that it takes 20 mins for the store to get up to full temp from cold and 5 mins through out the day to keep it up to temperature.

So it would need 7 people living in that house to keep that boiler fired for 2 hours and that would be if everybody had a bath every day. A shower would mean that the boiler would not be constantly on High fire as the flow rate will be less.

 

[Millsy 82]

I think combi's have there place 2 bed houses and flats and some 3 bed with only 1 bathroom then they are great. Anything over that then stored water all the way plus fitting a heat only boiler is a doddle and once everything is drained so is a cylinder.

 

[Pssst]

You are pushing at an open door here mate... I am fully in favour of high efficiency storage systems..

 

[stevetheplumber]

thyeres no way a cylinder is going to be more economical than a combi every time you heat a cylinder your never going to use all youve paid to heat and the best insulation in the world aint going to stop heat loss over time as in over night or between on periods
you dont show which aerea your working in but it sounds as if it might not be inner city stuff where space is at a premium and an airing cupboard is a luxury people want hot water 24 7 nowadays the days of waiting for the water to heat up to have a bath are gone so that cylinder needs to be on 24/7 to give the same service as a combi making it even less efficent
thats compared to a mains fed cylinder if you compare a combi to a gravity fed one the advantages are even bigger, dry loft, less system controls, mains preasure
most of the blocks of flats springing up in london now have district heating system with heat exchanger units in each flat

 

[GrahamM]

Agree with steve.
It is all down to the HW demand for the particular household. (not house)
A retired couple who plan to see out their retirement in their 4 bedroom house will save money with a combi.

Not everyone baths/showers every single day, but realistically you will be heating the cylinder every single day wither being used or not.
Hoe can this be cheaper than a combi?

 

[mike_s]

To answer stevetheplumber I am not comparing combi's to gravity systems, my point is that even if you have a gravity system and don't want the water in the loft, an unvented downstairs would be better providing you have the space and incoming water pressure of course.

Also, I don't really agree with waiting for the hot water cylinder to heat up as a disadvantage as I think that just means your stored hot water system isn't big enough for your requirements.

I have customers with a 300 litre unvented cylinder running 2 drench head showers, a jacuzzi bath, 5 sinks and with 6 living in the house they have never ran out of hot water.

The calculations I did I had my brother help me with who works for a renewables company so he does these calculations every day and are the approved methods. I will aim to get back with the exact calculations but even if they are not totally accurate which they can't be exact but I still feel sure that a stored hot water system is far more efficient.

You can heat a 300 litre cylinder up with a 3kw coil in 25 mins and it can stay hot for 2 days. A 40kw combi is running at 40 kw per hour without much modulation during a power shower or filling a bath.

 

[SimonJohns]

To answer stevetheplumber I am not comparing combi's to gravity systems, my point is that even if you have a gravity system and don't want the water in the loft, an unvented downstairs would be better providing you have the space and incoming water pressure of course.

Also, I don't really agree with waiting for the hot water cylinder to heat up as a disadvantage as I think that just means your stored hot water system isn't big enough for your requirements.

I have customers with a 300 litre unvented cylinder running 2 drench head showers, a jacuzzi bath, 5 sinks and with 6 living in the house they have never ran out of hot water.

The calculations I did I had my brother help me with who works for a renewables company so he does these calculations every day and are the approved methods. I will aim to get back with the exact calculations but even if they are not totally accurate which they can't be exact but I still feel sure that a stored hot water system is far more efficient.

You can heat a 300 litre cylinder up with a 3kw coil in 25 mins and it can stay hot for 2 days. A 40kw combi is running at 40 kw per hour without much modulation during a power shower or filling a bath.

There is no way you can heat a 300l cylinder in 25 mins using a 3kw immersion

Your Mathis is wrong. You need a 30kw to heat that much water in 25 mins

 

[stevetheplumber]

To answer stevetheplumber I am not comparing combi's to gravity systems, my point is that even if you have a gravity system and don't want the water in the loft, an unvented downstairs would be better providing you have the space and incoming water pressure of course.

Also, I don't really agree with waiting for the hot water cylinder to heat up as a disadvantage as I think that just means your stored hot water system isn't big enough for your requirements.

I have customers with a 300 litre unvented cylinder running 2 drench head showers, a jacuzzi bath, 5 sinks and with 6 living in the house they have never ran out of hot water.

The calculations I did I had my brother help me with who works for a renewables company so he does these calculations every day and are the approved methods. I will aim to get back with the exact calculations but even if they are not totally accurate which they can't be exact but I still feel sure that a stored hot water system is far more efficient.

You can heat a 300 litre cylinder up with a 3kw coil in 25 mins and it can stay hot for 2 days. A 40kw combi is running at 40 kw per hour without much modulation during a power shower or filling a bath.

thats rubbish m8 a 3kw imersion takes that long to heat a small cylinder a litre of water takes the same kws to heat to the same temp regardless of the power source combi or cylinder

 

[galaxy_plumbing]

for those more interested in the issue you might find this link helpful.

Energy calculator for water flow heating including costs

 

[Millsy 82]

Agree with steve.
It is all down to the HW demand for the particular household. (not house)
A retired couple who plan to see out their retirement in their 4 bedroom house will save money with a combi.

Not everyone baths/showers every single day, but realistically you will be heating the cylinder every single day wither being used or not.
Hoe can this be cheaper than a combi?

If you have an elderley couple in a 4 bed house it is difficult as you know the house will need a cylinder but to suit them a combi would be better so I always recommend on the property and what it would possibly need in the future

 

[mike_s]

OK I'm willing to admit my maths were probably wrong on the running costs but I'm confused how the manufacturers can state a 3kw coil on the cylinder and re-heat times of 25 mins. That is where I got it from. I will read that link posted. I still don't like combi's though and I will look into the calculations until I'm 100% sure of the exact way to calculate it.

 

[julesverne]

i reckon combis are ideal for a 3 bed house 1 bathroom but onwards and upwards of this you do need an s or y plan fitting with suitable sized unvented cylinder or open vented if you have the height for the tanks.