# Combi vs system boiler and tank

Discussion in 'Plumbing and Central Heating' started by frank999, 20 May 2020.

1. ### frank999

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I am trying to calculate the cost benefits of 'Combi' vs 'system boiler and tank'.

Looking at a System Boiler and Tank:

I am advised a 300 litre water tank (fed by an indirect system boiler) requires 2.4 kw of energy per 24 hours to keep hot.

Assuming 0.03p p/kwh cost for gas, the above equates to a cost of £0.072p per 24 hours, which again equates to £26.28 per annum to keep 300 litres of water hot.

Does that figure sound correct ? looks too good to be true.

I am also advised average hot water volumes for a 10 minute shower are 120 litres (12 litres per minute) x 0.70% (for the hot water portion) = 84 litres.
This assumes everyone takes a 10 minute shower, I take 5 minutes, but thats just me.

What over head of hot water should be kept in the tank, ie number of litres, 84 litres ... ie enough for one shower ?. working on 84 litres, the total capacity of the tank is 3.57 showers.

I am told a 24kw input recharges the 300L tank in 42 minutes, does that mean a single 10 minute shower of 84 litres could be replenished and be hot in 11 minutes (42 divided by 3.57).

Any thoughts appreciated.

Last edited: 20 May 2020
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3. ### DP

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Modern cylinder are rated at around 24kw and fast recovery ones can reach around 70% target in 15-18 minutes (cylinder vary for this)

If you already have combi or a sealed system boiler, Look into invented cylinder and combi supply hot water to kitchen sink and heating cylinder for baths. Shower off combi works as well

Installing cylinder is belt and braces job when boiler packs up.

If you can instal solar panels then unvented cylinder with additional coil equal hot water for washers.

4. ### flameport

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That is the heat loss per 24 hours, as in if it was hot already and left sitting there unused, that's how much heat it will lose in a day.
Some cylinders are better than others but that figure is fairly typical. It's a few pence a day in lost heat.

The myth about combination boilers being more efficient, only heating the water that's used and so on are just that - a total myth.
Most heat losses are from the pipework, which is why hot water pipes should be insulated.

For any given amount of hot water, the cost of heating it is basically the same whether it's stored in a cylinder or heated in a combi.

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5. ### bernardgreen

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But is better when fed from a cylinder of hot water as the hot water from a combi gets cooler when the flow increases.

6. ### frank999

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Thanks for your replies

Look into invented cylinder and combi supply hot water to kitchen sink and heating cylinder for baths

I've not heard of an 'Invented Cylinder' before ...

So the combi's CH flow feeds the Cylinder - the cylinder in turn feeds the hot water taps in the bathrooms and showers, and the CH radiator's.

The hot water for the kitchen taps is fed off the Combi then ? so you'd need a separate set of piping for the the kitchen taps, which is OK if your boiler is in or near to the kitchen.

In this configuration, which is more efficient, to have the CH flow from a cylinder or direct from the Combi. ?

7. ### DP

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Not if installation is correct

8. ### bernardgreen

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No matter how "correct" the installation is the flow rate of hot water from a combi is limited to that which the boiler can instantly heat to the required temperature for the shower.

The concept of the combi boiler was created for flats and small houses where there was no space for hot water cylinders. Then marketing people at the manufacturers and "quick job" plumbers started to sell them into houses where they were not the best option.

9. ### DP

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You said above. Once boiler is upto speed, delivery is stable for temperature.

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11. ### bernardgreen

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Hot water from a cylinder doesn't have to wait for the cylinder to get "upto speed"

That is only true if all related matters are constant,

any change in water pressure will alter the flow and hence the temperature of the hot water delivered,

increasing the flow rate by opening another hot water tap will alter the temperature of the delivered hot water,

and given that a boiler can only modulate down to a certain minimum burn rate a combi will have a minimum flow rate for delivery of hot water. Draw less than that minimum flow and the boiler will shut down on over heat. A small trickle of hot water can be drawn from a cylinder.

12. ### nwgs

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I’m going to convert my Intergas combi to an inverted now. 16L or how water just isn’t enough.

13. ### MeldrewsMate

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I don't belieeeve I've heard of a kw either, and 2.4 kw per 24 hours doesn't make sense....but allowances can be made.
I'll assume you meant kW (the Watt is named after James Watt, so carries the capital W).
A kW (sic) is a unit of power, the kWh is the conventional symbol for the kilowatt-hour, a unit of energy equal to 3600 Joules (guess 'what' or who the Joule was named after?).

Are you really considering a combi to compare with a 300 litre cylinder? A 300 litre cylinder is one of the larger domestic sizes, quoted as big enough to serve a 2 to 3 bathroom house. Perhaps if you chose a smaller cylinder (120 litre or so) your 7p per day cost could be shrunk to 3p or so, without loss of amenity, especially when the reheat time of a modern unvented cylinder is so low.
BTW don't get hung up on the size of the heating coil; size is good, but not everything (I think that's someone else's quote?). The heat transfer rate into the water is far greater when that 'hot' water is cold, and reduces dramatically as the water approaches 60 C or so (please note the Celcius does not take the 'degree' prefix of the Centigrade - despite what the BBC weather forecasters may say...plus I can'tfind that little o on my keyboard).

You've been given some great advice by others, but from the starting point of not being told that you already have a combi or a system boiler it's not so easy to give clear advice.

If you already have a combi then keep it, and you'll already have a cold mains supply to it. Install zone valves to separate the heating pipework from that feeding the (new?) cylinder. Retain just one hot tap to kitchen sink, basin, utility room, or whatever is closest to the boiler.

If you don't have a combi, then rejoice and fit/retain your heat-only boiler, fit a 120 litre cylinder, install some solar PV panels and an iBoost type diverter to give yourself truly free hot water for 8 months of the year.

MM

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14. ### DP

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Bernard, take it from me, a boiler that is set up correctly, once the tap is delivering rated water output, opening tap further will not increase flow from the tap. Running another tap WILL NOT increase throughput through the boiler. At minimum flow rate water will be hotter but will be under control of hot water thermostat. You need to sit down with boiler manual and understand how the boiler function. Your understanding of boiler operation needs more input. At minimum delivery, boiler will NOT overheat unless there is a defect or system is dirty

15. ### bernardgreen

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At minimum rated delivery the boiler will ( should ) not overheat, but what happens if the flow rate is less than the minimum flow rate that is necessary to remove the heat produced by minimum gas burn rate.

Is it not possible that a partially open tap could result in a flow rate adequate to activate the flow sensor but in-adequate to remove all the heat produced by the minimum burn rate.

or perhaps the flow sensor can measure the flow rate and only fire up the boiler if the flow rate is greater than the minimum rated flow rate.

16. ### frank999

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I don't belieeeve I've heard of a kw either

Excuse the capitalisation, kW is what I meant, but thank you for the pointer, press the 'Omega' button on the reply dialogues 'ribbon' - hover over it and the mouse will display 'Special Characters' ... '°' degrees is on the drop down menu..

No I don't have either a combi or cylinder system yet, if the standby cost of a cylinder is this inexpensive then it is worth having.

Does an unvented system need any special annual testing and certification.

I am limited with space, getting the boiler right next to the cylinder might be a bit tight, will a couple of meters distance make any difference to running costs, so I can site the boiler on a wall other side of a sink.

I have a cupboard 700x700mm that is 300mm deep at the base, but that widens out to 1100x1100 mt (x 1750mm height).

I have installed PV panel clips on the roof ready for Solar panels, presume this feeds a immersion on the tank ?

Is the suggestion of using a combi to feed some hot water pipes to sinks, and CH output to feed a cylinder a valid one ?

Last edited: 21 May 2020
17. ### DP

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No

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