Megaflow Running Costs

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Hi All

Looking for some advice on a Megaflow system;
I've seen a lot of information around the additional cost of setting up a megaflow system vs a combi system - however once installation is complete is there a noticeable difference in the annual running cost of the 2 systems?
The piece I am trying to work out is if running a combi boiler is cheaper than a system boiler + Megaflow (including both electricity and gas costs...)

Thanks
 
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Assuming both boilers are similar efficiency one would think running costs would be similar. I suppose there are some heat losses from the Megaflop cylinder but they are well insulated so probably not noticeable.
 
Is this just a "theoretical" exercise, or do you have a reason for wanting to know the answer?

If so, what's the reason?
 
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For a 150L unvented cylinder, Viessmann says heat lose over 24 hours is 1.2kWh, ie 6C drop over one day, and Vaillant being 1.6kWh, 9C drop.

1kWh = 6p gas now.
 
Combi boilers have losses too - starting up every time you turn the tap on to wash your hands.

Big advantages of stored hot water are that you have a backup immersion if/when the boiler breaks down, and you aren't limited to the combi instantaneous hot water output.
 
There is virtually no difference in the running costs.

The unvented gives a higher flow rate and that would encourage many people to use more hot water!

Tony
 
Thanks all
@D_Hailsham - it's kind of a theoretical question I guess
Standard 2 adult + 2 (young) kids family - doing some work on the house at the moment and I'm leaning towards installing a Megaflow as a future proof solution for when my daughter is older! Also I'm a big fan of when I need something (i.e. hot water) it's available and I don't need to worry about what someone else in the house is doing.
Obviously the installation itself is more expensive but after this I was curious to see if I would be signed up for much higher utility bills given it's not only the boiler now but also the Megaflow that needs to be powered...
 
it's kind of a theoretical question I guess. ... I'm leaning towards installing a Megaflow as a future proof solution for when my daughter is older!
A combi boiler and an unvented cylinder both depend on the incoming cold water flow rate and pressure to work. The only difference is that the combi heats the water instantly while the invented cylinder does not. So if you have a combi at the moment and experience problems running two outlets at the same time, you will probably have the same problem with an unvented cylinder. You need to measure the incoming cold water flow rate. A minimum of 20 litres/min is necessary for an unvented cylinder - 25 is better.

I was curious to see if I would be signed up for much higher utility bills given it's not only the boiler now but also the Megaflow that needs to be powered...
I'm not quite sure what you mean when you say the megaflow needs to be powered. The water in the cylinder is heated by the boiler; and the only time electricity is used is if the boiler goes wrong and you use the immersion heater.
 
The piece I am trying to work out is if running a combi boiler is cheaper than a system boiler + Megaflow (including both electricity and gas costs...)
Any difference is insignificant and not worth measuring. All systems have losses, and heating a certain amount of water instantly or heating it and storing it changes nothing.
Over a year, the majority of gas used will be for heating your house, hot water is a small percentage of the total.
Electricity is also irrelevant, the amount used for the pump and other controls is tiny.
 
So if you have a combi at the moment and experience problems running two outlets at the same time, you will probably have the same problem with an unvented cylinder.
You don't think that might be down to the boiler having to limit the flow to that which it is rated for? ;)
Yes, there is that possibility. But you won't know unless you measure incoming flow/pressure.
 
I disagree, I think there are differences in cost but what of it? Nobody ever factors in satisfaction because you can't put a price on it.

10 litres/min compared to 25, wake up. More flow rate and a 5 minute, 10 minute or 2 minute shower will use more water because the unvented will outperform a combi by a large factor yet be less effected by other taps opened at the same time. You will be just as clean with a combi but can't run large or multiple spray showers with one as well or even at all.

With a combi you have one service charge each year, there is a second for an unvented cylinder (or should be). If you're in a hard water area you will also have higher service charges.

The guy that mentioned the size of the water heating bill to the space heating is quite right, water is small in comparison. Say 2KW standing losses per day so 12px365 = £44/year less whatever the combi losses are.

Lag the how water pipes in the cylinder cupboard, I was astounded to learn what those losses are at the BRE.

Unvented is more expensive to install, to run and to service if done properly IMHO but that's what I'd install if I had to because of performance and backup with immersion heater. With a combi an electric shower is useful. The only thing better is a tank fed system with sufficient head and generous pipe sizes but that is unfashionable and to be modern is everything nowadays.
 

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