This prompted by a comment by a builder coming to discuss our proposed extension. We said combi, he said that people are tending to go for Megflows now. So is there any good reason to?
Rough very shortened recap of parts of the Wiki.
Q1. Do you need more hot+cold water flow than the maximum cold mains flow? [e.g. two or more bathrooms and teenage daughters]
Yes - stored cold water in a tank, and pumped hot+cold to the baths and showers.
No -
Q2. Do you want more continuous hot water than can be heated by your maximum gas flow?
Yes - stored hot water, e.g. in Megaflow or see (1).
No -
Q3. *Well, here's the interesting part. If you got this far any of the systems in (1) and (2) can meet your needs but so can a combi.*
So what is Q3 which decides between a Megaflow and a Combi?
Factors:
Combi uses less space, as there is no stored hot water, but has to be sized for maximum immediate demand. Conflict betweeen hot water demand and central heating demand. Combis more complex (everything in one box) so probably lower MTBF. Alleged shorter lifetime. But they do free off the airing cupboard which is potentially a big plus.
Megaflow can be heated by a smaller boiler (possibly more efficiently?) and once the stored water is up to temperature there should be no conflict between CH and hot water. Pressurised systems have higher maintenance - need testing and checking every year. Can run at the maximum mains flow if required by future changing demand. Can be heated by backup power source e.g. immersion heater if there is a problem with the boiler. They do require more plumbing and take up significant storage space in smaller properties.
So, leaving aside the space issue, it comes down to cost.
Which is cheaper to install. Which is cheaper to run. Which is cheaper to maintain over a 10 year period.
Answers on a postcard......
Oh, and I have deliberately ignored heat stores to try and limit the options. Also ignored additional heat sources linked to renewables such as PV and back boilers on stoves. Also ignored mixed solutions like having the combi heat a hot water store for the showers but provide DHW to taps.
Cheers
Dave R