Combi vs Megaflow

[David WE Roberts]

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

[John Rumm]

Yup, there can be...

Basically two things... flow rate, and "others". Where "others" are the specific advantages/features that you list later like having immersion backup etc.

Flow rate is the main driver for most I would guess. For either you need a decent cold water supply obviously. Ignoring the very exotic and elaborate combi solutions, its usually easier to get a decent flow rate of "hotter than required, and hence can be mixed water" from a store of water than from a combi, which will deliver its maximum rate at pretty much final use temperature.

Indeed, and there is the problem for many. Even using the full capacity of your gas main may not meet your immediate demand.

Yup, although it can be a problem with a store if its not large enough to meet immediate needs without re-heating.

Compared with a system boiler, then not really. For that matter, compared with a complete system of valves etc, you need the same basic components somewhere - so the only difference is the location of the bit that goes wrong!

Not sure there is much in that. Buy a decent boiler, and the life is likely to be similar in either case. However, getting the 30 years plus the old cast iron lumps used to manage does seem harder.

And leave it unheated, which may be a minus.

With modern boilers running at 90%+ efficiency, low water content, and pump overrun etc there is not going to be much in it, unless you have very low hot water demands. Also note that hot water generation will cost only a small fraction of your total heating bill.

Yup.

Although I can't see any reason you could not do that yourself...

Alternatively there are heatbank solutions that will provide the same benefits, but without the requirement for routing checking.

Combi, by a large margin.

Not enough in it to matter.

Probably not much in it.

PV etc seems to only make significant contribution to your hot water heating, and hence will not offset enough value to ever be economic in most cases.

That may make more sense if you have access to cheap fuel.

Probably only worth doing if the layout of the house is such that it solves particular problems.

[David WE Roberts]

Thanks John - I think cost of installation will rule here :-)

On the PV front there is a minor additional gain if you already have your snout well entrenched in the greenbribe trough. With proper circuitry (as suggested in a recent post) you can turn on the immersion heater whilst you are generating and make even more money. Not having suitable roof space we just look on in envy as others take our hard earned money.

[John Rumm]

In which case, the combi rules (which is why builders love them!). Boiler will be no more than £100 more than a system boiler, and you save yourself £600+ on the unvented cylinder.

Yup possibly, but the lions share of the gain is the bribe. The actual energy you need for hot water production over a year is not usually that significant.