Storage Combi boiler vs megaflow

KremboMeshuga

Hi, 
We're doing up our new house - it's a three bed semi to which we will be converting the loft. Water pressure is good and our plumber recommended we installed a storage Combi boiler, but a friend down the street said if we wanted support for two parallel showers we have to have a megaflow.
Can someone please help shed light? I'm not crazy about the space a megaflow takes. On the flip side, we have two teens so parallel showering is kind of a must... 

Browntoa

I'd go for the megaflo 

A storage Combi boiler is simply not going to give you the volume os water for 2 showers .

I've had both and prefer the megaflo system 

FreeBear

A combi with integral DHW storage will not be particularly efficient when it comes to gas consumption. It will need to fire up on a regular basis to keep the tank hot, although it should be possible to disable this feature.

Something like the Viessmann 111 will have an internal 46l tank. If you want bigger, the Viessmann 222 comes with a 100l tank, but is floor standing.

Megaflo tanks come in a variety of different sizes, so you could pick (for example) a 180l model that would supply oodles of hot water for multiple showers or a bath or two. That would need to be paired with a heat only (or system) boiler. If you go this route, avoid any recommendation to fit a 30kW boiler unless you really need fast reheat times. For a 3/4 bed property, 18kW will be more than enough to heat a tank of water and also supply sufficient power to the central heating system. A smaller boiler will also be quite a bit more efficient, especially if you get one that can modulate down to ~3kW.

Have a 3 bed semi myself with a Viessmann combi - Most of the time, it is running at 4-6kW for heating and only gets to 8-10kW at start up. Heating hot water during the winter, and it will run at full power, but DHW is just a small proportion of my energy requirements.

Albermarle

For the Megaflow to work properly in supplying two showers etc the mains pressure must be a minimum of 2 Bar ( I think) 
During a loft conversion we had a megaflow system fitted, but it has never been able to supply two outlets at the same time adequately. Our mains pressure is only 1.5 bar.
I think our pipe runs/configurations do not help though.

To reinforce the comments from @FreeBear.
We have a 4 bed older house ( incl loft conversion) and 13 radiators. Our 18KW system boiler has no problem heating the house and hot water. 

WIAWSNB

KremboMeshuga said:

Hi, 
We're doing up our new house - it's a three bed semi to which we will be converting the loft. Water pressure is good and our plumber recommended we installed a storage Combi boiler, but a friend down the street said if we wanted support for two parallel showers we have to have a megaflow.
Can someone please help shed light? I'm not crazy about the space a megaflow takes. On the flip side, we have two teens so parallel showering is kind of a must... 

How good is good, both pressure and flow?
If it is 'good' - say 3+bar and mid-20's+ lpm -  then I'm going to be controversial; fit a straight 38kW combi.

A storage combi surely relies on it being permanently preheated? Which partially defeats the point of a combi. And, once that reservoir is used up, you'll be back to normal combi power - the Veissman 111/222 is 32kW, for example, which is around 12lpm = weak showers.

Yes, a Megaflo is great, but you obviously don't want the space it takes up if there's no good reason for this.

A straight 38kW combi should deliver around 16lpm. That suggests to me that two showers will get 12lpm each. 

Because, 8lpm 'hot' + some cold to prevent scalding should be around 12lpm :-)

You obviously fit thermostatic showers, and if for some teenage reason 12lpm just isn't good enough, then tough whatsits. It is.

(And one shower running will literally be a blast - so if they'd rather stagger their ablutions to suit, then fine).

There - the benefits of a combi; endless DHW, and no wasted storage. 

Obvs carry out due diligence.

(We only have a 30kW combi, and had the teenager bit. It'll only provide one good shower at a time, but - guess what - folk have to work around that. And we did. And it was fine.)

Albermarle

There - the benefits of a combi; endless DHW, and no wasted storage. 

Although no hot water if the boiler stops working , unlike with a Megaflow as the tank will have a back up electric immersion heater.

Always pros and cons for any system. 

Rodders53

Look into a heat pump with grant to reduce price of the overall installation.

That'll need an unvented HW tank (I'd avoid the Megaflo brand personally - NB no w in the brand name) and maybe some larger rads.

WIAWSNB

Albermarle said:

There - the benefits of a combi; endless DHW, and no wasted storage. 
Although no hot water if the boiler stops working , unlike with a Megaflow as the tank will have a back up electric immersion heater.
Always pros and cons for any system. 

Absolutely. And another 'con' of the combi is its almost certain level of unreliability compared to a system boiler. 

The OP made it clear, tho', that they much rather not have a DHW tank. 

FreeBear

WIAWSNB said:

Albermarle said:

There - the benefits of a combi; endless DHW, and no wasted storage. 
Although no hot water if the boiler stops working , unlike with a Megaflow as the tank will have a back up electric immersion heater.
Always pros and cons for any system. 

Absolutely. And another 'con' of the combi is its almost certain level of unreliability compared to a system boiler.

Is a combi any less reliable than a system boiler ?

A combi will have a P2P heat exchanger, 2 way valve, plus a couple of sensors (flow & temperature). The rest of the internals will be no different from a system boiler.

Both modern combi and system (and this includes heat only) boilers have complex PCBs which can fail.

Both have pumps, expansion vessels, and 2 way valves - Heat only boilers will generally have these components externally, so it is possible that you don't need a Gas Safe engineer to replace these parts unless they are inside the boiler casing.

WIAWSNB

FreeBear said:

WIAWSNB said:

Albermarle said:

There - the benefits of a combi; endless DHW, and no wasted storage. 
Although no hot water if the boiler stops working , unlike with a Megaflow as the tank will have a back up electric immersion heater.
Always pros and cons for any system. 

Absolutely. And another 'con' of the combi is its almost certain level of unreliability compared to a system boiler.

Is a combi any less reliable than a system boiler ?

A combi will have a P2P heat exchanger, 2 way valve, plus a couple of sensors (flow & temperature). The rest of the internals will be no different from a system boiler.

Both modern combi and system (and this includes heat only) boilers have complex PCBs which can fail.

Both have pumps, expansion vessels, and 2 way valves - Heat only boilers will generally have these components externally, so it is possible that you don't need a Gas Safe engineer to replace these parts unless they are inside the boiler casing.

The main issues that beset combis on here are the P2P and the diverter valve. 
And mine's been no exception.

rustybloke425

Browntoa said:

I'd go for the megaflo 

A storage Combi boiler is simply not going to give you the volume os water for 2 showers .

I've had both and prefer the megaflo system 

I've seen a viessmann 111w run 7 hot showers for about 15 minutes before they started to cool down... so can you, its in a video on YouTube.