Do we really need a megaflow system?

Dedekind

Hello,

this has probably been answered already, but we are still not sure what's the best in our case. We have a 3 bed semi, currently with a gravity system, and we are updating that to either a combi-boiler, or a system boiler + megaflo.

We have 1 bathroom in the first floor and a toilet downstairs. We may do a loft conversion in the future, which would imply having a second bathroom in the loft. In terms of water mains pressure, the plumbers will probably measure it, but both them and us can see from the kitchen tap that it's very very high, so I think both systems are doable.

From what I see online, megaflo is usually recommended when there are at least 3 bathrooms? Also even if we do the loft and put a shower there, we are a small family (2 adults + 1 preschooler + 1 baby), so I don't see us simultaneously using showers for a long long time..  similarly, if someone wants to wash the dishes and someone else is taking a shower, we can surely wait for 5 minutes until the shower is finished.

I say this having lived with a combi boiler since I was born 35+ years ago.

We are updating this as part of a kitchen renovation, and our builders are giving us a reasonably good price for fitting the megaflo. So in terms of cost it mostly boils down to the cost of the cylinder, since I presume the boilers costs are similar.

Any thoughts/recommendations are more than welcome. Thanks.

GDB2222

If you have the space and you will be in the house a few years,  have a megaflo. It solves issues that you mentioned, and it will have an immersion heater so you’ll still have hot water if the boiler fails.

Jeepers_Creepers

Dede, what a crackingly detailed question.

The answer is - you choose.

You clearly have a very good handle on the pros and cons of each system. A few teeny tweaks to your info:

COMBI.

Pros: Endless hot water supply. Energy cost is only for the water you actually heat up. With good mains P&F, it'll provide superb showers (go for a 35kW at least). Cheaper to buy & install. Takes up less space.

Cons: Will only supply one proper shower at a time. Showers will be affected to some degree by other taps being used, BUT thermostatic showers will ensure showering can continue. (Plus you could run 2 showers if you really wanted to, if the flow to each is cranked down (eg. by isolating valves), but that's a pointless as each shower would be a bit pants). If boiler fails, you'll have no hot water (but I'd consider that a minor issue).

MEGAFLO.

Pros: Depending on mains supply, a few outlets can be run simultaneously. Cylinder room will act as an airing cupboard to some degree. Backup of an immersion heater if needed. System boiler will almost certainly be more reliable than a combi type.

Cons: Additional £1k+ installation cost. Cylinder takes up space. Extra servicing required (tho' minimal). Slightly higher energy bills.

I think that's it.

If your P&F are solid - say, 3 bar and 20lpm+ - then consider 38kW if going combi. It'll throttle back to provide how little CH you require, but the power is there for your gushing hot water needs if desired. Always nice having a reserve...

Bottom line, for 95% of the time, both will provide all the gushing hot water you could possibly want. Their respective limitations will only manifest themselves at certain short times.

For a lot of folk there is a deciding factor, like "I MUST be able to have 2 good showers running at the same in the morning". No brainer. Or, "Cupboard space is very limited - I need as much as possible..." Again, n-b.

You seem to be quite flexible.

coffeehound

Jeepers_Creepers said:

MEGAFLO.

Pros: Depending on mains supply, a few outlets can be run simultaneously. Cylinder room will act as an airing cupboard to some degree. Backup of an immersion heater if needed. System boiler will almost certainly be more reliable than a combi type.

Cons: Additional £1k+ installation cost. Cylinder takes up space. Extra servicing required (tho' minimal). Slightly higher energy bills.

Hi Jeepers.  You've stated as fact that a combi is more efficient in hot water mode than a system boiler heating a megaflo.  A big point; the sort of thing that could well determine people's substantial buying decisions.  Do you have any proof of this please?  

Also, I've read in multiple places that it is a bad idea for a number of reasons to over-specify the power rating of a boiler.

Doozergirl

I'd have zero qualms about putting in a combi in your house.  It's the most convenient and it's  a great deal cheaper.  I can't see the point at all of storing water for one bathroom that could be fed directly from the boiler.   

The loft conversion might be a consideration, but it's a cost that is for something that doesn't yet exist and it isn't a huge compromise if you don't mind waiting for a shower on the odd occasion.  

It's by the by, but you'll notice a huge difference from a gravity system anyway. 

Jeepers_Creepers

coffeehound said:

Hi Jeepers.  You've stated as fact that a combi is more efficient in hot water mode than a system boiler heating a megaflo.  A big point; the sort of thing that could well determine people's substantial buying decisions.  Do you have any proof of this please?  

Also, I've read in multiple places that it is a bad idea for a number of reasons to over-specify the power rating of a boiler.

I should have clarified - slightly higher energy bills are inevitable from any stored hot water system, simply due to heat loss from the cylinder. The boilers themselves will likely vary only by make and model.

Actually, come to think of it, you may have raised an interesting point; will a system boiler possibly be more efficient in condensing mode when heating up a hot cylinder than a combi boiler running 'flat-out' heating up water instantly?! Could be - I don't know. But I was just referring to the inevitable heat loss you'd get from stored hot water, that's all.

As for over-speccing the power of a boiler, this is a combi boiler we are talking about, and its hot water delivery rate is directly related to it's max power output. A 35kW combi will not under any conditions deliver 35kW of heat to your rads! (And it'll only deliver the full whack to the hot water if the tap is turned on full).

If I lived in a 2-bed flat, I'd still almost certainly fit a 35kW combi. They all automatically modulate down to - what? - 7kW? to suit CH demand. In a house with multiple bathrooms or occupants, where regular tap-turning is the norm, I'd go higher - 38kW - provided the mains flow and pressure justifies it. If plumbing from scratch, I'd also run 22mm pipe from the mains stopcock up until the point it tees off to supply the combi.

Jeepers_Creepers

Ditto as Doozer say. I went 'combi' as we converted the loft over 15 years ago (so losing the CWS), and also to gain some extra cupboard space where the old cylinder lived. It's 'only' a 30kW - in a 3/4 bed 3-bathroom property, but has served us well and gives great showers. Yes, if some twit turns on a tap or flushes a loo during a shower, the flow drops a noticeable amount but the thermo keeps oohing & ahhing to a min; you don't stop showering.

No regrets, tho' I sort of wish I'd gorn bigger. But no big deal

Dedekind

Thanks everyone!

I am more or less decided to go combi..

Jeepers_Creepers

If your mains F&P justify it, go 'large'    A 38kW jobbie will give you 16lpm.

Theoretically, split in two and with added cold to each (so you don't scald), that'll still give you 2 simultaneous showers that'll be significantly better than any leccy shower.

fezster

A combi may well be the best and most convenient option, but I'll add a couple of thoughts:

- A combi is almost always oversized to meet DHW demand. Depending on your heating requirement and the modulation ratio of the boiler, an oversized boiler can be very inefficient and will constantly turn on and off if the heating demand is smaller than what the boiler can modulate down to, which puts more strain on the boiler and reduces it's lifespan.
- A combi will usually be more expensive to buy and maintain than an equivalent heat-only boiler.
- You may want to consider just having a heat-only boiler and keep your existing gravity fed HW system, until such time that your HW demand increases and you can switch to an unvented.

More important than anything else is choosing a good installer. No matter how reliable or efficient a boiler claims to be, it'll only be so if it has been installed and setup correctly.

coffeehound

Jeepers_Creepers said:
I should have clarified - slightly higher energy bills are inevitable from any stored hot water system, simply due to heat loss from the cylinder. The boilers themselves will likely vary only by make and model.

Actually, come to think of it, you may have raised an interesting point; will a system boiler possibly be more efficient in condensing mode when heating up a hot cylinder than a combi boiler running 'flat-out' heating up water instantly?! 

That's what I was thinking.  With new tanks losing less than two units of heat per 24 hours, the system boiler wouldn't need to be a lot more efficent to come out on top.  Lots of variables wrt geometry and usage though.