Combis and flow-rate: Vaillant 838 Plus Ecotec

fathertobe · 25 Jun 2018

We currently have a Worcester Bosch 37CDi boiler in a three bed house. It's been fine for almost 10 years - very few repairs needed. However, you can't turn on a tap on while someone's showering etc.

We are about to start building work which involves adding an additional shower room. We've also planned on upgrading the main pipe from the street to the front of the house - which is bound to a very old, narrow lead pipe of some kind.

We assumed we'd need to upgrade the boiler too - and the vallaint 838 Plus is what has been suggested. (We have some space for a tank but it will involve nicking space from a bedroom. So the compact nature of a combi is attractive.)

But when I dig a little deeper into what we have and what is proposed, these boilers don't see very different... Same flow rates, 32kW vs 35kW.

Question:
1) Is this upgrade worth it?
2) Do people think this will serve our needs of running two showers simultaneously? Or do we (in reality) need to look at a separate tank?

bernardgreen · 25 Jun 2018

fathertobe said:
However, you can't turn on a tap on while someone's showering etc.

fathertobe said:
Do people think this will serve our needs of running two showers simultaneously?

You have answered your own question. Also with a cylinder that is fitted with an electric immersion heater you can still have hot water when the boiler breaks down

fathertobe · 25 Jun 2018

Thanks for replying. So why do some combi boilers say they are capable of running two showers concurrently?

manortim · 25 Jun 2018

fathertobe said:
Thanks for replying. So why do some combi boilers say they are capable of running two showers concurrently?

Modern regs regarding water use.... Means new homes have flow limitors on hw.

For example...

House has a 20L per min incoming mains volume.

2 showers running 10 L per min outlet limitors.... Will need circa 4 L per min of hot water at circa 45 degree.

Going into the boiler.... Circa 12L per min... Take away the restrictions of the heat exchanger.... And you'll get maybe 10/11 L per min on the outlet side.... At circa 45 degree

So.... The boiler can provide flow to 2 showers.

But.... That's best practice and unlikely real world in a conventionally NON combi home.

Ecotec 838.... Urgh.... That's like having a 500 hp V8 Mustang and fitting remould tires....
Just pointless.

If you've space.... Fit a cylinder and an intergas boiler.... Using pin 5... You'll have alternative flow temps and fully modulating temps.

Boom... 21st century tech

muggles · 25 Jun 2018

Agree with the above - a boiler and hot water cylinder is what you need, and an Intergas ECO RF with 10 year warranty would be a great boiler choice. If you haven't got space for a full size cylinder, the combi superflow from The Intergas Shop might suit you, although a full size unvented cylinder is always the best option where possible

fathertobe · 25 Jun 2018

OK. Leaving aside intergas vs other makes for the moment, do I actually need a new boiler - or should I just have an unvented cylinder added in? NB: THe current boiler is working fine.

Also, I guess upgrading the mains from the street would be advisable before anything else, no?

muggles · 25 Jun 2018

If the current boiler is in good condition, just add an unvented cylinder. As for whether a mains upgrade is required, or likely to actually achieve anything, you need to get your current incoming flow rate and pressure tested

fathertobe · 26 Jun 2018

Thanks. Looks like adding an unvented cyclinder is my solution. I know my current flow rate/pressure is poor and neighbours report significant boosts after upgrading.

(That said, IF it's theoretically possible that my current combi would serve my needs without an unvented cylinder, then logically if feels as though I should get main upgraded and test. If the boiler can't serve two hot taps at the same time, then I should add cylinder.)

fezster · 26 Jun 2018

All depends on the shower's you have - have a read of my post here: https://www.diynot.com/diy/threads/combis-dont-work-in-larger-houses-still-true.503563/#post-4156441

fathertobe · 26 Jun 2018

Great! Thanks - as I understand, two showers might or might not work with a combi only (depending on how much water they use and time of year), but collective wisdom for now seems if you want to guarantee it working, fit an unvented cylinder.

John D v2.0 · 26 Jun 2018

manortim said:
10 L per min outlet limitors.... Will need circa 4 L per min of hot water at circa 45 degree.

Only if your cold water incoming is already 30 degrees, you'd need more like 60c hw at 4l per min. Or 7l per min at 45c

manortim said:
Going into the boiler.... Circa 12L per min... Take away the restrictions of the heat exchanger.... And you'll get maybe 10/11 L per min on the outlet side

Where does the other 1-2l of water go? Also Some boilers have flow restrictor in rather than it being the heat exchanger.
As a non heating installer, I'd say you can get two shower from a Combi with a water saving head but it depends more on being able to get 16l/m on the incoming supply even if you have the right boiler. Not sure if you can fit an accumulator before a Combi?
OP stated they can't turn on a tap when the shower is on but it's not stated whether that's due to flow or temperature.

manortim · 26 Jun 2018

fathertobe said:
Great! Thanks - as I understand, two showers might or might not work with a combi only (depending on how much water they use and time of year), but collective wisdom for now seems if you want to guarantee it working, fit an unvented cylinder.

No.

If you want it guaranteed working...

Employ a decent heating technician who can explain what's needed.

No point fitting anything until you know what's what.

You don't build a house without foundations

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Combis and flow-rate: Vaillant 838 Plus Ecotec

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