New combi sizing - overspecced??!

[Dan Robinson]

If you are principally worried about shower performance then you only need to push the performance a little more.

Dynamic what? Flow rate? Flow is dynamic by very definition, it's just Tony had a fixation with it. No one else has a foggiest what he's referring too.

 

[Phil12345]

Hahahaha I've read in a lot of places that if your flow is less than 20/22l/min then benefits of unvented are gone.

Yes mate my little un has a little bath every now and then but we are shower people and like a proper one, not rip your skin off but that triton running well was what we had in our last house.

That's also what skewed us cos we had a combo there and it was just fine! So to the an ignorant knob head like me I can't help thinking well it did the job there how bad can it be?? I know I know!!

 

[Phil12345]

I thought he meant 22l at 1 bar and when one guy did exactly that I had about 11!! So I assumed I was on a loser for uv

 

[Phil12345]

See this is what no one can tell me and every time I ask it's silence. It's the reason I'm still confused.

Will my 14l/min be enough once upgraded to 22mm to run an invented properly?? If it might not how can I justify £3000 on fitting one....

I know it's a better system, you're excellent advice has convinced me of that, I just don't know if that 20-22l/min requirement is true (everyone swears it is) and whether I'm likely or unlikely to ever achieve it???

If it means only 1 tap can use at a time with my 14 or maybe 16l/min why not a combo???

 

[PullerGas]

There are alternatives but most RGIs do not like anything out of the normal.

This provides mains pressure showers without the need for G3 qualified plumbers to install and maintain it. The cylinder is a heat store for the shower as well as the hot water supply to taps in bath and basin where mains pressure can be too much pressure.

A 6 kW boiler can take 20 minutes to heat the cylinder and then the shower can extract 30 kW from the cylinder for a 4 minute shower. ( These times and values will vary depending on size of cylinder and the boiler output )

It is not a sludge bucket ( as some heat stores are ) because the water in the cylinder is not the water circulating around the radiators and hence does not have chemicals and products of corroding radiators in it. That dirty water ( shown dark brown ) is confined to the lower coil in the cylinder.

View attachment 96887

Looks like there's a lot missing from your design BG, are you still expecting full fees??!!!

 

[bernardgreen]

Looks like there's a lot missing from your design BG, are you still expecting full fees??!!!

The system works for me so the design would appear to have everything that is essential,

I do not however expect myself to pay myself any fees.

The description(s) that I post here are not and never will be the full design documentation. That said I am happy to discuss it privately with people who are open minded.

 

[bobdogavitch]

I know I was shot down last time I said it but a combi really will be your most cost effective option imho. I don't think you'll get the benefits of an UV cyl given your current incoming main even with the upgraded pipe. You could always add a small unvented cylinder to do the bathrooms at a later date if the fancy takes you/when finances allow if you can re-route/connect the appropriate hot pipes and you'll have a nice new boiler to connect it to. Do we know what flow rate your shower/shower head needs to achieve the performance you'd like? Also, in my experience incoming cold main temp rarely drops below 10 deg C so factor this into your decision. You're shower will work fine with 50deg water. You can turn down the DHW temp on the boiler as there's little point in heating it up in the boiler to cool it down again in the shower. Also look for a combi where the maximum heating output can be tailored to suit and that modulates to a low KW output. As discussed on a previous thread you'll struggle to find one that goes below 7KW.

 

[Phil12345]

Cheers Bob I know invented is the tits but for me i am not convinced it's worth it. As you say can still add uv later.

As a practical example I just got excited that I need to do washing up in the kitchen and I'd remembered to set the hot water to on in my vented tank. Just measured it coming out hot.....5l/min.....shower is the main thing for me but life's too short to fill a kitchen sink at 5l/min surely!

How would you spec a boiler for an increase in flow rate predicted? Eg got 14l/min cold at the moment, if uninstall a new combo it will probably need bigger gas and I will probably run 22mm straight to it either on its own or instead of the current 15mm main. Both will increase flow rate won't it? So how do I know what combo to pick? Or should I just go 35kw and that will cover it?

 

[PullerGas]

I know I was shot down last time I said it but a combi really will be your most cost effective option imho. I don't think you'll get the benefits of an UV cyl given your current incoming main even with the upgraded pipe. You could always add a small unvented cylinder to do the bathrooms at a later date if the fancy takes you/when finances allow if you can re-route/connect the appropriate hot pipes and you'll have a nice new boiler to connect it to. Do we know what flow rate your shower/shower head needs to achieve the performance you'd like? Also, in my experience incoming cold main temp rarely drops below 10 deg C so factor this into your decision. You're shower will work fine with 50deg water. You can turn down the DHW temp on the boiler as there's little point in heating it up in the boiler to cool it down again in the shower. Also look for a combi where the maximum heating output can be tailored to suit and that modulates to a low KW output. As discussed on a previous thread you'll struggle to find one that goes below 7KW.

A very good point mate. I've been in the trade a very long time & listening to all these retired engineers & Google warriors on here makes me laugh.

Even a fairly cheap combi like a Heatline can have the CH output reduced to about 9KW. There's a general view by these 'so called' experts in here, that all combis are slung in by Cowboys. This is complete rubbish!! Like all appliances, they have limitations.

 

[Dan Robinson]

They do indeed. As it happens my next two installations are open vented cylinders being changed to Intergas Eco RF 24 boilers.


But if there is the room, and the cost difference is £200 then it is a complete no brainer for me.

 

[Phil12345]

So you don't reckon the 14l/min flow would be a problem Dan? I am absolutely no expert whatsoever but absolutely everyone else says if it's not 20-22 all benefits of an unvented are absolutely gone whatever you do.....

 

[Phil12345]

I'm willing to believe you mate but you are literally the only person who has suggested that it would be worth it on 14l/min. I'm no fitter though! You want sales training I'm your man....!

 

[bernardgreen]

You are right Dan it wasn't all the advice I received, but it was about 50/50 split on here and RGIs that I have had round.

Remember that you do NOT need an RGI to install the water side of the plumbing. An RGI is only needed to connect the gas service and commision the boiler.

If you go for an un-vented cylinder then you will need to employ a G3 qualified plumber to install and commision the cylinder and the associated safety devices. Then there will the need to arrange and pay for the annual testing of the un-vented system's safety devices.

 

[Phil12345]

And the quotes are £2900ish for a brand new combi with all work required or about £200 more for changing cylinder and keeping my 16 year old boiler. Which is a risk of course

 

[Phil12345]

Take your point Bernard but I'm the kind of guy that likes to understand what I'm buying them get a few guys round, choose 1 I like and get him to do the lot. As long as he is qualified I.e G3 if I go uv. I can't be bothered to muck about with different guys on a job like that! I like the guy I choose to have enough in it for him to be interested!!