My open flow rate is 38 litres per minute.

[Mr.B]

I was recently politely reminded not to tag my question on to another's thread, so this is a new thread.
I have, for ages, thought I would fit a new 'heat only' boiller on to my existing, 28 year old, open vented system and have been round the houses in some depth with a good deal of head scratching going on. Can't help it, it's the way I am.

But maybe I'm barking up the wrong tree .... there's just us two in a fairly modern, well insulated, four bed house on an estate with one shower room that very rarely gets used plus a bathroom with a bath (with a shower over it which we both use in the morning ) ... and very little further use of hot water during the day (dishwasher for most of the dinner stuff). Hardly any likely occasion for two outlets to be used at the same time.

I checked the open pipe flow a few yrs ago and it was 36.5 litres per minute and the pressure gauge I borrowed showed 4.5 bar with no other taps open.

Today the open pipe flow with no other taps open is 38 litres per minute. I haven't got a pressure gauge handy.

An installer is booked to come round in a fortnight but I'm just doing my usual research as he has tentatively suggested removing the two tanks in the loft and the HW cylinder in the airing cupboard and pressurising the system with either a Worcester or an Ideal combi (he's local and has done others in similar houses the neighbourhood so can picture mine in his head).

He's an accredited installer for both WB and Ideal (a MAX installer for Ideal).

My simple question is .... given that I'm a bit obsessed to have a stainless HEX - if I went for the 32kw Vogue Max, changed the two Mira shower valves from gravity to pressurised, connected the combi 15mm hot water outlet to the existing hot water pipework (a mix of 15 and 22) and moved the new combi boiler close to the most frequently used shower (no great inconvenience) ...... does 38 litres per minute sound a decent figure? My suspicion is that it's plenty but it's nice to have some confirmation in the back of my mind.

Ta.

 

[muggles]

No combi on the market will do 38l/min. The Navien NCB700 54kW would be an excellent choice for you. 25l/min hot water, option of secondary circulation so positioning is less of an issue, 12 year warranty, stainless heat exchanger, and very keenly priced.

NCB700 ON Gas Condensing - Navien UK

The game-changing NCB700 ON is our powerful combi crossover, capable of delivering hot water to three bathrooms simultaneously.

navien.co.uk

 

[Mr.B]

Understood, Muggles - ta .... I was just asking if the 38 litres per minute from the 22mm supply that I have run adjacent to the boiler position is a good figure for the boiler, not whether I couild achive 38 lpm hot water flow from the boiler.

The reason I only mentioned those two manufacturers (WB and Ideal) is that they are the two makes that this particular installer uses and I have trust in him ... he has a good rep and is local and employs half a dozen blokes so I would hope not to be left high and dry if a problemj occurred.

He used to do Vaillant but had stopped.

When I mentioned Viessmann he reminded me that not so many places stock parts and not so many blokes are familiar with them, he was speaking with a view to my getting it fixied if it went wrong (notwithstanding that Viessmann themselves could/would attend under warranty plus the Viessmann accredited installer would return to fix/service it (all the time he remains in business and remains active).

There aren't all that many Viessmann accredited installers locally but as it happens one of them has had experience of Navien (but I got the impression he prefers Viessmann.)

Incidentally, has your preference for Intergas lessened at all? There are still no installers locally and I don't see Intergas spoken about in this place these days like it was a year or so ago.

 

[muggles]

Still specifying Intergas now and then but they've not innovated and moved on much in the last few years, and HQ is in a bit of chaos. The Xclusive is a decent boiler but can be a bit on the loud side. The Navien is very keenly priced for what you get with it.

 

[Johntheo5]

You said you were getting 36.5LPM with a open pipe and the PG showed 4.5bar, are you saying it was reading 4.5bar with this flowrate or was the 4.5bar the static pressure before the open pipe reading?.

 

[Mr.B]

I've not overlooked your comments, Muggles, and have been looking at it.

Couple of points though .... I did mention that after we've both showered in the morning (separately, I might add) there's very little call on the hot water for the rest of the day and evening. So the recirculating hot water would seem extravagant if it constantly circulates hot water round the circuit (but I can't claim to have any real knowledge except I could see on the drawing that there is a second pump in the boiler for the purpose of recirculartion).

It's one hell of a beast though, ain't it? lots of kilowatts and heap plenty flow.

I was also a bit puzzled to see your recommendation and suggestion that it might suit us as I spotted that in August 2018 (during that 20 page battle of wits between Hardwork and Dan Robinson) you weren't remotely in favour of it and didn't have a good word for it.

 

[Mr.B]

You said you were getting 36.5LPM with a open pipe and the PG showed 4.5bar, are you saying it was reading 4.5bar with this flowrate or was the 4.5bar the static pressure before the open pipe reading?.

I was trying to say that I have installed a 22mm supply from immediately beyond the blue MDPE intake in our kitchen and taken it straight to the position where a boiler would be installed. This is to give a boiler its own supply without any disruptions ... and the flow from that supply today is 38 lpm. A couple of yrs ago, when I fitted it, I checked and it was giving 36.5 lpm and when I borrowed a pressure gauge at that time it read 4.5 bar.

 

[Johntheo5]

That was a fantastic dynamic pressure at that flow rate.

 

[Mr.B]

Maybe I still haven't explained it ...............The 38 litres per minute was measured by opening the tap that's fitted at the end of the 22mm run. It's a washine machine tap with a three quarter thread on it. I connected a washing machine hose to it and discharged it into a bucket for 15 seconds - multiplied by four and that gave me 38 litres per minute.

I did the same a few years ago just after I fitted it and got 36.5 lpm. I then disconnected the hose and connected a pressure gauge to it and got 4.5 bar. 4.5 bar isn't fantastic, it's roughly normal for this area. When I moved in 18 yrs ago it was higher than that and I fitted a PRV and brought it down to just over three bar. Saves the hose popping off every five minutes. So, to summarise, the 22mm supply to the proposed combi boiler in my garage supplies 38 litres when unrestricted and with no other taps open - I don't know what the pressure is today coz I don't have a gauge to connect to the end of the supply but the rest of my house runs at just over three bar because I've got a pressure reducing valve installed that brings it down from, say 4 or 5 bar..

 

[muggles]

I've not overlooked your comments, Muggles, and have been looking at it.

Couple of points though .... I did mention that after we've both showered in the morning (separately, I might add) there's very little call on the hot water for the rest of the day and evening. So the recirculating hot water would seem extravagant if it constantly circulates hot water round the circuit (but I can't claim to have any real knowledge except I could see on the drawing that there is a second pump in the boiler for the purpose of recirculartion).

It's one hell of a beast though, ain't it? lots of kilowatts and heap plenty flow.

I was also a bit puzzled to see your recommendation and suggestion that it might suit us as I spotted that in August 2018 (during that 20 page battle of wits between Hardwork and Dan Robinson) you weren't remotely in favour of it and didn't have a good word for it.

Different boiler. Wasn't a fan of the original Navien range but this is from the new range released last year

 

[Mr.B]

Aha. Ta.

Have to say it's a pretty spectacular thing compared to others regarding it's flow, and if recirulation on the HW was beneficial to you then it's a rare beast to provide that.

You won't remember but I was dead keen on fitting a heat only Intergas before Covid and did also go to see their local rep in a plumbers merchants who was demonstrating an Xclusive combi (and giving away sausage rolls).

And have continued to be convinced I wanted a new heat only connected to my 28 year old vented cylinder along with 28 yr old F&E and CWSTank, separate pump and three way valve ... but, as mentioned, am now wondering if that's such a good idea. I'm beginning to thank a combi might suit our circumstances better and just want to try to pick the right one (and the right installer).

 

[Johntheo5]

A few points to ponder, if installing a combi, its output is largely determined by your required showering flow rate, 30kw will give 13.0LPM at 40C from a winter cold of say 7C, a 36kw will give 15.6LPM and so on, some of these boilers have a relatively poor turndown to ~ 6 or 7kw which in a CH system with zoning etc is far too high causing frequent boiler cycling, Viessmann do make boilers with a very big turndown to ~ 3kw or lower, others, like the newer Vaillants I think (in the 30kw range) do likewise, maybe down to 3.5kw or so, so bear this in mind.

 

[Mr.B]

Valid points.

I've become quite conscious of Viessmann in recent days and have learned that they have stainless HEX, good turn down rates, no opentherm on the heat only I WAS contemplating (but probably not any more IF I'm going combi). I was visited by an accredited Viessmann installer and was confident he would do a good job. He was about 7 miles away so not too far.....I'm currently awaiting a reply to my two emals and am sure it will arrive in due course.

Everyone on here say the boiler isn't as important as the installer. If I wanted to get a very local, trusted installer (who is only accredited for WB and Ideal) then it would be a different installer to the Viessmann one. Rather than an ali HEX Worcester I would prefer a stainless HEX Ideal Vogue (probably 32kw) which modulates down as per the figures below. That particular installer doesn't fit Vaillant any more (he used to). If he fitted an Ideal Vogue combi I would get a 12 yr warranty (as he's MAX accredited).

If I went Viessmann I would be aware they were less familiar to most repairers than Ideal and the parts availablity with Ideal is, obviously, better. But hopefully the warranty would cover all or most of that with whoever I went with..

Regarding shower flow (on the first floor of a normal configuration house) ... I have been used to the shower flow afforded by a gravity, vented system courtesy of the cold water storage tank in the loft so I can't imagine a combi would be any less powerful, even in winter ... and I would opt for the 32kw rather than the 26kw. I don't think the next one up (40kw) would be necessary (16.4 ltrs per min at 35 degrees rise) but only modulates down to 5.7.

The table here shows the 26kw model and the 32kw models.

26kw CH output (kW) min/max @ 70°C	3.7 - 18.0	4.6 - 26.0
32kw CH output (kW) min/max @ 40°C	4.0 - 19.3	4.9 - 27.9
DHW flow	26kw = 10.6 l/min @ 35 degree rise	32kw = 13.1 l/min @ 35 degrees rise

 

[Mr.B]

I just checked. My gravity / Mira thermostatic shower produces a fraction over 5 litres a minute from cold. Not sure if it slows a bit as it warms up (don't think so).

 

[Johntheo5]

What make/model is your present heat only boiler and what is it's minimum output?.