How much do combi boilers reduce flow - DHW alternatives

[Buzzark]

Hi - sorry for long post but it's better to have all the facts up front right?

We are planning to convert out loft which means I have to rethink our hot water supply options.
For getting rid of the tanks and therefore gaining the most space, a combi seems the most obvious choice.

I want a powerful shower though and our last house we had a nice big Worcester CD35 II fitted and I was dissapointed to find that the combi really limited the flow.
e.g. we had 19 l/m flow at the cold tap, I phoned WB pre sales and spoke with them before hand and also spoke with the installer. (who came highly recommended)
I was told that was more than enough for full flow but wasn't told;
A. The combi (all of them?) would be very restrictive 19l/m in became about 11l/m out, although it was outputting at about 63 degrees.
B. The 35 CDi II has a flow restrictor in it to prevent more that 12l/m (12.5?) passing through the boiler anyway - quoted 14.7l/m outputs @ 35c rise are apparently only "theoretical for comparison".

The shower was pretty decent but wasn't quite what I was lead to believe, I even had the installer back and WB came out too but only to confirm this was the case and the boiller was in fact operating normally.

So... we've since moved house and we need to decide what to do with water again. I HAVE a decent pumped shower pushing out 16l/m with decent pressure but mains flow at this property is a poor 14l/m at the moment. There's a 15mm stop tap fitted on the 25mm MDPE incoming main which won't be helping things and this will be getting replaced next week after which we'll get a better idea of what we have to work with.

The big questions at the end of it is how much do combi's typically restrict flow?
Is my experience normal in what happened on the last install regarding flow reduction?
If I want around 18-20l/m through a shower (2/3rds hot, 1/3rd cold?) how much flow/pressure do I need overall - I realise there's a lot of variables in that one, not least of all the standing and dynamic pressure etc. but just experience/guidance will help!

I know that unvented cylinders or thermal stores are also an option and if the boiler wasn't old and "behaving badly" in a non energy freindly way (requiring replacement) then I'd just go straight for one of those.
I believe that either of those options seem to have very little flow restriction for a given pressure. Coupled together with a new boiler though it sounds a pretty hefty amount.

Thoughts and suggestions welcome - no fighting please!

 

[Doctor Drivel]

Combis restrict flow to what the temp rise spec' says: 10 l./min, 14 l/min, 19 l/min, etc, usually at a 35C temp rise..

Get a high flowrate combi. The stored water version are best. Alpha CD50 is good (wall mounted). The floor mounted versions are good too: Viessman 333, Powerrmax, Gledhill Gulfstream (can up to 3 bathrooms), ACV Heatmaster, Glow Worm models, W-B HighFlow, Vokera, etc.

 

[Dan Robinson]

Said it before - I'll say it again:

Atmos Multi 24/80+. Best of both worlds.


Just enjoyed a 15 minute long shower at 25l/min all from 24kW.

 

[Buzzark]

Thanks DD - are you saying that normally if it's rated to e.g. 20l/m and that it only requires a 20l/m feed to achieve that?

What seemed to happen with the last one we had was we fed it 19l/m, it was rated at 14.7l/m but we only got 11l/m - on top of which the installer and 2 WB engineers said afterwards that they were rated at 14.7l/m @ 35c rise but they had a flow restrictor fitted limiting max flow to 12 (or it could have been the 11 we got out of it..)
I'm not saying you're wrong, WB and the installer fed me so many different stories in the end that I wasn't sure what was fact and what was BS.

Some of the thermal stores and unvented's have specs which state input and output flow e.g. 30l/m @ 1 bar input = 29.5l/m @ 1 bar output.

PS is the Gledhill Gulfstream a themal store/boiler combined - seem to think I've read that?

 

[Buzzark]

Said it before - I'll say it again:

Atmos Multi 24/80+. Best of both worlds.


Just enjoyed a 15 minute long shower at 25l/min all from 24kW.

I'm sooo envious! I normally get the cold shower just after my wife gets out in the morning - still on a gravity fed grade 4 cylinder at the mo but that's part an parcel of the same problem. It takes about 90 mins to heat and I was going to upgrade and go to fully pumped before we thought a bit further ahead and a loft conversion. My good showers are only in the evening or weekends until we get something done!

I'll be checking out all these suggested models so far. The Atmos isn't one I've heard of before so I'll have a little look for that now.

Edit:Atmos Multi 24/80+ - wow that looks like an awesome piece of kit, great specs too! It does look a little like a bomb though
Interesting design, definately appeals to the techie in me. One for the shortlist I think. At a list price of £1885 does that come pretty much complete? It seems to have most of it within the box already.

 

[Dan Robinson]

They are not well known, not cheap either. But when you factor the labour and materials for a system boiler and unvented cylinder they are actually very good value.

Key selling point for us is the reheat time of 11 minutes.

See other current thread for links.

 

[simond]

Combis restrict flow to what the temp rise spec' says: 10 l./min, 14 l/min, 19 l/min, etc, usually at a 35C temp rise..

Get a high flowrate combi. The stored water version are best. Alpha CD50 is good (wall mounted). The floor mounted versions are good too: Viessman 333, Powerrmax, Gledhill Gulfstream (can up to 3 bathrooms), ACV Heatmaster, Glow Worm models, W-B HighFlow, Vokera, etc.

Contrary to what is posted here (eg: by Dr Drivel), most combis are rated at 35 degrees for comparison purposes, but set up for around 42C temperature rise. Therefore when measuring flow rate with a flow cup you can expect less than the brochure suggests, allow at least 20%.

The flow restrictors are set up to reduce flow to prevent lukewarm water being delivered and assume a 42C approx temp rise. It is the estimator's responsibility to point out the disparity betweeen the specs and real World flow rate.

To the best of my knowledge when he says the fast flow combis listed are 'good' his experience is limited to poring over brochures and the internet. For instance, no one with actual experience in the trade would be likely to use that word to describe a Gledhill or Powermax

 

[ChrisR]

Buzzark if your property only has 14 litres/minute coming in, which will be less by the time you've added other plumbing, you aren't going to get the shower you want unless you have a gravity (open) cistern and a pump.

SImon will be quick to say that an accumulator is an option, if your pressure is good enough. It still takes an awfully big accu to match a cistern and pump, but there are siting/noise factors etc to consider.

Before you even THINK any more you need to find out what your water pressure is (and varies between. it'll be lowest when you want it most). From what you've told us, I doubt it's good.

 

[Buzzark]

The flow restrictors are set up to reduce flow to prevent lukewarm water being delivered and assume a 42C approx temp rise. It is the estimator's responsibility to point out the disparity betweeen the specs and real World flow rate.

Yeah, that sounds like the WB engineer said when he came to check the install.
Of course, WB's pre sales pretty much said 19 in, 19 out on several occasions speaking to different people (but at lower temp than the 35c rise). I should have known really, they couldn't tell me why I should buy a CDi over the Si at the time either, both apparently were almost the same on reliability (actually they said the Si came out about 0.5% MORE reliable according to their figures), the same on efficiency and likely to last as long as each other - actually, that's still a question I haven't got to the bottom of!
To my knowledge, restricting the flow at any point before the outlet creates a large loss in dynamic pressure. The loss being dependant on the width of the opening of the outlet compared to the restrictor. we had to drop to a pretty small showerhead to generate the desired pressure, or at least something acceptable. The supply before the restrictor seems almost irrelevent and I can't help but feel that with the restictor removed you'd get higher pressures at the same tap opening as long as you didn't ask the boiler for more than in could heat.
Similar to what I suspect is happening with out mains pipe with the 15mm stop tap blocking our flow and pressure...

For instance, no one with actual experience in the trade would be likely to use that word to describe a Gledhill or Powermax

Just so I know - not good in what way? Performance, reliability etc?

 

[Buzzark]

Buzzark if your property only has 14 litres/minute coming in, which will be less by the time you've added other plumbing, you aren't going to get the shower you want unless you have a gravity (open) cistern and a pump.

SImon will be quick to say that an accumulator is an option, if your pressure is good enough. It still takes an awfully big accu to match a cistern and pump, but there are siting/noise factors etc to consider.

Before you even THINK any more you need to find out what your water pressure is (and varies between. it'll be lowest when you want it most). From what you've told us, I doubt it's good.

Thanks Chris
Yes, I'm aware that that's not enough for anything mains fed. I've spoken to the engineer who'll probably be doing the install for us and he's going to replace that 15mm stoptap with the correct 25mm version after which we'll see where we stand. Pressure in the area is apparently good and the new incoming supply to the house was laid around 2 years ago, so it really does seem this is the problem. I was going to buy a pressure guage and take a measure but I may as well wait for the engineer to take the measurements.
To gain a more accurate representation on the flow I measured at the bath tap which is closer height wise than the downstairs taps. The flow to the shower is really all that matters, if that's good - fillsing a bath etc will be acceptable.

I'm also making arrangements with a loft specialist and we'll see if it's feasible to keep the cold water tanks - I'd rather not waste that potential space though. Don't mind keeping a hot water vessel (of whatever type we end up with) in that space as it doesn't have to be roof level.

 

[ChrisR]

The stopcock is the equivalent of about 3 metres of pipe. It won't make that much difference.
Some bath taps are horrid though - try an outside tap at the same time and add everything up.

 

[Doctor Drivel]

Combis restrict flow to what the temp rise spec' says: 10 l./min, 14 l/min, 19 l/min, etc, usually at a 35C temp rise..

Get a high flowrate combi. The stored water version are best. Alpha CD50 is good (wall mounted). The floor mounted versions are good too: Viessman 333, Powerrmax, Gledhill Gulfstream (can up to 3 bathrooms), ACV Heatmaster, Glow Worm models, W-B HighFlow, Vokera, etc.

Contrary to what is posted here (eg: by Dr Drivel), most combis are rated at 35 degrees for comparison purposes, but set up for around 42C temperature rise.

No 35C.

To the best of my knowledge when he says the fast flow combis listed are 'good' his experience is limited to poring over brochures and the internet.

Have you seen a fast flowrate combi?

For instance, no one with actual experience in the trade would be likely to use that word to describe a Gledhill or Powermax

A combi is a one box solution. The Gledhill and Powermax are that. Look at Sedbuk.

You don't even know the definition of combi. That is sad.

 

[Doctor Drivel]

Fit a new 25mm stoptap and a dedicated supply to the combi - no tee offs, except the cold to the showers which you take off just before the combi on the combi supply - any prerssure variations around the combi will afect the shower too, to prevent serious pressure hot and cold pressure imbalances. . All other cold supplies are teed of at the stop cock in their own dedicated line, which may be 15mm.

 

[Agile]

Buzzard, you are missing a very important point!

Its no good measuring the open pipe flow rate! You need to measure the dynamic flow rate when leaving a pressure of 0.5 or 1.0 bar remaining the pipework.

As you have learnt the boiler will create a flow restriction and so will your shower.

I recommend taking the dynamic flow at 1 Bar for full performance or at 0.5 bar for reduced performance.

The flow restrictor can be removed and will increase the flow rate by a few percent. However its essential that the mains supply to the combi is through full flow valves. The stopcock could be replaced by a full flow valve if you are struggling but that is not exactly correct under the water regs.

The ultimate solution is to have a new service pipe laid from the street in 25 or larger size pipe. Thats usually a budgetary £1000 or more.

Tony

 

[ChrisR]

Contrary to what is posted here (eg: by Dr Drivel), most combis are rated at 35 degrees for comparison purposes, but set up for around 42C temperature rise.

No 35C.

Which is drivel. Simon is right.