Poor hot water flow

Ian_P · 10 Sep 2010

We are currently trying to get enough flow to supply two showers in the house (one 1st floor, one 2nd floor). After a previous post and getting Northumbrian Water out, it transpires that the pressure is fine but flow is a problem. I bit the bullet and had a new supply pipe fitted and am now waiting for a new communication pipe to be fitted.

The thing I don't understand is that when I have the bath tap and sink tap both open on cold water there is plenty of flow but when I move them both to hot (they're both mixer taps) the flow drops to the point that there is only enough water to get a decent flow from one tap and the other is down to a trickle. (The same thing happens in the bathroom on the top floor as well with a different style mixer tap and a shower)

Could you let me know what the possible cause of this could be?

kevplumb · 10 Sep 2010

combi boiler

Ian_P · 10 Sep 2010

Sorry - should have said its a combi boiler, old and knacked. The whole purpose of trying to improve the water flow is to justify spending more on a bigger bolier and so we can run both our showers.

Ian_P · 11 Sep 2010

Having previously searched the internet and found nothing I discovered the answer to my question which is that my 24kw combi boiler can only produce enough heat for one shower (pretty much).

There's a few things I'm still not clear on:

i) Lots of the boilers talk about flow rates in L/min to raise the temperature by 35 degrees. Does the boiler always look to achieve a set temperature and therefore will you get lower flows in winter than summer or does it just provide whatever heat it can achieve and will you get the same flow in winter and summer but just at varied temperatures?

ii) When measuring the flow rate for my system should I measure just the ouput from one tap in a minute or should I measure the total amount of water I can get from the system in a minute (e.g. open up the tap in the kitchen, the bathroom etc)

iii) I've got a feeling that the gas pipe supplying the boiler is too small (this is what the BG guy said when he came around). What is the effect of this? Does it just reduce the amount of gas to burn and therefore we don't get as much heat or does it actually effect the flow rate?

Thanks for any help

Ian

ChrisR · 11 Sep 2010

i) They vary, but most have a flow restrictor of some sort to keep the flow down so you get water hot enough for a bath (45º) in the winter.
about 8 litres/min fir a 24kW. On some combis you set the maximum temp the water is allowed to get to, or the gas is turned down internally.

ii) Measure everything. Add the simultaneous flows, per temp, together. If one tap is much better than another you hae a restriction.

iii) Maybe no effect on an older boiler, but can stop a new one firing. Can also reduce the heat getting into the water.
Measure the Gas Rate (see FAQ wot I rote), with the boiler on full HW.

Ian_P · 11 Sep 2010

Many thanks for your help, I finally feel like I'm making some headway on this problem.

looneyfitter · 11 Sep 2010

all new combis will require a min of 22mm pipe for the gas, 28mm for part of the run dependant on the length of the run.

If your going to pay out for a new boiler matey, id be looking st the viessmann 200w 30kw or upgrade to the 35kw(not much extra) Worth every penny. What size have you upgraded the main too? I pressume you have lead at present?

Ian_P · 13 Sep 2010

Thanks for that info. We upgraded because of lead and although I don't know what size the new pipework is we have a good flow of water with both showers running so I am assuming that it's now a case of how much water the boiler can handle.

I was skeptical about the Viessman suggestion as I'd never heard about them before but must say they look excellent and I really like the sound of the weather compensation - we have a problem in our house with the house feeling noticeably colder when the radiators are off - I think due to temperature gradients creating drafts (and yes, I've done everything to try an reduce these). I've rung two installers from the Viessman website and they are coming around this week and next to quote. The second guy said "it's refreshing to get a call from someone who wants to fit quality and not the same old stuff" so I'm looking forward to seeing what they say. Maybe I'll change my mind when I hear the damage!

looneyfitter · 13 Sep 2010

The new 2010 Viessmann 200w 35kw with the weather comp option is £1000+vat dead. Then to flue at £50 notes. I have to say tho, that is with my discount.

Id be prepared to pay around £1600-1800 notes for a combi-combi swap plus the gas upgrade.

HTH

Ian_P · 13 Sep 2010

Sounds reasonable to me. I'd been quoted £1600 to fit a Glowworm (can't remember the size, I think it was 37kw) which I wasn't convinced about - more the type of boiler than the cost. If it comes in near that then I would be impressed.

looneyfitter · 13 Sep 2010

Id have come and done it for you matey but im a bot of a trek away. The 100w is also a good boiler. But for the few etra pennies well worth going for the 200. Make sure oyu get the NEW VERSION to mate. You may have to wait another month for it tho as VIESSMANN are not releasing it until there stocks have been sold. I got mine because im an installer and know the rep well.

Not a fan of GLOWORM at all.....

HTH

Ian_P · 13 Sep 2010

thanks again - I've got the first guy around on Friday so I'll let you know how I get on.

Agile · 13 Sep 2010

Is the "new" 200W being supplied with a "new" vitronic 200 already fitted???

Rather than a constant temp unit that has to be thrown away???

I see some confusion will reign while both old and new versions of the same numbered boiler and controller are being supplied.

Tony

looneyfitter · 13 Sep 2010

No mate you have to choose one or the other.

Ian_P · 24 Sep 2010

The first guy has been around and he said that he thinks the 100 is a cheaper and equally suitable alternative to the 200-W and suggested going for this. He also suggested considering the 222-F with it's storage tank as he doesn't think a 35kW boiler is able to get a decent enough flow for two showers.

Lastly he was less than convinced about weather compensation - I didn't really have a chance to dig on this so can't explain why.

My gut feel is that I would rather go with a 200-W (because I get the impression they're a slightly more sophisticated system) with weather compensation (because I think the physics of these are great!) and accept the fact that it's a compromise as the 222-F is probably the ideal solution for our house. I might not be able to have two showers running felt pelt but at least I'm likely to have a warmer house.

Quotes were about £1800 for the 100-W with weather comp and magnathingy (permanent thing to clean the system) or about £3000 for 222-F as above.

Anybody massively disagree with my view?

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Poor hot water flow

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