Combi hot tap shuts OFF!!


Postby bellrope » Mon Jun 10, 2013 2:37 pm

Newly installed W/Bosch Greenstart Junior in loft, three hot taps, one first floor bathroom sink, two ground floor, kitchen / utility. Turning either ground floor hot tap completely shuts off first floor hot tap in bathroom? W/Bosch been to check boiler, all is OK and within limits.

15mm pipe to all taps from boiler, both grounjd floor taps can be on with tiny drop in pressure but impossible to get first floor hot tap to run at all?
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Postby proptech » Mon Jun 10, 2013 8:54 pm

Hi bellrope

If you mean that the downstairs tap is 'robbing' the upstairs one, then there's not a lot you can do about it.
The Junior 24 can only supply something like 8 or 9 litres/minute, the Junior 28 something like 10 to 11 litres I'm sure the engineer has checked this.
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Postby bellrope » Tue Jun 11, 2013 5:44 am

Boiler and installation is about 2 months old, Worcs/Bosch Engineer been to check boiler operation, spent 1.5 hours and confirmed all was fine and that the problem was with my "pipes" but other than that he had no idea why the ground floor taps were "robbing" my first floor taps? He said in all his years he had never witnessed this happening, only ever an equalisation of pressure across all open taps.

Plumber returned, tried several tests to ascertain why this is happening as all 15mm pipe runs from boiler to all taps, two ground floor hot taps will equaliser pressure, first floor hot shuts off. You can run first floor hot tap and cold water mains everyehere else and no change in first floor hot tap flow?

We live in two storey, ex council semi, three hot taps, electric shower only (no bath). At NO point have Worcester or Plumber advised incorrect size / power boiler, we even have one less radiator for this size boiler? Flabbergasted now.
bellrope
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Postby proptech » Tue Jun 11, 2013 7:37 pm

The thing is that with a combi, The water is 'throttled' down in order to achieve the temperature rise required. This being measured by flow rate, rather than pressure. So if the ground floor tap is providing that flow, then there is no more for the upstairs tap.
The only time there is equal pressure is when all taps are closed. Therefore with ground floor tap open and supplying the designed flow, the pressure upstairs could be zero.
I suggest that you could get a rough idea of the flow rate yourself without a flow meter. just use a marked bucket, and a stop watch.
I don't think you can expect the boiler to supply two taps at the same time.
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Postby bellrope » Wed Jun 12, 2013 8:47 am

Many thanks for all the reasoned responses. Worcester Bosch Tech support have confirmed that our boiler is way too small for our needs, in terms of outlet flow. I need to measure the flow of every tap / shower and decide if I can afford a combi that can deliver that level of flow to support, say, half of them open at once?

Now got fight on hands with installation company as they never surveyed site / our needs at all and thus just fitted the smallest / cheapest boiler to seem competitive.....
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Postby proptech » Wed Jun 12, 2013 8:14 pm

Thanks for the feedback. at least now you know what's going on. I think you really need to have the installation carefully assessed. A bigger boiler may not be the way forward if the one you have is correct for the heating load. You really need the advice of a good heating engineer.
Also is there anyone you know who also has a combi boiler, you could see how theirs works. For many years I worked on the old gas water heaters, the largest only provided roughly the output of your boiler, people seemed happy to expect only one tap to function at a time. The good old days !

I hope you get it all sorted.
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Postby Atticus Finch » Sat Jun 15, 2013 10:24 am

bellrope wrote:I need to measure the flow of every tap / shower

You need to know the incoming cold water flow rate and pressure. Pressure needs a special gauge but it can be estimated.

There are two places where you can measure the flow rate: Garden tap or Kitchen sink cold tap. The garden tap is preferable as many modern kitchen taps have restrictions in them.

Use a marked bucket and a watch with a second hand to measure how long it takes to fill a 10 litre bucket. Do it at both taps and take the higher one as the base line. This will tell you the incoming flow rate. Now turn both taps full on and measure again. If there is a big difference in the flow rate your pressure is not high enough to service two outlets at the same time.

Don't forget that when you have a combi boiler all outlets (hot and cold) are served from a single incoming supply. So flushing the WC will affect someone in the shower if the flow rate and pressure is not high enough. You really need a minimum flow rate of 20 litres/min to supply more than one outlet at a time.

As mentioned earlier the Junior can only supply a max flow of about 11lpm, which is good enough for a person on their own, but not for a family.

You say that all pipes are in 15mm, including the one to the bathroom, and the flow is OK to the other taps but not the bathroom. How long are the pipe runs? It may just be that there is a high pressure drop which would be reduced by using 22mm pipe.
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Postby htg engineer » Thu Jun 20, 2013 9:00 pm

That's normal for a combi, if you're wanting to use more than one outlet at once you need to be looking at a system boiler.

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Postby bellrope » Wed Jun 26, 2013 6:30 pm

Quick update, company returned to fit 29kw worcs combi boiler, made no difference, top taps still shut off once ground floor hot tap turned on, he left stating nothing else he could do. I suggested that was not good enough and that the amount flow required a much larger flow rate boiler, to which he stated that the boiler he had just installed would heat the street.

He suggested he remove the combi, fit a regular boiler along with a pressurised hot water tank, to which I said OK and he said he would work out the price for me, I asked why and he said because I would have to pay for it to be done......I suggested he go away and think some more about that and come back with another suggestion.
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Postby htg engineer » Thu Jun 27, 2013 7:11 am

Why does this cause an issue for you? there's thousands of people with combi boilers that learn to live with it. Combi boilers will happily supply one outlet, when other taps are used there will be a reduction in flow, sometimes resulting in no water at all from some outlets.

If you're trying to run more than on bath or shower at the same time, then a combi boiler is not for you.
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Postby bellrope » Thu Jun 27, 2013 7:33 am

Mainly as not at any time did the heating engineer explain this can happen and neither does Worcester Bosch agree it should happen if the property is correctly surveyed prior to installation and the appropriate Combi boiler chosen for the property and the intended use. Neither of which were done in our instance.
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Postby htg engineer » Sat Jun 29, 2013 8:40 pm

Did you ask for a combi boiler or did you ask for a new boiler/heating system and this is what they recommended/fitted?

The customer normally requests a combi as they're a lot cheaper to buy and install than a system boiler. A system boiler and a decent cylinder will cost twice as much as a combi boiler.

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Postby bellrope » Sat Jun 29, 2013 9:51 pm

Company has agreed to send engineer to meet installation company on site to try to understand what is going on and what solution is.
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