Minimum flow rates for dhw

9 Sep 2014
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United Kingdom
We've recently had a new combi boiler fitted to an extended two bathroom house four bed house.
The house is heated using electric underfloor CH but the DHW services the two bathrooms, downstairs wc, kitchen and three towel rails.
The boiler is a Viessmann 100-W 35kW with a flow rate of 12.3l/min.
The problem is when two baths are in use the flow rate is halved to approx. 6l/min which is not acceptable.
The DHW system was a contractor designed item and they are insisting together with Viessmann that the boiler is sufficient for the property.
Are there minimum flow rate requirements or codes of practice that I can reference to prove the system installed in not adequate or is it a subjective issue?
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Another case of people thinking combi's are the be all and end all of heating systems. thing is people don't realise their downfall until it is too late and hot water performance is less than satisfactory.
Unfortunatly there is not much that can be done to improve your flow rate, unless you change to an unvented cylinder which will give you the performance required to run 2 baths at once (provided your incoming flow and pressure are a minimum of 24lpm).

The problem is when two baths are in use the flow rate is halved to approx. 6l/min which is not acceptable.


That is vaguely amusing!

YOU accepted, and probably specified, a combi with a flow rate of 12 ki.min.

Then you whine when it performs exactly according to its specifications.

Anyone in the trade knows that a combi in not suitable for anywhere with two bathrooms!

But you seem to think that you know better, probably to save your money, and we have to do what the customer wants because "the customer is always right" and we lose the job if we don't!

I lose jobs because I will only install what is right for a property.

The DHW system was a contractor designed item
Tony, did miss that bit? You then assume that he probably specified the boiler.
Then you go on to insult the man. I despair sometimes.
Don, the boiler is suitable for single point operation, as are most combis, and I can only suggest you stagger your bath times. Not a bad boiler.
Obviously if you stipulated that you require two baths to run at the same time, then you may have a case against the contractor.
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I read that but did not really know how to interpret it.

Regardless, I always expect anyone making a significant purchase of over £1000 to take a significant interest in what is being offered and to ask all the relevant questions and consider any other alternatives.

Luckily I would never be in that situation as I always recommend a storage system for a two bathroom property and always explain the pros and cons of any system and any other alternatives.

Tony, what’s with the arrogant attitude mate?
Forums are intended to provide/share information are they not?

A combi system was specified by the architect with no flow rates just temp rise
The contract administrator approved the installed system for payment which we did
The contractor designed the system and no information re. flow rates was to my knowledge provided

The Water regs of 1999 in Table 4.1 give recommended design and minimum flow rates from fittings and appliances but these seem very high. Also the regs go on to state that for combo boilers the rates may not be achievable and the system should be designed accordingly.
Can the rates given be used to show the fitted combo does not perform to the requirements and/or are there other sources of info?

You seem to have been way too trusting to the architect!

The boiler power will determine the flow rate at any particular temperature rise. It is a common practice to quote flow rates at a 35 C temperature rise. So your boiler will give more than 12 li/min in warm weather and less in cold as the temperature of the incoming water varies.

In my experience most architects have very little knowledge of heating and hot water systems and should not be specifying them.

If yours did then normally a heating contractor would usually try to put them right. But some are so short of work so will do what has been specified without question ( often knowing that further remedial works will be needed to give them more work ). The contractors also know that legally all they need to do is exactly what is in the specification so they will be blameless ( well legally at least! ).

So in my view any case you have is against the Architect and not the contractor who only did what was in the specification.

The good news is that Architects usually have good professional indemnity insurance. So making a claim is likely to be easy and fruitful.

In my unpaid opinion you need a storage system with a minimum capacity of 50 li plus 50 li per person. Probably a 300 li unvented. BUT the incoming mains supply has to be capable of adequate dynamic flow rate. If not it may need to be upgraded.

Dan, whilst you live in a 2.5 person household I think that you have a 300 li cylinder! Even though a 210 li would be adequate in your case.

The storage capacity required is based on the number of people and not the number of bathrooms.

So that the entire household can have a bath each is quick succession using the stored water without running out of hot water and during which time the boiler can continue to provide heating without interruption.


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