# hot water at below minimum flow rates on a combi?

#### mymedi

Hi, everyone,

I was wondering if someone might be able to comment on whether the following setup would work?

Every combi boiler has a minimum required flow rate at which it fires - if the flow is below this number (2.3-2.8 l/min or so) the boiler doesn't fire in order not to overheat.
However, this creates the issue that if the tap is only half-open, the hot water never comes - it stays cold because the boiler is not working. But if you have two taps half-open, then everything works fine.

To give an example, when washing up in the kitchen sink I don't want water splashing all over, so I run the tap just enough for it not to be too strong. At this rate, the flow (as measured by the flow cup, hot water only) is just under 2 l/min...

How would I go about making the boiler work even when the flow is this low?
I was thinking of fitting a flow gauge onto the hot water supply line to read the current flow. If the flow was within a range of 0-3 l/min, the gauge would activate a brass pump that is connected to a loop branching off the hot water supply and returning not sure where - maybe to the waste drain (I know it's wasteful...)?

What this crazy setup would do is effectively act as a second half-open tap - when the flow is low, it would boost it, thus making the boiler fire and heat the hot water...

So, would this work?
And if so, what specific parts/brands would you recommend for the flow gauge and for the pump?

Many thanks!

The minimum flow rate is not to prevent the boiler overheating.

It is based on the design of the flow switches and the boiler makers assumption that no normal person wants less than 2 Li/min of hot water.

To avoid waste you are not expected to wash under a running tap anyway.

There is a further problem and that is the modulation range of boilers is only about 3:1 so they cycle if delivering less than a third of the max output causing variations in the temperature.

Better for you to stop using a running tap. But if you must then drill it out or replace it to allow a higher flow rate before splashing.

Tony

At such a low flow rate then providing the boiler registers the demand it would simply cycle the burner on/off leading to erratic temperature control.

If you really want to run a hot tap at such low flowrates your only option is to select a boiler with a very high modulation range, for example the Vokera Unica i. This has a turndown ratio of 10 to 1 and will still operate the hot water at very low flowrates (2 litres/min). Alternatively you'll need a hot water cylinder.

I would be inclined to fit a small electric storage heater under the sink - 10 to 15 litres, the smaller the better. This is fed from the combi. When you draw off small amounts the water from the electric heater is used. Draw off large hot water volumes and the combi cuts in pushing hot water in to the electric heater, so it does not cut in. This also gives instant hot water at the taps. The cost of running the electric heater is small as a lot of the time it is being fed with hot water from the combi. A pump also uses electricity and does not give instant hot water at the tap.

Here is a 10 litre Ariston undersink heater for about £100.
http://www.heatandplumb.com/acatalo...6vwRr0GI-6ETaqWltK73AZJQcfkMx2rZxvsB73Jvw_wcB

AlexCarp,

You have mentioned low loss headers and thermal stores in previous posts, now the electric under sink heater.

Some of your postings have been attempting to belittle others.

This is beginning to follow a pattern. I'm wondering whether you have been on this forum under some other aliases?

Such as Big Burner, Dr Drivel, Watersystems etc?

AlexCarp,

You have mentioned low loss headers and thermal stores in previous posts, now the electric under sink heater.
Yes. Good aren't they !
Some of your postings have been attempting to belittle others.

This is beginning to follow a pattern. I'm wondering whether you have been on this forum under some other aliases?

Such as Big Burner, Dr Drivel, Watersystems etc?

Some of your postings have been attempting to belittle others.
What world are you in? Undersink water heaters are common, it may be the perfect solution. It is better than others for sure. Did you think of it? No.

There was no point to your post whatsoever. Why did you write it?

You can have the full credit for mentioning undersink heaters, it is genius and I'm surprised no one thought of it before. Especially the many manufacturers of said items.

I wrote the post because I was reasonably confident you are the same person who has been thrown off this forum umpteen times under different usernames.

I am now more than reasonably confident.

He's as transparent as air Simon.

You can have the full credit for mentioning undersink heaters, it is genius and I'm surprised no one thought of it before. Especially the many manufacturers of said items.

I wrote the post because I was reasonably confident you are the same person who has been thrown off this forum umpteen times under different usernames.

I am now more than reasonably confident.
Thank you for giving me credit. Are you Sherlock Holmes? There are strange ones here for sure.

After reading quite a few interesting threads (really enjoyed the Low Loss Header discussion!) I realize that AlexCarp may be either a complete wind-up merchant, or just a plain idiot, but I actually found his suggestion here quite interesting!!!

An under-sink electric storage heater fed by a combi is not in my opinion the most stupid thing I've ever heard... Gives a buffer of instant hot water at as little flow-rate as you like, and is replenished by the combi, whose supply may well be cycling hot and cold at very low feed rates as the boiler cycles! This solution would also make an awful lot of sense where a tap is a substantial distance from a boiler, where it may normally take over a minute to achieve a flow of hot water as a Combi winds up!

Just my two-peneth!

In that case you will be sorry to hear that he has been banned!

The rest of us are celebrating!

Would it not just be easier to fit an aerator on the tap? Then it doesn't splash everywhere? Sometimes the best solution is the easiest and cheapest .

In that case you will be sorry to hear that he has been banned!

The rest of us are celebrating!

So now he'll just come back using his what, fifth or sixth new pseudonym?

Seems like good fun trying to spot his posts though... It's like DIYnot's own 'Where's Wally' (Yes the name Wally is probably very appropriate!)

The name I preferred was the "Dr Drivel".

Its a pity really because he was apparently a real commercial gas boiler design engineer.

Unfortunately, after retiring a long time ago he seems to have lost all sense of reality!

Tony

The name I preferred was the "Dr Drivel".

Its a pity really because he was apparently a real commercial gas boiler design engineer.

Unfortunately, after retiring a long time ago he seems to have lost all sense of reality!

Tony

-I guess that explains why there was actually a thread of technical sense running through some of his postings.

-Oh well, they say 'If you're not on the edge, you're taking up too much space!'

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