Combi with stored water tank? To solve water flow issues.

[NY]

Yes. I would rate hot water solutions in the following order (most preferable to least preferable):

- mains-fed combi

- mains-fed cylinder

- tank-fed combi

- tank-fed cylinder

Only consider tank-fed if there really is nothing that you can do to increase the flow of cold water to the cold taps and (via the boiler) the hot taps. I regard a mixer tap in the "fully cold" position as a cold tap, and likewise for it in the "fully hot" position being effectively a hot tap.

If there is any pressure-reducing valve for a water softener, it could be the reason for poor flow rate of hot and cold water. I hadn't realised that there was a statutory minimum flow rate below which the water company has to provide a solution. I did some work (not water-related) for a couple who apologised for the time it took to fill their kettle when they were making me a coup of coffee, and said it was due to them having a spring-fed rising main - and this was in a reasonable-size village, just off a trunk A road, so not a farm miles away from anywhere.

[NY]

Our rising main goes through a water filter (a large blue plastic cylinder containing a tubular cylindrical filer element). When the input to this developed a hairline crack, I temporarily connected the input and output pipes so we still had water in the house, while I searched for the correct replacement adaptor. And I measured flow rate like that and with the filter (once I'd got the adaptor). In both cases, it was very similar - from memory I think it was 18 litres/min without the filter and 17 with it.

[John Rumm]

Not even close :-) The external volume of that is about 177L if my sums are right.

(my Unistor is about 5'2" tall, and 22" diameter, and that hold 210L)

The cold fills from the bottom, and normally they are designed to not introduce much turbulence - so they tend to remain quite well stratified without too much mixing.

[John Rumm]

ISTR there was at one time a water regs requirement that you don't in effect pump directly from the mains supply - which is what you would be doing if pumping from an unvented cylinder.

(you can also get "accumulators" basically a big mains fed cylinder with a large pressure bladder in them that acts like a large expansion vessel these can supply mains water at much better than mains flow rate for a period of time. Handy in cases where there is a low flow rate, but adequate pressure, and not much prospect of changing the supply)

Yup, sounds like a nice identifiable task with easy to verify results - ideal for delegation!

[John Rumm]

In my previous place I replaced the old (very poorly done) gravity system with a 35kW combi when I did a loft conversion[1] (the old cylinder and header tanks in the loft, and so needed to go).

[1] The cold main could do 17lpm, and the boiler could get close to that in the summer - somewhat less in the winter - but it was in most respects an improvement. Showers were much better (the old system tended to draw in air and splutter - it was also carefully designed to run out of cold water before hot, and scald you if you were not careful!)

The current place with a 32mm "fire hose" MDPE water main and 6.5 bar pressure is very well suited to the unvented cylinder however, and that trounces any system I have used in the past for flow rate delivery.

[NY]

On 05/03/2023 20:11, John Rumm wrote:

The volume is pi.r^2.l, isn't it?

22" diameter is 11" radius or 11*2.54 = 27.9 cm. 5'2" is 62" or 62*2.54 = 157.5 cm.

So volume is 3.14 * (27.9)^2 * 157.5 = 384,963 cm^3 = 384 litres.

Maybe the internal volume is considerably less than the exterior, but surely not to that extent.

I have "fond" memories, long before the days of mobile phones with calculators and access to Google, of my dad and I working in our holiday cottage, trying to calculate the approximate volume of a hot-water cylinder (no helpful label stating is volume) to estimate its weight and hence decide whether a couple of baulks of 2x4" timber would take its weight. We had a tape measure which was only calibrated in inches. We had no calculator. We rounded the figures and ended up with an approximate volume in cubic inches. That is where we came unstuck. Neither of us had the remotest idea (even very approximately) of the conversion factor from cubic inches to gallons, from which we could have used the approximation that a gallon of water weighs 10 lb. (The answer is 277.419 cu in / UK gallon - trust the imperial system to relate linear and volumetric measurement by a number which is an obscure number that is not even an integer!) We ended up converting the lengths to centimetres (1" is approx 2.5 cm - maybe even 1" = 2 cm is good for a vague approximation; pi is about 3), getting an answer in cubic centimetres and hence litres/kilogrammes, and then converting back to pounds because my dad could think better in pounds. We just needed to know "is it 50 lb, 100 lb, 1000 lb" to decide whether the wooden battens would break and or rip out the screws that fastened it to a timber frame acting as a room partition. What we should have done is go up the road to the phone box, phone my mum at home and get her to do the calculations - never thought of that.

We must have estimated and judged correctly because the cylinder is still in place, above the stairwell, some 50 years later.

[John Rumm]

Indeed.

I don't actually know what the shape of the containment vessel is inside the outer cylinder is - but would not be surprised if it had domed or dimpled ends to cope with the pressure - so that will lose some volume. There is also fairly significant lagging in there, since the case is not even warm to the touch. Also with a fast recovery cylinder there will be some volume occupied by the large heat exchanger coil - presumably they don't include the volume of that in the 210L.

Yup sometimes it is too easy to focus on the problem and not think about simpler ways that save actually having to solve it.

Probably long enough to count as "safe" then! :-)

[John Rumm]

I think the required minimum is something in the region of 1 bar pressure and a flow of 9 litres per minute.

[Robin]

Some companies may promise more but the England-wide legal minimum is in Statute:

"An English water wholesaler must maintain, in a communication pipe serving premises supplied with water, a minimum water pressure of seven metres static head."

reg. 17G of the Water Supply and Sewerage Services (Customer Service Standards) Regulations 2008 as inserted by The Water Supply and Sewerage Services (Customer Service Standards) (Amendment) Regulations 2017

But a much more accessible guide to what companies must do is Ofwat's guaranteed standards scheme (GSS) :)

[David]

<snip>

Mildly confused yet again. From up thread "The boiler is a Worcester Bosch 38 CDi which is rated at

19.1 l/min for a 30C rise.". So, especially in the summer, to take mains water from 10C to 40C I would need a flow of 19.1l/min. Which I assume should usually be possible given that this is a common combi boiler.

Ah, well.

Dave R

[John Rumm]

I think you might be reading more into the spec than you should.

The comment on flow is just a measure of the *maximum* heating performance of the boiler give then laws of physics... basically just the sum:

38kW = 38,000 J/Sec

x 60 = 2.28 MJ/Min

Divide by the specific heat capacity of water, and the temperature rise:

2.28x10^6 / 4200 / 30 = 18.1 kg/min (or lpm)

So that is the maximum amount of water it can deliver in a minute at that temperature rise. I can however do less.

Depending on what temperature you have set for DHW, as you reduce flow rate (either because you have throttled it with a tap, or it is limited by the supply), the temperature will rise. When it reaches the set point temp, the boiler will reduce output power to keep it under the limit. If you reduce it so far that it would exceed the set point temp and the boiler has modulated down to its minimum output, then it will cycle the burner on and off to maintain some semblance of average temperature not exceeding the set point.

So with a flow of say 9 lpm it could heat it by 30 degrees needing only

19kW, or heat it by 60 degrees using the full output power.

[Andrew]

And any decent heating engineer will (should) have checked the flow rate before recommending a combi in place of a stored system. If only.

Did he run 22 or even 28mm copper for the gas supply to the Combi (which is not located where the original boiler was) ?.

[Andrew]

If your incoming supply has sufficient pressure and flow then you should ?not need a shower pump. Combining a pump with an unvented cylinder sounds a bit iffy. Do the manufacturers of unvented cylinders have any special instructions for such a combination ?.

Potentially your shower pump could try to extract water from the cylinder faster than the incoming flow rate from the mains.

[Andrew]

Err, it depends on the number and age of occupants in the property (so by definition its size).

A combi would not be sensible in a 5-bed house with 2/3/4 teenagers in-situ. And if the water goes off, that's it, no hot water, no toilets, no cold tap in the kitchen.

[David]

Ta.

Next task is to look up the flow rate for the mixer showers.

I am sure I have the details somewhere safe.

A quick look at the first hit on a Triton bar mixer gives:

"Water minimum flow rate For best performance within the specified running pressure range a minimum flow of 8 litres per minute should be available to both inlets."

Hot supply temperature 55C -> 65C.

Flow temperature with a running bathroom sink hot tap settled to 60C but there is another notch on the temperature control.

This suggests a flow rate of significantly less than 19l/min! That is, if mains cold was going through at 19l/min running the boiler flat out I would expect the boiler temperature to be around 30C-40C if I have understood the numbers correctly.

I think I will wait until the weather is a bit warmer to turn the boiler off and start tinkering with the cold input, although I can run the central heating with the mains turned off, I assume.

At least I am finally engaging with the issue.

Oh, and I have a diagram of a horizontal indirect vented cylinder from a manufacturer. If this is of interest I could post this to the web page. '.doc' format - would this be acceptable?

Cheers

Dave R

[The Natural Philosopher]

They dont need em. Unvented cylinders come with pressure regulators by law I think and a lot of safety valves. Pumps come with pressure and or flow switches. The combination should stop the pump when the pressure is high. The tanks have a membrane or a bubble trapped in them or an external pressure vessel to avoid massive pressure build ups when heating

Indeed. You need a big header tank with pumps

[NY]

I read that spec as saying that if your flow rate is less than 19 litres/min, the boiler will be able to heat it by 30 degrees at that flow rate. If the flow rate is greater than 19 l/m, the boiler will not be able to manage that amount of heating and the water will be cooler.

It's not saying that your flow rate must be >= 19 l/m for the boiler to work.

[NY]

Is diameter of gas pipe really a limiting factor with combi boilers? My parents have a holiday cottage which is heated by bottled propane (2x47 kg cylinders connected together, with another pair in reserve). The gas pipes are micro-bore (smaller than 15 mm water pipe) and yet the boiler seems to work OK with the relevant pressure-reducing/cylinder-changeover valve. It's a combi which is used for heating water for the sink and washbasin; an electric immersion heater, cylinder and header tank is used for heating bath water. The shower is electric, though I'm not sure what power.

[NY]

I agree that resilience in the event of failure of supply is useful to have. If I had a tank, I'd only use it for toilets and maybe washbasins, but not for anything where a decent pressure was required.

My first house had a cylinder whose base was about 1 foot higher than the bath taps, with an integrated tank joined onto the top of the cylinder, so the tank was about 3 feet higher than the taps. The flow rate of hot water, even with 22 mm piping to the hot taps, was puny. A tank in the loft (maybe

10 feet above the taps) would have been better.

[alan_m]

For domestic natural gas no more than a 1 millibar drop between meter and appliance so yes the diameter matters but a boiler may work satisfactory with a greater pressure drop - but the gas installation may not meet regulations.

A propane cylinder may start off at 10+bar and the regulator reduce it to a _much_ higher pressure than available at a mains gas meter hence you can use micro-bore type pipe. Gas meter pressure is nominally 21 millibar.