How to set flow rate through HW cylinder coil

[Lower]

I moved the flow and return pipework to my hot water cylinder at the weekend to create more room in the airing cupboard. The return pipe is fitted with a gate valve to act as a flow restrictor and i used thte gate valve to isolate that leg of the pipework.

Unfortunately, i lost count of the number of turns it took to close the valve so i can't simply set it back to where it was.

Can anyone advise me or point me in the right direction on how to set the flow through the how water cylinder coil correctly? I can't find much on the web to advise me on how to do it. I have a Worcester Bosch Greenstar 35 CDI system boiler.

Thanks in advance.

 

[fezster]

Two trains of thought on this:

1. Fully open to give HW priority so that the cylinder heats up as quickly as possible.

2. Balanced so that the heating is still able to function concurrently with the cylinder. This is trial and error. With both HW and heating running, close the valve down until all of the radiators start getting hot and get sufficiently hot (if not, close further).

 

[Notch7]

Two trains of thought on this:

1. Fully open to give HW priority so that the cylinder heats up as quickly as possible.

2. Balanced so that the heating is still able to function concurrently with the cylinder. This is trial and error. With both HW and heating running, close the valve down until all of the radiators start getting hot and get sufficiently hot (if not, close further).

Sorry to re open an old thread, but this is exactly the same thing I’m thinking about.

I’ve had a hot water priority system fitted with Ideal vogue boiler and an ultra fast recovery cylinder. The heating engineer has included a gate valve to slow the flow through the cylinder.

The gate valve is set sort of 3/4 shut, when the cylinder stat calls for heat the boiler sets flow temp to 80deg and return temperature starts around 65 deg then climbs as cylinder gets closer to its temp setting.

Should I have the gate valve fully open? I am thinking the faster flow rate would shorten recovery time.

 

[MNW67]

I am thinking the faster flow rate would shorten recovery time.

It is something I have also wondered about.

A faster flow rate certainly means higher coil output.

But it would mean a higher return temperature and a smaller dT. Modern boilers are designed to work at dT20. It has previously made me wonder whether the higher return temperature might cause the boiler to modulate down prematurely. And then my head explodes!

 

[ericmark]

I would not think it is that simple. It is with my oil boiler, but a gas boiler modulates, so if return water too hot, boiler starts to cool down.

So it will depend on the hot coil, as said I have oil, and it takes 20 minutes from turn on, to the return water temperature turning off boiler, but tank is not hot, so then the boiler will cycle on/off around 10 minutes off, and 10 minutes on to start with, so 19 kW boiler so seems hot coil can sink about 10 kW, but as the cylinder heats up, the on time gets shorter.

That is summer with no central heating but since my boiler does not modulate, hot return water does not matter.

Winter likely the water around the hot coil can be very slow, as loads of time to heat up the DHW, so only needs to be cracked open.

Summer is where it matters, the less time the pipes are hot the better, in my case summer does not matter, as use an iboost+ and spare solar, so boiler turned off.

However there can't be one answer as too many variables. But to keep water hot will mean for me half hour per day, warm every other day, so letting it heat DHW first will not delay heating much.

 

[Exedon]

I’ve had a hot water priority system fitted with Ideal vogue boiler and an ultra fast recovery cylinder. The heating engineer has included a gate valve to slow the flow through the cylinder.

How's the hot water priority been done ?
X plan using a normal open valve?

 

[Notch7]

How's the hot water priority been done ?
X plan using a normal open valve?

it has a normally open valve on heating and normally closed valve on hot water

Im not 100% sure of how the controls are wired, I know the boiler has 2 switched lives one for hot water, one for heating -and the heating flow temp is determined by the weather comp

 

[SimonH2]

It is something I have also wondered about.

A faster flow rate certainly means higher coil output.

But it would mean a higher return temperature and a smaller dT. Modern boilers are designed to work at dT20. It has previously made me wonder whether the higher return temperature might cause the boiler to modulate down prematurely. And then my head explodes!

It would stop the boiler condensing as well.
For maximum heat transfer (and hence fastest recovery time for the cylinder), you want maximum flow rate - delta-T across the boiler and coil will be low, but delta-T between coil and cylinder contents will be maximised and so power drawn from the boiler will be max. Unless it's designed to compensate for the poor* design of boilers and heating systems by controlling the return temperature instead of flow temperature, then it'll only start modulating down when the cylinder is getting hot.
If you want maximum efficiency, then you want reduced flow rate to increase the transit time and hence reduce the delta-T between coil and cylinder contents. The result will be that the return temperature spends a lot (if not most) of it's time low enough for the boiler to condense. However, in practice that's likely that you hit the lower flow rate limit of the boiler before you reach the optimum flow rate.
* I won't repeat it all again.

 

[MNW67]

Unless it's designed to compensate for the poor* design of boilers and heating systems by controlling the return temperature instead of flow temperature, then it'll only start modulating down when the cylinder is getting hot.

There have been discussions before on here about whether boilers modulate based on flow temperature or return temperature. I have always thought flow temperature was the more obvious. Others seem to disagree. Is there any hard info?

 

[Exedon]

it has a normally open valve on heating and normally closed valve on hot water

Im not 100% sure of how the controls are wired, I know the boiler has 2 switched lives one for hot water, one for heating -and the heating flow temp is determined by the weather comp

Normally wired so cylinder stat demand feeds normaly open valve and closes it and feeds hot water connection on boiler.
Boiler will then run at 80C until demand satisfied,
You want the hot water heat up to be as fast as possible to get heating back on and boiler condensing again.
Have you got the outside sensor fitted?

 

[Notch7]

Normally wired so cylinder stat demand feeds normaly open valve and closes it and feeds hot water connection on boiler.
Boiler will then run at 80C until demand satisfied,
You want the hot water heat up to be as fast as possible to get heating back on and boiler condensing again.
Have you got the outside sensor fitted?

many thanks

Yes the outside sensor is fitted -this afternoon its showing outside temperature at 11 deg and when the room stat is calling for heat the boiler was firing at around 55 deg flow

the cylinder stat called for heat earlier on and I noticed the boiler started up and ramped up to 80 deg

yes I believe the cylinder stat is wired so it closes NO valve on heating

 

[Exedon]

We have now done a fair few systems like this including a small new build site all working well.
My own system is 4 zone (2 are underfloor) plus hot water x plan vogue system boiler outside sensor
Flow temperature in low 50s seem normal
I've a cylinder with high recovery coil even running a full bath dhw runs for less than 10 minutes.

 

[MNW67]

I am thinking the faster flow rate would shorten recovery time

Below are some figures from another manufacturer for coil outputs against flow rate. The increase in output isn't linear across all cylinder capacities, and I don't understand that. I would have thought it was a straightforward relationship. There is a limit to how fast a flow rate you can achieve as the resistance of the coil starts to rocket (depending on its diameter).

If we assume your boiler was initially running at 100% modulation i.e. 18kW, and you said the initial dT was 15C, then the flow rate you are currently getting through the cylinder would be 17 litres per minute. To be sure, though, we would need to double check the boiler modulation against the dT. I vaguely recall another poster with a Vogue could get historic data in the form of a chart. Or you could do another test.

 

[Notch7]

Below are some figures from another manufacturer for coil outputs against flow rate. The increase in output isn't linear across all cylinder capacities, and I don't understand that. I would have thought it was a straightforward relationship. There is a limit to how fast a flow rate you can achieve as the resistance of the coil starts to rocket (depending on its diameter).

If we assume your boiler was initially running at 100% modulation i.e. 18kW, and you said the initial dT was 15C, then the flow rate you are currently getting through the cylinder would be 17 litres per minute. To be sure, though, we would need to double check the boiler modulation against the dT. I vaguely recall another poster with a Vogue could get historic data in the form of a chart. Or you could do another test.


View attachment 332461

the hot water cylinder I have is a McDonald ultra high recovery open vented

I cant find any info on it regarding coil output versus heat rate. As far as construction goes the coil is 2m2 and it has integron (ribbed) coils.

standing loss doesnt seem great its quoted as 2.27kw loss in 24hrs, recovery time at 18kw and Dt50 is 23 minutes.





It seems to me that hot water cylinders are pretty inefficient if they have a standing loss of over 2kw in a 24 hour period, given that heating is need for 6 months of the year and hot water is needed 12 months of the year

I guess it wouldnt hurt to make my airing cupboard well insulated to minimise standing loss. I might insulate hot water pipes too we typically run the hot water tap often to wash hands, do a bit of washing up etc and that means it might be worth minimising standing losses in the pipes supplying the taps

Given that any gas boiler doesnt go into condensing mode when heating hot water, it seems to me this is the least efficient part of the system, I guess since most of the energy sage is heating there isnt much incentive to make hot water side more efficient, the key is to get it done fast and make the boiler most efficient for heating

 

[Mister Banks]

You could set the thermostat on your HW cylinder to 45c and then set the temperature on the boiler to 55c when feeding the HW cylinder then the boiler will condense. You just need to increase the cylinder temperature to 60c (and of course the boiler temperature to say 70c) periodically to kill off any bacteria. Another way if you boiler can't increase the water temperature would be to set an immersion heater to come on using a timer.