How to set flow rate through HW cylinder coil

[Johntheo5]

It is set on curve 15, I think, the menu isn’t all that clear.

If I understand correctly I need to have a higher curve number to raise the flow setpoint.

Judging by the graph there needs to a compromise between keeping flow rate set point at highest outside temp sufficient to minimise short cycling and the flow rate set point at the minimum temp staying low enough so boiler return temp is below 56deg so boiler condenses.

If its curve 15 (the 4th one) then the boiler flow target temp should be 30C at a outside temp of 11C, if the target temp is 45C at a outside temp of 11C then that corresponds to ~ curve 20 (the 9th one).

That’s very useful, I’ve been trying to work out the maths from that but struggling to work it out.


Great thanks for confirming can be a bit confusing,

Even after 50 years or more I still don't know whether they should be written in upper or lower case or a mixture of both but I think I do know how to calculate them, I write them down in lower case.
kwh is kwX(time in hours) so if a 18 kw boiler ran at full (power) output for say 40 minutes then the energy is 18X(40/60), 12 kwh.

HW Cylinder losses are often displayed on a energy label (rating) in watts, w, which in itself is a bit confusing since the display is actually power, you might see say, 75 w on the label which is a energy loss of 75*24/1000, 1.8 kwh per day, probably a bit less confusing if they included both on the label, but they all do include the energy loss in the spec sheet.

Some HW cylinders now have a vacuum casing around them which give very low heat losses.

 

[Notch7]

Indeed. I think sometimes people overlook the comfort advantage and concentrate purely on the efficiency.

As part of your set up, do you also have a room thermostat. If so, which one?

The plumber fitted a basic heating pack, the room stat is the Drayton RTS1

I imagine the room could be causing cycling.

I might change to the Tado wireless kit V3+ kit.

overlook the comfort advantage

Yes I’m surprised, I had read about people mentioning comfort but it actually makes a very noticeable difference, it’s a gentle warming heat, the radiators are warm to touch and it’s possible to sense very slight radiant heat


Today I’ve got:

7 deg Weather comp
20 deg room stat
48 deg flow temp
43 deg return temp

My weather station says
8.2 deg outside temp
19.7 deg room temp (same room as heating stat)

 

[SimonH2]

Re TRVs throttling back and the return temp obviously falling, then because the flow rate through the boiler also falls the flow temperature will again tend to rise so boiler will again modulate down, if it can, until it cant reduce the power any further, see example below.

Except ...
Few systems actually work like that except at relatively high load. Especially when the boiler is grossly oversized because it has to be to act as a moderately effective combi, it cannot handle the low flow rates when the TRVs are throttling back. Hence while the return temperature from the rad loop falls, the return temp into the boiler once the bypass flow is mixed in RISES as heating load falls. AIUI, many boilers detect this (reducing delta-T) and reduce the flow temperature - the idea being that by reducing the flow temperature, it will better match demand. It will still need to start cycling once demand reduces below it's minimum range.
And where the rads are "a bit marginal", it could well make the difference between condensing at a lower overall system temperature or not condensing with a high return temperature.

Assume 10kw of T50 rated rads operating with a heat demand of 6.88kw (at 20C rooms temp) and assuming a target flow temperature of 65C, the boiler will operate (assuming no bypass), flowtemp/returntemp/dT/flowrate, 65C/50C/15C/6.574LPM and now assume the heat demand falls to 5.6kw, the flowtemp/returntemp/dT/flowrate, will now be 65C/39C/26C/3.086LPM

Which on some boilers would cause a lockout on high delta-T, 65-39 = 26˚C. Hence why a bypass is needed to prevent the flow rate getting that low.

, if for some reason or other the boiler couldn't or didn't modulate down then the dT would rise to, 6.88*860/60/3.086, 31.96C resulting in a flowtemp of 39C+31.96C, 70.96C and burner shutdown, any bypassing will not stop this occuring since the bypass is at the flow temperature.

No, the bypass will not stop the flow temperature rising, but it will prevent the flow rate dropping so as to create a delta-T well outside of the boiler spec. TBH, it's been a long week and a long day so I CBA to try and work out what the bypass rate, and boiler return temperatures would be for the delta-T to be limited to 20˚C.

BTW - if you do know of a boiler which is specified to operate with a delta-T of 32˚C then I'd be interested as it would improve the efficiency, and simplify the system, with my thermal stores and both my own house and rental flat are overdue upgrades.

That’s very useful, I’ve been trying to work out the maths from that but struggling to work it out.

kW is a measure of power, or energy flow rate. Analogous to to selling 10 boxes of eggs per hour (i.e. rate of movement of boxes of eggs 10 boxes per hour).
kWHr is a measure of energy. At the above rate, if the shop were open for 3 hours, then they'd have sold 30 boxes of eggs (10 boxes per hour * 3 hours = 30 boxes)
Like most engineering or scientific formulas, you can swap things around according to what you are needing to work out.
In the above, if you knew that you'd sold 30 boxes in 3 hours, then the rate of selling them is 30 / 3 = 10.

Getting back to your figures, the manufacturer states 2.27kWhr (energy) in 24 hours (time). So the energy flow rate, or power, is energy/time, or 2.27kWhr/24Hr = 0.095 kW, or 95W. Had they quoted the loss rate (95W), then you'd calculate the loss per day by 95W*24Hr = 2280Whr (the difference is just due to rounding), or 2.28kWhr. "k" here is simply the SI multiplier (or prefix) "kilo", 10^3, or 1000.
As an aside, kilo is the only commonly used SI prefix greater than unity that is lower case, others (e.g. M for mega, or 10^6) are upper case. All prefixes below unity (e.g. m for milli, or 10^-3) are lower case. Upper case "K" is the unit of temperature, the Kelvin. Hence why case matters - and I have to admit that I'd been using "Hr" when I should have been using "hr" .

 

[Johntheo5]

. AIUI, many boilers detect this (reducing delta-T) and reduce the flow temperature - the idea being that by reducing the flow temperature, it will better match demand. It will still need to start cycling once demand reduces below it's minimum range.

That would be a step in the right direction, as I (think) stated, the newer Vaillants (series "6" anyway) can have the dT user set to a max of 20C but they achieve this by reducing the pump speed up to to a minimum of 50% and do not reduce the flow temperature.
The Vaillants have a very high flow internal ABV which defaults to 2.5M and can only be adjusted to a max of 3.5M resulting in very high bypass flows as the boiler flow falls with zone valves, TRVs etc throttling down, I have seen several posts where the users set (balanced) the rads to give a 18C to 20C dT with a system return of say 40/45C only to find that the internal bypass is opening so much (even when set to 3.5M) to give a actual boiler return of ~ 55C, not very encouraging re condensing effect.
To put it it context, a boiler, at 5kw output with a flow/return/dT of 65C/37/28C will have a flowrate of 2.559LPM and will require a bypass flow of 1.024LPM (at 65C) to give flow/return/dT 65C/45C/20C with a flowrate of 3.583LPM and still give reasonable condensing.

Again, I wonder why the return temperature control is not utilized more, or at least tried, on the Vaillants, we would also then know if the flow temperature is controlled to achieve this, it may well be?.