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"
.