Ok, small capacity condensing boiler with a small range of modulation,
10-15kW ish.
Point 1, chill out on achieving a specific drop when balancing, you are looking for the same drop, not a specific drop.
Follow the FAQ method bearing that in mind, no matter what the absolute drop is. Also, TRVs should be fully open while balancing and open windows if a room is overheating as that will confuse the balancing.
I previously made the mistake of throttling down all my rads in an attempt to get a magic 20degC drop but the boiler wasn't happy as it was running with a restricted flow. Following helpful advice here I re-balanced the system using the FAQ method of opening everything then only controlling flow on rads that need it and the result is a happy system.
My flow temp is set by comfort level, currently around freezing outside and my flow is 62 with a return of 50 and the system is maintaining a comfortable room temperature so no need to increase the boiler stat. If it gets a bit more baltic I will bump up the flow temp, 70 will probably run a return of 55 but if I needed to I would run the flow at 80 (returning say 60ish) but not worry about any loss of condensing efficiency as it is a short lived event for extreme outside temperatures.
See above, once the system is balanced, turn it down as low as can be while still maintaining a) desired room temperatures b) adequate time to reach desired room temperature c) avoidance of boiler short cycling
Sorry, for not being Ed, this is free advice so you have to take who's available ;-)
OK, with the pump on lowest setting, I get a drop of about 15 deg C across the boiler. I've balanced the rads to a 15 deg drop. Will check today and see temps.
If the boiler makers says a 20C drop is fine go for that if possible. Keep temperatures as low as possible.
Too late now, but a superior boiler like a Broag (well priced) with OpenTherm control system with integrated weather compensation is the best way to go. They go full belt when re-heating DHW and low temps dictated by the outside temperature. They give superb economy.
For a supply water temperature between 70 and 90 °C, it?s quite common to design the plants for a ÄT = 20. This magic value has been adopted for many years and translated into local units (20 °C in continental Europe and 20 °F (11 °C) in the UK and USA, for instance). However, to reduce the return water temperature in district heating or when using condensing boilers, a higher ÄT is adopted. The design ÄT depends mainly on the habits in each country, but it can be optimised according to each specific plant.
The important thing is not to have a radiator that returns water very quickly and hardly cooled down back to the boiler. As for the actual difference between the flow and return temps, you really need to worry, you'll have little or no control over it any way.
The boiler, if it has an integral pump, will try to manage things so that the differential temperature is neither to low or too high.
Let things settle, 20 mins at least, measure flow and return temps on all rads, restrict the one or two rads that have the smallest difference (unless the difference zero between stone cold in and stone cold out!). The one(s) you restrict will also have a temperature that is near the boiler flow temp.
Frankly most systems I've worked on come good with 2 rounds of restricting and one of checking.
Ok. managed a 16 degree drop leaving one lsv open as per a Phil Addison post. Not sure of the reasoning there.
Downstairs rads, newer ones would handle a 20 C drop. Upstairs are old single panel no fins, due to be changed next year or sooner, wouldn't handle 20 c drop.
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