Balancing radiators - what should the temp drop across rads be? (Baxi 600)

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TL;DR - I want to balance the radiators, but not sure what sort of temperature drop I should aim for across each rad. I've looked in the boiler user guide and also the installer guide, but I can't find anywhere that says what the flow and return temps need to be.

Background:
I have 2x radiators in the lounge and both are hot to the touch with no cold spots anywhere, but one of them doesn't seem to put heat into the room even though on the affected rad I can measure a temp drop of ~17 degrees between lockshield and TRV - it's on full blast with the TRV always open, trying to hit our target temp... but the room often feels cold. We moved in during the summer, but since it's been so cold over the last couple of months we're looking to try and improve things.

The space I'm trying to heat:
The room is open plan with 2 sections, and total volume of the room is:
(3.0m x 2.8m x 2.4m) this is "the nook"
plus
(4.5m x 3.9m x 2.4m) this is "the lounge"

That makes total volume 62.28m3 (I think! Please forgive me if my maths is wrong! :D)

The lounge & nook are downstairs: it's a really old property without cavity wall on the lounge side (which is maybe the problem), with suspended floor. It has a non-functioning chimney - there's a chimney breast, but everything above the ceiling level has been removed. I've used a "Chimney Sheep" to stop warm air escaping to the room above. We have modern double-glazing in the nook, and brand new double-glazing in the lounge. There's a door out to the conservatory which loses loads of heat - so we have a curtain drawn across it pretty much permanently (new doors being installed soon, but we'll have to make do with the curtains for now).

The radiators we already have
1x type 21 (0.6m x 0.6m - unpainted) on an internal wall in the nook (thoroughly cleaned behind, and between panels)
1x type 22 (0.9m x 0.6m - unpainted) on an internal wall in the lounge, (thoroughly cleaned and dusted behind, and inside/between the convectors)

The problem
The radiator in the nook is doing ok, but the lounge one never seems to get above 21deg (21.5 if we're lucky) - we're using Honeywell HR92 TRVs to measure the 21deg figure. The HR92 in the nook happily gets to our target temp of 22 and beyond, but even increasing boiler temp doesn't seem to help for the lounge (CH temp on boiler is currently at about 50% - I increased it to nearly full the other night, and I got maybe an extra 0.5deg, but I want to see if there's a better (cheaper) way to improve the heat output from this rad). I know I can adjust the offset of the HR92 if I wanted to - but it'll still feel cold in the lounge, and that's what I want to fix. The lounge takes ages to warm up, and often feels cold (while the nook isn't affected by this problem). The heating comes on at half 6, and the Honeywell tries to maintain a target temp of 18 during the day (boosted to 19 when it's below 10deg outside), up to 21 in the evening (or 22 when it's cold out). On a weekend the target temp is 21 (boosted to 22 when it's cold out) the whole time. The heating has been running like this for a few weeks now.

Maybe we're going to be limited by potential heat loss through the walls and floor? It's been dead cold outside lately, but wondering if anything we can do to improve heat output, and I wanted to try balancing the rads in case that makes any difference.
 
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You could add 3-400watts output by increasing the flow through the radiator (balancing it to have a 9-10 degree difference between flow and return)

Before you do, check the temperature difference accross the flow and returns of some rads in the warmer rooms and see what they are, you might find they are around 10 degrees and your cooler room radiator at 17 is indicating a restriction in the flow through it.
 
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You could add 3-400watts output by increasing the flow through the radiator (balancing it to have a 9-10 degree difference between flow and return)

Before you do, check the temperature difference accross the flow and returns of some rads in the warmer rooms and see what they are, you might find they are around 10 degrees and your cooler room radiator at 17 is indicating a restriction in the flow through it.

Awesome - thank you, and I'll give that a go. I don't know much about this, and I was labouring under the (false? :D ) impression that increasing the flow would make it cooler! :) But as you suggest - I'll get my IR gun on the flow & return on other rads and see what sort of drop they're getting.

I read a couple of FAQs on this site, plus have searched through and read several posts on balancing rads - I found one that said the drop across the rads would depend on the boiler. I got the impression that I should be finding some numbers from the boiler user manual and then using that to set the drop across the rads. Is the drop always going to be around the same (irrespective of boiler) - will it always be in the 9-12 degree range? (Google suggests it's 12 degrees, but I'll go with your suggestion of 9-10 degrees).
 
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Techincally, old standard efficiency boilers and systems were designed for 11 degree drop, condensing boilers for 20 degree, but in reality you would very rarely ever see 20 degree difference on them for various reasons. Partly as the radiators would need replaced to suit the 20 degree drop and never were.

Easy way to think of it - If you run water at 70°c into a radiator, and you run the water through it really fast, its not in there long enough to loose much heat so will come out with a high return temperature, say 65°c, if you slow the water flow down, then the water is in the radiator for longer, so its loosing more heat to room before it leaves and you get bigger drop, the return could be 50°c then.

Having a big 20°c difference suited condensing boilers, as the return temperatures were cooler, giving it more of a chance to create condensation in the boiler (which makes them more efficient) However a radiator with flow 70 and return 50, has an average surface temperature lower than a radiator with a flow of 70 and return of 60, so cant put as much heat into the room.

Typical older systems had say a 10-11°c drop, so if one radiator had a 17-20°c drop then that would be an indicator that there may be a blockage/restriction to it, or if its the furthest away radiator that possible the pump wasnt strong enough to flow the water at the correct rate.
 
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This system is reasonably new, I think. The boiler was installed in Jan 20 - we've had a bit of work done since we bought the place, and virtually all the rooms upstairs now have new rads. The old rads (all downstairs) are fairly new - all either type 22 or type 21 (but they were in real need of a clean down the back!)

This Baxi 600 boiler - I think it's a condensing boiler, right? Does that mean we should be looking at a bigger drop across each of the rads to make the boiler more efficient? Or should we be sticky with 9-12 degrees? (I've yet to check the temps across other rads, but I'll report back :) )
 
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See what the other rads are like first. Like I say 20°c is fairly uncommon in reality, but yes the lower the temperature the boiler runs at the more it condenses. Same effect can be had just turning the boiler thermostat down a bit when its not deadly cold outside, they only need the full temperature in the worst weather, rest of the year the lower temperature you can run it at the better.
 
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