Radiator Balancing - is there an idiots guide?

@HydroSam 22.6l/m is 7.84kW @ ΔT5 - so likely to be an issue if thats max output flow rate.

Edit just seen you have an older unit EDLQ011CV3, sorry!: For the newer models, the expectation is 102kPa for ~30L/m (10.43kW at ΔT5) flow rate (it’s how I knew I had a blockage or something on mine):

Let us know if you’re running ΔT5 or otherwise (you mention further up moving to ΔT7).

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I have been running with dT5. I mentioned above I could change to dT7 because of my pipework restrictions

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Here are the relevant pump/pressure/flow rates for your model - note it states capable of 46l/m (!) (which is 15.98kW @ ΔT5) at just under 30kPa

Newer Daikin Altherma 3M seem to have a significant upgrade on pump power I think comparing to the older “2” model:

Below, red is where you’re at from your reported numbers, green is max flow rate given low enough pressure drop, and blue is (the minimum) where you need to aim for at ΔT5 to get 11kW out of your unit.

At 62.3kPa, you should expect ~31L/m - I assume the “62.3kPa” figure has been given as the max pressure to get that flow rate for ~11kW - so you still need to get to 31L/m @ΔT5 to get ~11kW output (or increase ΔT)

Data book reference:

EB-DLQ011-016CV3_CW1-EKCB-CV3-EK2CB-CV3-EKMBUHC3V3-EKMBUHC9W1_EEDEN18_Data-books_English.pdf (7.9 MB)

So it looks like I’d never be able to achieve the max output this heat pump is capable of with significant alterations to the pipework? Eek! Good job I only needed 5kw when it was -6C last winter. The house was warm enough but I wouldn’t have got what I needed if the MCS calcs had been correct.

In danger of taking this thread off on a tangent…


I’m about to embark on balancing my system. I’ve a gas boiler, but my dT is very tight as my flow is down at 40 at the moment. Boiler can’t go any lower than 43 (it’s a 35kW beast) and the return is 37, so it’s pretty tight.

Lock shields - I think they are Danfoss, for the most part, but haven’t a clue if they are any good. The rads weren’t sized - they were eyeballed by the plumber.

System is currently all open and I’ve instrumented each radiator, so I can track the flow, return and
mean temperature. (https://youtu.be/YUV8U6BC5oQ)

Here is a timeseries from Influx for today. As you can see, the warmer radiators are those at the top and they are closest to the boiler. I’m doing to start experimenting and actually tracking how the system behaves when I start to close down each lock shield.

The information in this thread has been very interesting and I’m hoping it will come in handy!


Hi @tomasmcguinness welcome. :smile:

For a condensing gas boiler post ~2005, in case you were not aware, the target ΔT a modulating pump (assumed) inside the boiler will be hunting for is around ΔT20 (vs ΔT5 for standard heat pump config) as gas boiler condensing efficiency is all about driving the return temperature as low as practically possible (whereas for heat pumps, flow temperature is what drives efficiency)

Also, given the modulating nature of the pump, running the system at max output will likely run the pump at max output, and should be considered to avoid a feedback loop of shutting down flow, and then the pump reacting and vice versa.

Your system in the video looks pretty neat, and would save a lot of time! Good stuff. Would be good to try it out!

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See below, you may have kopped lucky Tom!

Tut tut, those pesky “plumbers” !!! :joy:

I think the lesson here is to not be chasing dT of 5 across the rads? Is the happy medium for them to be the same as each other?

The one radiator I have fiddled with is the one in the bathroom, which is small, old and closest to the heat pump. On 1/4 turn of the lock shield, the dT is higher than average, but the mean temperature is lower than the rest. This suggests that I’ve throttled it down too much? Maybe give it another 1/4?

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Love your radiator monitor, Tom. That would be so much easier than running around the house with an infrared thermometer, and one can’t have too many charts. :grin:


Looking back at my notes / spreadsheet, there’s a direct correlation between the size (output) of the rads and the amount I’ve opened them.

3 examples from my rad list:

  • Small rad (600 x 600 K2) - 0.3 of a turn
  • Medium (1400 x 600 K2) - 0.5 of a turn
  • Largest rad (1400 x 600 K3) - 0.8 of a turn

But these are turns on my IMI valves, calculated from their HyTools app.
Your number of turns may well be different.

I posted a video and article from heat geek earlier in the thread where they show the difference a quarter turn can make across a variety of lock shields. It’s wild.

If you used the IMI hytools app and put your rads into there, you’ll get to see the recommended flow rate for each one. If you note them all down you should see your own sliding scale?


Hi Stephen,

I didn’t realise it would try to drive the dT to 20. I’m using a system boiler and I believe the pump speed is fixed to deliver a certain pressure. The data coming out via the EMS bus shows the pumps modulation doesn’t really change.

I’ve been running at 45 flow for a while now and just dropped it down to 40 last week to see how the boiler would cope. It can’t go low enough at 7 degrees outside. Once the weather gets a bit colder I think it will manage.

The heat meter I had installed shows the boiler can run long and low, but the thermostat jumps in to turn it off rather than it cycling, which is good.

I’ll be curious to see how it all behaves once the weather gets down to 0 or below. My limited understanding of heating predicts the pipework won’t deliver the needed heat once the boiler is required to put in 8kW or 10kW. I have 22mm primaries, so I’m good up to about 6.5kW I think.

I do want to start experimenting with the pump settings just to see how the radiators are effected.

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I suspect you may know of this fantastic resource for learning, but in case not, take a look here:

It may be possible your pump is not connected / integrated with the boiler system such that it is setup in a fixed way. You may want to investigate the pump configurations available if they are separate to the boiler controlling/modulating it.

The other thing is that as you’re already running really low flow temp, then return maybe can’t actually drop that low as the flow rate would be just too slow.

Also, your 22mm primary pipework is good for 18-24kW at a dT of 15-20C on a gas boiler:

For a heat pump, yes an optimum target of ΔT5 is approx ~6kW on 22mm pipework.

When I checked the specification, the pump speed defaults to a fixed pressure and I know the flow rate is around 16l/min (got this from both my heat meter and my DIY cheap ass flow meter)

The pump is integrated into the boiler (WB Greenstar 8000 Life) and my ems-esp device reports the modulation speed. It’s always around 80%. This kinda confirms the default setting. There is another option available which will provide a speed based on the flow temperature. I want to try and balance my system using that fixed speed. 80% seems quite high to me, but I’m not basing that on anything :slight_smile:

I’ll read that article you linked to.

In terms of pipework, I have one circuit with four radiators on there and that’s my main concern right now in terms of heatpump upgrade. However, I feel that if the trunk gets upgraded to 28mm, the four 22mm branches which come off that will be able to deliver enough heat.


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I would have thought your (any) boiler will work more efficiently if it can modulate power and therefore flow rate to target optimum return temperature given that’s where gas boiler efficiency is lost/won.

As can be seen below, condensing efficiency is not binary, the lower the return temp, the higher the efficiency:

Source: Condensing Theory: How Do Condensing Boilers Add Efficiency

The other point is that with a fixed speed, I would assume it’s always using fixed power (if pressure drop doesn’t change such as TRVs closing) - so the modulation of flow based on temperature may also use less electricity (if it can modulate down)

Back to balancing, you really want to get the return as low as possible (close to and up to ΔT20) on your current gas boiler. If you try (and it is!) sending ΔT5 water back by balancing to ΔT5, you’re really running the pump too fast, and losing out on efficiencies: both the pump using too much electrical power, and the boiler not condensing as much as it could. Which goes back to the point above re: modulating mode vs fixed.

Of course this all changes when ΔT target is 5 upon moving to a heat pump!

It does modulate power. I can see that too. You’re saying it should be changing both power and speed at the same time?

I was trying to achieve dT5 so I’m confident that my pipework and house can handle a heat pump. With my dT of about 7, the house is comfortable and the heating runs for long stretches.

Are you saying I’m missing a trick and that I really should widen the dT across flow and return as I’m using a boiler? Wouldn’t that mean more power required?

I’m going to have to experiment, aren’t I.

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Right, I missed this point: that’s a fair experiment!

Yep, specifically for a gas boiler, you’re artificially holding the return water higher temperature than is needed, and losing some efficiency. The pump will modulate down and run flow slower, thereby using less power. Efficiency of the boiler should go up a few % as well as more condensing occurs in the heat exchanger of the boiler even as the flow temperature may raise a little given widening of ΔT Win win (for a gas boiler!)

From heat geek article linked in post above:

As you can see, for each litre of water condensate produced, 690watts of power is recovered as heat into the heating system, vs released as steam out of the flue:

I’ve added a condensing boiler chart in above post to show condensing efficiency is not binary.

Experimentation is great for observation and learning. Keep doing it and report back!

Edit - resized images in post - BT, Moderator

Ah, so it’s not just enough to have the return below, say, 50. You want it as low as possible to recover more heat?

This is from the manual. It has a fixed pressure, which kinda matches what I see I think.

I will switch that the dynamic mode and see how its behaviour changes.

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:ok_hand: spot on. This is specific to gas boilers though to be fair.

Also note the following if you want (or need) to work out heat output or flow rate by rearranging the formula given your flow and return ΔT will widen, if your heat output is to remain the same, something else has to change.

Source: https://heatpumps.co.uk/technical/pressure-drops-flow-rates/

Per other places this has been mentioned, you’ll probably need to run the boiler at max output (heat) - so the modulation of the pump doesn’t affect (as much?) the balancing operation via feedback loops.

There are servicing modes on gas boilers that do this as well from memory. Worth looking at that too.

Reading through all these posts you can see how complicated we like to make things. Your heatpump manual will give you an ideal flow rate around your system but that’s based on perfect conditions which most of us haven’t got. So it’s a starting point to which you aim to achieve best efficiency and lowest running costs, is it worth the investment in better lockshields to chase a degree or two because what you setup today will differ tomorrow. It’s a compromise you are after, keep it simple.

I don’t have a heat pump. I’m just tinkering with my gas boiler :slight_smile: