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:
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?
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!)
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:
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.
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’m struggling to get my head round the basics still…
If my heat pump is delivering a dT of 5K, but I’m only seeing 3K across my radiators, should I instead be looking at the speed of the pump that’s circulating from the low loss header to the rads? What effect will that have on the performance of the whole system?
[Edit: I checked the speed setting on the pump (Grundfos UPM3 AUTO), and it’s already on the “Lowest relative pressure curve” for radiators]
All the rooms get warm at the same rate, so maybe I don’t need to worry about about balancing?
You are looking for a dt of 5 across your primaries which mean’s potentially your nearest rad maybe only 1 degree and your furthest rad will be 5 degrees because you are losing convected heat from your rads into the room. So as the water is flowing it slowly loses heat around your circuit that’s why flow rate is so important for efficiency along with volume.
All rads will be ΔT 5 degrees near as makes no odds; as the losses from the distribution pipework are minimal compared with the output from the radiators.
I would have to disagree slightly in as much that if you had 2 pipes and no radiators then yes you could obtain and keep your deltaT of 5, but when you take into account convected heat and heat loss through the pipe work you will vary the balancing to take that into account. Not all rooms will take the same amount of convected heat out some more than other which means that your return will be cooler enlarging your deltaT. You cannot dismiss those factors in your balancing particularly at low flow temps. It’s not so important when you have a gas boiler pumping out at 75 degrees
Dredging this topic back up based on my recent attempts to balance our system.
We have recently (finally) added a large K2 vertical radiator in our hallway.
So I thought I’d better balance everything.
The main things I’m worried about are the two towel rails and two plinth / kickspace heaters we have as without balancing these are probably going to be passing the majority of the heat that comes in their flows straight through to the return. I believe this could cause the flow temp to go up and up causing early cycling.
Although this is also a concern with any of the radiators that aren’t able to deliver enough heat to the rooms.
However… The big problem is I can have the system all open when I balance but most of the time all of our bedroom radiators will be closed or only slightly open as these have TRVs to limit the temperature. This is especially needed in our house as currently the first floor and attic are well insulated but the ground floor (with suspended timber floor) isn’t and they’d overheat otherwise.
So as soon as I set the TRVs for these rooms all my hard balancing work in the rest of the house goes out the window.
So I’ve been wondering if I’d be better to balance with these radiators out of the equation (closer to normal running of the system).
Whether auto-balancing TRVs could be an option, particularly for the rads I’m most worried about.
And whether actually I’d be better doing like @Rachel and trying to control my system pump to target a system wide ΔT would be a better use of time.
There are too many variable going on here for me to work out what I should be doing and how much of a difference it would make.
I struggle with this a bit too! The main reason I wanted the bigger pump I was asking about in another thread (arrived today - a ducapump 8m head PWM controlled), is becuase I’m not getting enough flow to some of my more remote radiators. So whilst I was able to control the 5C delta at the LLH, that didn’t mean I was getting 5C drop across all my radiators and by the time I’d wound back the flow in the nearest ones as far as I can sensibly go, I’ve found that the delta T tends to settle at 7-8C and I still have some cold-spots on the two triple radiators I have. I should be able to get about 30% more flow and hopefully get rid of the cold spots and perhaps allow the system to run with a lower flow temperature, but I think I may still have a balancing problem.
I’m wondering how much my system is responsible for high pumping power on my EDLA09. The specs for the pump are really helpful - the curves on the Danfoss site suggest my pressure drops are huge. How much of this is almost-closed lockshields, and how much is the pipework, is yet to be determined!
Slightly off topic, but an interesting observation for you.
Modern K2/K3 radiators often come with a detachable side panels and slotted top. Worried that these might be constricting convection, I set up a “chimney” (a cylindrical carton with a thermocouple set in the centre that would sit on a radiator), and also two thermocouples on the radiator supply and return (with heat transfer cement and insulation) to check that they did not vary for my test. Then I took the three readings without and with the top/sides.
To my slight surprise, and with constant flow/return temps, the “chimney” reading actually increased (by about 1degC) after I put the top/sides back on.
I conclude that the manufacturer’s claim (that the top/sides do not affect convection) is actually correct , so that’s one thing we don’t have to worry about…
Excellent work, I have wondered about that.
Our vertical K2 radiator is one with a (mostly flat) panel on the front. When I was looking into it I think the power output is reduced slightly compared to the normal version. I guess that’s because you’ve covered over the ridged front that would have a larger surface area. However I suppose it does mean the rad has a higher thermal mass and so could level off fluctuations in flow temp or between cycles.
spot on. This is specific to gas boilers though to be fair.
, so that’s one thing we don’t have to worry about…