How long SHOULD a flow temp take to come up?

im gojng to seem thick again here.

My combi boiler (without hot water tank) is rated to 24Ki have about 9 rads most are smaller but three bigs ones in three rooms with high ceilings that I know take about 90 mins to come up 2 degrees to temperature.

My boiler flow temp after setback starts off at about 30 deg and in 15 mins comes up to about 41 degree with 32 degrees return flow temp - why does it then take about another hour to get up to 50 degrees anda further hour after that to come to 60 degrees? Is this something the boiler is doing with the return flow water deliberately or just the laws of physics if the rooms are large and cold enough (eg law of diminishing returns) - this is the same case whether TADO is on or not.

Cheers in advance for instructing this muppet


  • Montage
    Montage ✭✭✭

    There's no way it's applying anything like 24kW to do that.

    Might be worth providing a summary of your system now, or just stick to one enormous thread for your Tado adventures.

  • gary333
    gary333 ✭✭✭

    The boiler sounds like it's been range rated. What boiler do you have? It sounds like it might be set at 6kw or lower or the pump is on very slow.

  • hipponax1973
    edited January 26

    @Montage @gary333

    My summary : see bottom for further detail. also this video of the issues im having adjusting the power of the boiler.

    24K boiler combi boiler - set at 45% maximum output for the burner (see below) - i don't seem to be able to change this much (see below) *

    Flat is 1500 sq feet, with 12 foot high ceilings throughout. All rads are horizonal and are appropriate for the 7 room size despite the v tall ceilings. BTU ratings every everything throughout would be max 19kw.

    there are three SMART TRVs on rads in rooms we inhabit the most.

    I estimate there to be a moderate amount of heat loss as victorian structure with original walls and stained glass windows (backed by double glazing).

    Despite it NOT being a condensing boiler, TADO tends to run the system low and slow - but even when TADO is disconnected the power doesn't appear to be enough and its takes ages for the temp to come up. the main thing TADO seems to do is cut the gas earlier than no TADO


    Going into the settings ive found its rated at 45% gas burner output - whats puzzling about this is that that when i set it at 70% it becomes nice and roary, but only whilst im in the adjustment mode -as soon as i come out of the adjustment settings (even though its saved the new rating of 70%) it goes back to the old quiet burner and is as slow as ever to heat things up. This is INDEPENDENT of whether TADO switched on or not.

  • gary333
    gary333 ✭✭✭
    How are you measuring the flow and return? Has the pump been set too slow (if it’s adjustable)?
  • i have a couple of @eezytiger's thermometers and have measured it directly = there is always about an 8-10 deg difference (hence why @eezytiger 's method of rad balancing might not work for me) UNLESS i power up the burner in testing mode and then there is a short period where the flow is much higher than the return eg 20deg but then the return temp catches up. Theres a factory setting in the boiler of a 10 deg difference which you can adjust from 0-30 degs. Im not sure it modulates the pump speed in order to achieve this and with regards the pump the only adjustment i can see is the run time after the burner cuts out.

    Thanks for all help in advance.

  • eezytiger
    eezytiger ✭✭✭


    Here's a graph of my flow temperature rise this morning when raising the house temp by 1C. Flow temp increased by 20C over a period of 30 minutes. This was with the boiler range rated to 9 kW (its minimum possible output) and supplying nine radiators, all of which were wide open from the Tado perspective, but throttled by balancing with the lockshield valves.

    Since my boiler is range rated I would never expect to see Delta T 20 between flow and return at the boiler. It is possible with the boiler at full blast (24 kW), but not with only 9kW. In fact I usually see between 8-10C at the boiler, exactly as you do.

    As for balancing the rads, this is why I only monitor the return from each rad. There is no point chasing a Delta T of 11C at the rads when I don't even get that at the boiler. All those old numbers quoted by old school plumbers are with respect to boiler flow of 70-80C. Expectations need to be lowered when you're running everything cooler.

    There are a couple of YouTube videos by Tomkat Training which go into radiator balancing at length. If I recall correctly, after doing things the old fashioned way, I think he finally recommends my method.

  • hipponax1973
    edited January 29

    Thanks @eezytiger sorry for the repeated questions just having difficulty understanding why mines duff:

    1)does this still apply if my boiler is non-condensing and efficient to 78% or should i run it at 70 degrees?

    2) if my ceiling is 3 metres high (the shelf in the distance is 1.6 metres) is that more likely the reason the flow return takes so long to come up or still more likely because the range rating is too low? eg if i set the burner hotter will it make the flow temp come up more quickly?

  • johnnyp78
    johnnyp78 ✭✭✭
    If I recall correctly @GrilledCheese2 or @Montage has already told you that as you have a non condensing boiler it needs to be run at a higher flow temp. Have you not done this? As much as you want your boiler to behave like a condensing model, it won’t.
  • hipponax1973
    edited January 29

    @johnnyp78 i think before i managed to put this all in one thread, if i recall correctly ive stated several times that im not trying to run it like a condensing boiler and for some reason i cant get it to run at a higher % of output than 45%. My question i put several times as well is

    a) does higher output mean quicker overall system heating up times. The plumbers aroujnd this neck of the woods think you should never have the house below 19 deg so theres not much to heat up to and therefore a slower burn is better therefore second Q

    b) is a slow burn over a longer period of time eg start 2 hours before you wake up more efficient than a quick burn time nearer the time? The latter is more attractive for things like using geofencing etc and the area under the curve seems similar. And then what happens when you have a higher burn rate when you are temperature surfing / Coasting?

  • johnnyp78
    johnnyp78 ✭✭✭
    edited January 29
    If your radiators are running hotter for a longer period of time then your house will be hotter, assuming you have some level of insulation. It won’t necessarily get hotter over a shorter period of time, especially as you don’t know how to increase your boiler’s output. Until you can work out how to increase your boiler’s output this is all academic.

    From everything you’ve said it sounds like your boiler either has a fault or has been set up incorrectly. Rather than spending money on fixing it I would be looking at replacing it with a much higher efficiency condensing boiler or possibly a heat pump, if you have the appropriate insulation and radiators. You seem to be going round in circles.
  • hipponax1973
    edited January 29

    @johnnyp78 if i knew how to increase the boilers output would this get the house hotter over a shorter period of time? I have no choice over the boiler as the house is rented and an engineer is going to come and look at the boiler to see about increasing power from the 6kw it currently runs at. And going back to my original q above with the diagram, would it drastically cost more to get the boiler to burn hotter over a shorter period of time? THanks in advance

  • johnnyp78
    johnnyp78 ✭✭✭
    If you could increase the boiler’s output then the house would reach the same temperature you are achieving now in a shorter period of time. Whether it would be able to heat to a higher temperature depends on your plumbing system and insulation. As your boiler is non condensing I suspect it would use less energy to heat the house at a higher flow temp but I can’t be sure.
  • thanks @johnnyp78 -i’ll see what i can do
  • @hipponax1973 you don't say what your boiler is. It's a 24kw non condensing but what's the make and model?
  • hipponax1973
    edited February 1

    its a buderus U154-24K - the model only exists on the continent - **UPDATE**engineer has just been round and they will come back next week to uprate it from the 6k output at the moment to the 14k its meant to be. Unknown to me you have to do something with the gas flow nozzle and a manometer to get it to work at that rate instead of just switching up the dial otherwsie it only gives you the minimum output.

    @eezytiger quick q on your rad balancing method - im trying to get the nearest rads to the boiler to match the boiler return temp - they are about 3-4 degrees below at best - would this suggest theres a higher throughput somewhere else in the house that i need to throttle back?

  • gary333
    gary333 ✭✭✭

    Sounds like I made a good guess that it was running at 6kw :)

    Glad they are sorting it. Makes you wonder if that was done (limiting performance) because the gas pipe was undersized.

  • yes it’s a bit strange as the hot water burner comes up to 100% right away without issue…
  • @gary333 my guess too. Also makes me wonder as being non-condensing doesn't meet current requirements for new installs if this is a newish install.
    Limits who can do servicing and makes getting parts very difficult too.
  • eezytiger
    eezytiger ✭✭✭


    Before deciding on what to do you need to check your Delta T at the boiler. You said it was 8-10C, which is just like mine. If that's still true then......

    If the rads nearest the boiler are not passing water through fast enough then you probably need to open them up a bit more. Balancing should start with the rads nearest the boiler and then you move further along the system. So if they're too cold now they need more water.

    Unfortunately, because this is all about "balancing", each change you make will have an impact elsewhere, so it is an iterative process, somewhat. Sure, closing down other rads might restore the balance, as would opening up the closer radiators. The correct approach will be determined by your Delta T at the boiler. Do you need to allow water to pass more quickly through the whole system, thus raising boiler return temp or do you need to slow the passage of water through the rads so that boiler return temp is lowered? That answer will inform the decision to open the closer rads or further restrict the more distant rads.

    BUT - if you're going to have your boiler output changed there may be a knock on effect to pump speed and a change in Delta T at the boiler. So just consider that you might need a bit of extra tweaking after the boiler adjustment.

  • that makes sense - the return at the boiler is 38 constant now off a 48 output. Ive taken all the TRVS off to do the balancing.

    I started by using the old listening system to close them all off and then open slightly until i can hear the hiss of water escaping the bottom of the rad - basically the three rads next to the boiler (which is situated equidistant about 2 metres from each) come on very quickly and had higher return temps which is presumably why rads 8 and 9 (more than 30 metres of piping away) were pretty cold.

    Current temps of rads

    1 - 4 deg below boiler return

    2- 3.7 deb below boiler return

    3- 3.8 deg below boiler return

    4 - 4.5 deg ABOVE boiler return

    my plan at this point is to have all the rads about 3 deg below boiler return and then start the process again until they are closer - this is shirts an pants business though isnt it bloody hot here.

  • eezytiger
    eezytiger ✭✭✭
    You can't have all the radiator returns lower than the return at the boiler. It's impossible physics, unless you have a bypass short circuiting the flow straight back into the boiler.

    Radiator return temps should be roughly equal to the boiler return or a fraction higher, allowing for heat loss on the return pipe from rad to boiler.
  • @hipponax1973 are you sure the flow and returns are on the sides you expect? The TRV isn't necessarily on the flow side.
    That may explain the temp above the boiler return .
  • yes but in discovered rad 6 and 7 was wide open both ends bypassing everything and returning very warm water -still fiddling as we speak
  • @eezytiger it might be because all the rad pipes are painted in insulating paint?
  • eezytiger
    eezytiger ✭✭✭
    @hipponax1973 Suppose the return temp from every radiator is 35C. How is it possible for the return at the boiler to be 38C? The water won't get hotter after it leaves the radiators.

    The only way for the boiler return to be higher than the radiator return is if the pipes are short circuited and not giving off sufficient heat within the radiator(s). As you've said above, two rads were wide open - thus short circuiting the water. Once you've balanced all the rads correctly then everything should be more or less equal on the return side from all the rads and at the boiler.

    However, since you don't have a condensing boiler you don't need to strive for the lowest return temp possible at the boiler. Your aim in balancing the system is to make sure that each room receives sufficient heat and you don't end up with cold rooms at the end of the pipe run.
  • Sooooooo...update (the last one disappeared for "approval" so luckily had it as a cut and paste.)

    @eezytiger @johnnyp78 @mojomoomoo @gary333 thanks for all the advice

    First round of balancing:

    Boiler 35 deg

    R1 - - 36.6

    R2 --36.6

    R3 -- 36.8

    R 4-- 36.0

    R 5 -- 36.0

    R6 -- 36.2

    R7 -- o-ring on the outflow valve rotten, fell apart and blew - flooding - screwed down tight, capped off awaiting new o-ring - 33 deg

    R8 - 33.5

    R9 - 36 deg

    Second round of balancing - tightened up R1-4 and fiddled as best I could with R789 suddenly R5 was 4 degrees above and R6 stone cold

    re did

    3rd round

    All rads within 1 deg except R7 which still remains 3 below and R7 i cant get any warmer than 1.5 below.

    Delta T at the boiler now 14 (?likely due to underpower of boiler and all TRVS wide open) but is balanced otherwise.

    As mentioned above engineer coming next week with gas valve manometer to up the power by 8KW. (PS there IS a setting in the boiler where you can do DELTA T 10, 20 or 30 depending on choice but i think thats out of my mental bandwidth at present) and ill get him to replace the o-ring

    Does this count as a success?

    PS the boxing tape here is great for the thermometer electrodes and i got much higher readings than the elastics i was using

  • eezytiger
    eezytiger ✭✭✭

    @hipponax1973 Congrats. You've cracked it.

  • yes ten hours of my life i’ll never see again…’thanks once again!