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View the thread, titled "Rate of heating in a living room?" which is posted in UK Plumbers Forums on UK Plumbers Forums.
Its the heat demand and the boiler minimum output that determine cycling or not.
The return temp to the boiler shouldn't make any difference to the boiler output or cycling time for the same heating demand, if you had 4 rads of total output of say 6 kw with a return temp of 50C (from 70C) then (assuming the boiler can modulate to 6kw or lower) the flowrate through the boiler is 4.3 LPM and the boiler deltaT is 20C giving a boiler flow temp of 70C.
if a ABV opened to give a flowrate of say 4.3 LPM (at 70C) you will now have 4.3 LPM at 70C mixing with 4.3 LPM at 50C resulting in a return temp of 60C but the flowrate through the boiler is now 8.6 LPM resulting in a boiler deltaT of only 10C but the boiler flow temperature will still be 70C..
That is also a reason for installing ABVs, to give a reasonable boiler Hx deltaT.
The return temp to the boiler shouldn't make any difference to the boiler output or cycling time for the same heating demand, if you had 4 rads of total output of say 6 kw with a return temp of 50C (from 70C) then (assuming the boiler can modulate to 6kw or lower) the flowrate through the boiler is 4.3 LPM and the boiler deltaT is 20C giving a boiler flow temp of 70C.
if a ABV opened to give a flowrate of say 4.3 LPM (at 70C) you will now have 4.3 LPM at 70C mixing with 4.3 LPM at 50C resulting in a return temp of 60C but the flowrate through the boiler is now 8.6 LPM resulting in a boiler deltaT of only 10C but the boiler flow temperature will still be 70C..
That is also a reason for installing ABVs, to give a reasonable boiler Hx deltaT.
So, I've placed black electrical tape onto the two 22 mm pipes under the boiler and that has made a substantial difference when taking the temperatures. I let the boiler run for 40 minutes before taking any temperatures.
Firstly, with just the one radiator in question switched on. On/Off below refers to the "flame" symbol coming on or off on the boiler screen. Boiler temperature on-screen ranged from 48 C - 67 C. So, on average it's turning on and off every 2 Minutes & 25 seconds.
The flow/return at the rad was 45.8 C and 40.6 C (I also used black electrical tape here).
Time:
0:00 On
0:02 Off
0:05 On
0:07 Off
0:10 On
0:12 Off
0:15 On
0:17 Off
Below are temperatures I was getting on the flow/return under the boiler at various intervals:
50.9 - 39.4
47.0 - 40.9
45.2 - 37.9
61.6 - 39.0
49.7 - 38.1
60.3 - 38.5
Separately below is with the radiator still on, but with another couple of rads on the same floor turned on. I left it about 40 minutes or so to allow the other rads to warm up. Boiler temperature on-screen ranged from 57 C - 67 C. So, on average it's turning on and off every 3 minutes 43 seconds..
The flow/return at the rad was 47.5 C and 42.7 C
Time:
0:00 Off
0:03 On
0:08 Off
0:11 On
0:15 Off
0:19 On
0:23 Off
0:26 On
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One Rad:The boiler would appearing to be firing for ~ 40% of the time with a average flow temp of 52.5C & return of 39C, based on a min output of 8 kw (post above) then theoretically a average boiler output of 3.2 kw, even if the boiler efficiency is very low while cycling its difficult to see that rad producing its rated output, very difficult to determine anything from a relatively small heat load with a boiler with such a high minimum output.
The second test would indicate firing for 62.5% of the time so theoretically a more believable output of 5 kw with 4 rads on.
I don't think you can really do any more to figure out what the true output of that rad is.
I think OP means that is the total run time. Not sure how long it rests before starting up but it’s in the boiler program. He has a big short cycling problem because boiler is oversized and system is not balanced. Rapidly rising return temp cuts it out. He has 2 jobs, balancing all the radiators and glazing.
Once more, the radiator is PERFECTLY FINE! Boiler should not be stopstarting like this at all. It is getting too much hot water in the return and cutting out. Balance all your radiators. Set boiler temp to 55 deg and see how long it runs. Slowly increase running temperature until it runs for at least 15 minutes before cutting out.
Draw curtains.
Seems to be a consistent 3 mins off time, certainly agree with new window but all the balancing in the world IMO will not improve the output of that rad as its measured deltaT was only 4C.
Also the shockingly poor modulation means that a min 8 kw heating demand must be maintained to allow the boiler to run continuously.
Also the shockingly poor modulation means that a min 8 kw heating demand must be maintained to allow the boiler to run continuously.
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Presumably the deltaT figure which you refer to is data which a radiator manufacturer can easily provide if I were to ask for this figure from a different manufacturer before purchase (and hoping that the figure which they provide isn't an over optimistic figure). What sort of figure would I be looking for, ideally?
Do you think this poor modulation issue is related to the radiator/balancing, or would it be specifically an issue with the boiler?
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Slowing flow through this radiator will drop its average temperature by 10 degrees or so, 40 degrees hotter than the 11 deg room instead of 50 deg so 20% less heat emission but for 100% more time. If a more continuous flow is delivered through it it will emit more heat overall. OP obviously wants a new radiator but he’ll be disappointed and poorer. He’s certainly not listening to me.
I don't want to appear that I'm not listening. Ultimately, I have no plumbing background. I'm listening to two different opinions here and, yes I can get the window sorted, but aside from that I don't know if I need a different brand of radiator or if it's just a matter of getting the system balanced or if there's a problem with the boiler? There are no boiler error messages on-screen.
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I suggest you get a heating engineer in to look at it for you. It shouldn't take much to work out what's wrong for someone experienced who is actually there observing.
The manufacturer couldn't care less what the radiator deltaT is, his/her only responsibility is to provide or advertise a radiator based on a "50 degree" standard, its quite simple.... a 50 deg rad is the (mean radiator temperature) minus the required room temperature. So if you require a deltaT of 20C (desirable for boiler efficiency) and if you accept that 50C is a reasonably low return temp you would run the boiler with a flow temp of 70C and you then may have to balance the rad or rads to achieve this return temp of 50C but you will then have a (70+50)/2)-20, a 40 deg rad with a output of only 75% of the 50% rad so you should oversize your rad by 1/0.75 or a factor of 1.33, on the other hand, if you are still running with a boiler temp of 70C but if the flow rate is increased to give a deltaT of 4C then your rad becomes ((70+66)/2) - 20, a 48 deg rad with a output of 94% of the 50 deg rad. So back to your famous rad, I think you ran it at 60C with a deltaT of 4C so you should have had a (60+56)/2)20, a 38 deg rad or 70% of its 2.4kw rating at 50 deg or 1.7kw. I think you then got the output up to, can't remember but 2.0 or 2.4 kw?? (see old posts) by raising the boiler temp to 75C.
You can do your own calcs here or just look up a rad factor table if more convenient.
The calculated output = ("known deg" rad)/"50 deg" rad))^1.3, what this means is if you have a 40 deg rad, the calculation is (40/50)^1.3 and so on, you may think that you should have 90% output but you only have 75%.
Now you know as much as any heating engineer does or should.
The boiler is operating as designed, my poor modulation referred to it 8kw output as most 30kw boilers will IMO easily get down to 5 or 6 kw
Would agree with above re different type of rad as there are numerous posts re the (non) performance of "tall" rads, IMHO, all these rads should have a diverter fitted or be plumbed TBOE.
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If it helps you, I pretty much agree with John.g.
Balancing will cost you nothing, there’s a thread here on how to do it.
if I saw this system like this without your input, I would conclude someone has turned up a radiator in a cold room, and made it worse.
Thanks guys, your input is very appreciated. As I mentioned earlier the flow for this rad is definitely going into the lockshield, and ideally it should be going into the TRV (which is not bi-directional). So I'll get that sorted and then I'll get the rads balanced so they have 15 C drops.
Thanks for mentioning the diverters, as I hadn't heard of those. TBOE (the best of everything?)
IF you put TBOE (top & bottom opposite ends)in the search box you will find a lot of info, its supposed to be the best way to plump a rad and it might but its certainly the ugliest. Someone discovered that if you plumbed in rads (like the majority) BBOE (bottom bottom opposite end) that the circulation was just as good almost.
Its almost uncanny how it (BBOE) works, if you have a cold rad with boiler at normal temperature and open the rad valve(s) the hot water will move straight up to the top of the rad along the first 6 ins or so and then gradually heat the rad but from the top down. It would be interesting if you were to shut off one valve on your rad, let it cool down, then reopen it with a hot boiler and see does it behave like the above, it should/might (from the hot inlet side) start heating up the first one or two columns and then gradually move along the top and start heating the remainder from the top down, I think this is why they sometimes fit a diverter plug because it forces the water up the first few columns and it flows downwards then gradually loosing heat.
Its almost uncanny how it (BBOE) works, if you have a cold rad with boiler at normal temperature and open the rad valve(s) the hot water will move straight up to the top of the rad along the first 6 ins or so and then gradually heat the rad but from the top down. It would be interesting if you were to shut off one valve on your rad, let it cool down, then reopen it with a hot boiler and see does it behave like the above, it should/might (from the hot inlet side) start heating up the first one or two columns and then gradually move along the top and start heating the remainder from the top down, I think this is why they sometimes fit a diverter plug because it forces the water up the first few columns and it flows downwards then gradually loosing heat.
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I've just seen a picture of TBOE at the link below, and yes that is ugly.
What is a heat sink radiator?
A heat sink radiator should be at least 10% of the KW output of the “maximum output to water” of the stove (more if the stove manual says so and I have seen figures as high as 20%).
www.stovefitterswarehouse.co.uk
I'm doing that ASAP.
For what it's worth, I've asked the radiator manufacturer and they state that the rad does not require a diverter (or have an in-built one).
That's the point " (or have an in-built one) ", as I said above, the diverter is normally only perhaps say 2 columns in from the flow pipe to force the hot water to the top of the rad and it then works normally like any other rad. In the absence of any markings then you don't know if its installed correctly or not (if diverter fitted).
Carry out that test I suggested, shut off one valve, let rad cool down of if already cold, shut the valve and heat up your system, then reopen the valve fully and see if the hot water is moving up the first few columns or is rising up gradually through the remaing ones, also the return shouldn't be too hot until the whole rad heats up.
Carry out that test I suggested, shut off one valve, let rad cool down of if already cold, shut the valve and heat up your system, then reopen the valve fully and see if the hot water is moving up the first few columns or is rising up gradually through the remaing ones, also the return shouldn't be too hot until the whole rad heats up.
It's 3 bars per column. It appeared to go up the central bar directly above the feed. Then go across the top directly across all at the top, then cascade down heating all at same time. Hopefully, that sounds ok, if not I can repeat and see if I can determine any further detail.
I didn't notice the return heating for some time
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It indicates to me that a diverter is fitted but even if it isn't the rad is performing exactly as I would think it should, ie hot water moving upwards in the first few columns and then flowing downwards through all the remainingcolumns.
To recap: First test gave 56.4C/54.2C flow/return which based on rated output = 1.54kw
second test: 57.1/53C = 1.7 k
third test: 75C but return not measured but assuming 4C deltaT = 2.4kw but room took 4.5 hours to heat up from 12C to 21C
The 2Kw electric heater heated the room from 11C to 20c in 2.5 hours.
There is obviously a big difference between rad/electric tests which clearly indicates the the rad isn't emitting its rated output, if indeed its rated output is correct but difficult to imagine a manufacturer giving out "false" information.
So, all you have left is to change the TRV/lockshield and balance the rads and increase the deltaT to 15C. I
don't know if the advise is to balance just the 4 rads on that zone or not, presume, initially, yes.?
To recap: First test gave 56.4C/54.2C flow/return which based on rated output = 1.54kw
second test: 57.1/53C = 1.7 k
third test: 75C but return not measured but assuming 4C deltaT = 2.4kw but room took 4.5 hours to heat up from 12C to 21C
The 2Kw electric heater heated the room from 11C to 20c in 2.5 hours.
There is obviously a big difference between rad/electric tests which clearly indicates the the rad isn't emitting its rated output, if indeed its rated output is correct but difficult to imagine a manufacturer giving out "false" information.
So, all you have left is to change the TRV/lockshield and balance the rads and increase the deltaT to 15C. I
don't know if the advise is to balance just the 4 rads on that zone or not, presume, initially, yes.?
An update: I now have the flow pipe connected into the TRV and the return to the lockshield.
I've ascertained the order in which rads heat up on that floor. After doing, I've turned all the TRVs up full and all the lockshields closed off. Then, with the rads allowed to cool, starting with the first rad on that floor, I've opened up the lockshield as little as possible but which still allows the flow through the rad. I presume this will allow the largest temperature drop possible?
By doing so, in one room I got:
a) 58.7 C at the flow end with a 6 C drop (this is a vertical rad, but not the same brand as the vertical radiator which this thread relates to),
b) 55 C at flow end with a 15 C drop, flat panel rad
b) 55 C at flow end with a 15 C drop, flat panel rad
d) the main radiator in question: 51 C at flow end with 7 C drop
I did notice, when I checked in to look at the boiler that it was no longer switching on/off every 3 minutes any more. Actually, I didn't notice it off so I'm not sure if the cycle time has increased greatly or if wasn't switching off, I'd need to check that. Any time I looked in, it seemed to be holding steady at 62 C without fluctuation.
It didn't seem to be possible to get the 15 C temperature drop. I was being very careful to open the lockshields as little as possible whilst still allowing flow.
I've ascertained the order in which rads heat up on that floor. After doing, I've turned all the TRVs up full and all the lockshields closed off. Then, with the rads allowed to cool, starting with the first rad on that floor, I've opened up the lockshield as little as possible but which still allows the flow through the rad. I presume this will allow the largest temperature drop possible?
By doing so, in one room I got:
a) 58.7 C at the flow end with a 6 C drop (this is a vertical rad, but not the same brand as the vertical radiator which this thread relates to),
b) 55 C at flow end with a 15 C drop, flat panel rad
b) 55 C at flow end with a 15 C drop, flat panel rad
d) the main radiator in question: 51 C at flow end with 7 C drop
I did notice, when I checked in to look at the boiler that it was no longer switching on/off every 3 minutes any more. Actually, I didn't notice it off so I'm not sure if the cycle time has increased greatly or if wasn't switching off, I'd need to check that. Any time I looked in, it seemed to be holding steady at 62 C without fluctuation.
It didn't seem to be possible to get the 15 C temperature drop. I was being very careful to open the lockshields as little as possible whilst still allowing flow.
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That is a great achievement IMO to get the boiler running continuously, at 8kw (or more), the "other" vertical rad is emitting 64% of its rated output, the two flat panel rads, 46% each of their rated output and the vertical rad in question also 46% of its rated output or 1.1 kw.
Have you established that the boiler is running continuously?, also a rough idea of the two flat panel outputs or type with dimensions.
I assume you throttled in the vertical rad lockshield to give that 7C delta?, wonder what it is with no throttling which would tell if reversing the valves does give a higher flow.
Have you established that the boiler is running continuously?, also a rough idea of the two flat panel outputs or type with dimensions.
I assume you throttled in the vertical rad lockshield to give that 7C delta?, wonder what it is with no throttling which would tell if reversing the valves does give a higher flow.
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This is a big improvement. Need to see if your lounge feels warmer now. Why have you not balanced all the rads in your home?
If I use 60C as the inlet temperature to all the four rads and assuming that they are all rated at ~ 2.4 kw each then based on their measured deltaTs, the two vertical rads should emit 1.6 kw each and the other two 1.4 kw each giving a total of 6.0 kw which will allow the boiler to run for a reasonably long period, (based on 8kw) so the question is, is 1.6 kw sufficient for the problem room.
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