Flow temperatures

[Johntheo5]

If the flow/return temps dT just before the boiler are 10C to 12C and only 7C in the boiler then there must be a internal bypass open?.

 

[Johnytuono]

I'm going off the menu system in WB showing about 7c difference (sometimes less). All rads (even the towel rail) are balanced to around 10 to 12c difference. Internal bypass is something that I don't know anything about though John, not even sure if my WB Si Compact (2014) has one or even is it something a householder, like me, can alter?

On another note, am I actually doing right at achieving a Delta of 10 to 12 c at the rads, would I be better going for something like 5 to 8 c difference thus giving me a higher mean temperature and obviously higher rad output and then able to use lower flow temps with reasonable heat up times?

EDIT : just looked at the manual and indeed it has an internal bypass.

 

[Johntheo5]

Boiler makers normally state the max flowrate (at max output) based on a dT of 20C mainly because they can install smaller heat exchangers and still have a reasonable pressure loss through the heat exchanger, if the boiler is run at a dT of 10C then this will increase the plate heat exchanger pressure loss to the square of the flowrates, as you can see by the calcs the dT will decrease automatically as the rad duty decreases but not a problem as the total flowrate is still relatively low due to the low boiler output required.
I have included a few numbers based on a dT of 20C at 100% rad output which translates to (at the rad outputting 36.36%, 800W) and flowtemp/returntemp/dT are 46.6C/39.33C/7.27C vs 44.78C/41.14C/3.64C at the original dT of 10C at 100% rad output, so the return temp is a bit lower, 41.14-39.33, 1.21C, so not much difference at these modest rad outputs but the boiler would have to be run at 80C with a return of 60C at 100% output.

 

[Johnytuono]

Again, thanks for that John.
The flow rate of 1.43 lpm, what flow temp differential at the rads to achieve that please as I notice in the charts you sent, the max delta I had (10 to 12c) was only at 100% rad output. I wasn't aware the delta would decrease with a lower flow temp. Also, in the charts, are the shown return temps at the boiler or rads?

EDIT: I've been balancing at the rads with a flow temp of 48 to achieve around 10 to 12c, should I first balance at the design flow (75c) to achieve 10 to 12c at the rads, then, when I decide to use a lower flow temperature the delta would automatically decrease as in your charts?
Sorry if I'm being a pain, just trying to get my head round it, as certain people say a lower differential (higher means temp so higher rad output) and certain sites are saying a higher diff temp (20c) which will obviously give less rad output and a lower means temp.

 

[Johntheo5]

The dT has to reduce with reduced rad output at the same flowrate, as the rad output decreases the flowtemp remains the same but the return temp starts rising until if there was no rad output, it would be the same temp as the flowtemp the dT would be zero. The return (& flow) temps are at the rads but should be very close to the boiler flow&return temps, they might loose a degree or so in pipework losses due to radiation but as stated the return temp at the boiler can and is different in your case? because its mixed with hot water from the internal by pass to give a higher indicated boiler temperature.
Yes,lower dT means higher output (for the same flowtemp) and higher dT means lower output, as you say the mean rad temp is lower at the higher dT.

I would suggest just increasing the boiler flowtemp to 75C for now and when the boiler flow and return temps have settled down, take the readings and also take them at the outside of the boiler.

 

[Johnytuono]

So really I should have started balancing at, say, flow 60c to achieve a return difference of 10/12c as this would give me a decent flow rate, then, when I reduce the flow temp the flow difference would be less too. I was trying to achieve that differential at a flow of 48c (which would obviously be too slow a flow rate)?

Btw, can anything be done about this internal bypass affecting the reading? We do have a towel rad without a try on and it is always open.

 

[Johntheo5]

So really I should have started balancing at, say, flow 60c to achieve a return difference of 10/12c as this would give me a decent flow rate, then, when I reduce the flow temp the flow difference would be less too. I was trying to achieve that differential at a flow of 48c (which would obviously be too slow a flow rate)?

Btw, can anything be done about this internal bypass affecting the reading? We do have a towel rad without a try on and it is always open.

I would just take a set of readings it for now at whatever the max flowtemp you decide to use, IMO it should be done at 75C because a T50 rad will only emit 100% output at 75C with a dT of 10C, with a required room temp of 20C, but its up to yourself.

Can you post on the MIs for your boiler, will have a look at that bypass then.

 

[Johnytuono]

It won't let me upload the 30 mb file and can only compress it to 17mb and still too big

If it helps, its a WB Greenstar si compact 30kw (2014) so has the 3 speed pump

 

[Johntheo5]

Downloaded the eManual, the minimum heat INPUT is 7.15kw so ~ 6.3kw minimum output, (Max 23.5kw output to CH)

"7 rads + towel rad
Roughly 8-9 kw rad output"

So will cycle regularly if the heat demand/rad output is < say 75% of max.

Don't think you can do much about that internal by pass, there's no adjustment from what I can see.

 

[Johnytuono]

ok, thanks for looking John, really appreciate it.
Have balanced the rads at 60c flow with 10 to 12 c differential at the rads. The boiler, when fully heated up reports back at 60 flow and 55 return. The pump is on speed 3 though now, maybe alter that and re balance at 60 c flow? Milder whether I will run at 50 /55 (or even less). I'm assuming when the flow temp is lowered then the temp differential at the rads will be less?

 

[Johntheo5]

Here you are, a few readings, based on flow/return temps of 60C/50C, a T35 rad, 62.9% Output.

 

[Johnytuono]

The anti rapid fast cycle time (ARFCT) seems to work differently on the WB. I emailed WB and they said the ARFCT is only the time between the Nest calling for heat and a new heating program starting i.e if I set the ARFCT to, say, 10 mins, then if my room temp dropped below the SP then it wouldn't restart until the ARFCT of 10 mins had passed. This seems different than what I thought it was, I thought it was when the boiler had hit the flow SP+5 then it was off for 10 mins ARFCT? Does this sound right? I timed the off time after SP+5 and it wasn't 10 mins, more like about 3 mins.
Also, with running a low flow temperature, am I better off putting the pump on speed 3 rather than the speed (2) that I have it on now.
Thanks in advance.
Btw I had flow set to 45c today and it hit room temp of 19c but obviously cycled a few times because of hitting SP+5

 

[Johntheo5]

The anti rapid fast cycle time (ARFCT) seems to work differently on the WB. I emailed WB and they said the ARFCT is only the time between the Nest calling for heat and a new heating program starting i.e if I set the ARFCT to, say, 10 mins, then if my room temp dropped below the SP then it wouldn't restart until the ARFCT of 10 mins had passed. This seems different than what I thought it was, I thought it was when the boiler had hit the flow SP+5 then it was off for 10 mins ARFCT? Does this sound right? I timed the off time after SP+5 and it wasn't 10 mins, more like about 3 mins.
Also, with running a low flow temperature, am I better off putting the pump on speed 3 rather than the speed (2) that I have it on now.
Thanks in advance.
Btw I had flow set to 45c today and it hit room temp of 19c but obviously cycled a few times because of hitting SP+5

The above (ANTI RAPID FAST CYCLE) at 10mins is to limit the cycling time to 6 cycles/hr and really has nothing to do with the normal anticycling time when/if the heating demand falls below the boiler min output, Evohome and other fancy control systems use the same ARFC philosophy, Evohome's is settable, if you only want 3 cycles/hr you set the time to 20 minutes.

 

[Johnytuono]

Thanks John, So is this what I'm setting or is it as the WB 'technicians' said, purely to stop another heat demand from Nest within 10 mins? I must admit, the answer from them did have me confused, although I had it set at 10 mins and it restarted after about 3 mins after hitting flow SP+5, unfortunately I didn't see if it was running for 7 mins or not. Would the 10 minutes always equal 10 mins (in total on/off)?
Again, sorry if this is basic stuff but just trying to get my head around it? Also, What do you think of the pump speed on a lower flow temp, should it be on 2 or 3?
Thanks again

 

[Johntheo5]

I don't know how clever the boiler is if a "normal" anticycle occurs and how it affects the set anticycle time, maybe it restarts another 10 minute antcycle time after the normal anticycle time has elapsed.

Re pump speed, in general, I would think that you require ~ 3M to 3.5M pump head with all rads running so you would have to look at the pump curves and modes, you may find that using a constant pressure mode is better if fixed speed 2 is only sat 2.5M which means you would have to possibly go to fixed speed 3 which is 6M on a 6M pump and will result in a ~5M head with all rads running.