Flow temperatures

[Johnytuono]

Worcester Bosch Si Compact 30 Kw
7 rads + towel rad
Roughly 8-9 kw rad output
Ok, I keep hearing all the talk of trying lower flow temperatures and seeing the advice about trying different lower temperatures to see if your system would heat up ok with them.
So, I thought I'd try at 48c and work my way up.
Running Nest our schedule is
0715 to 1730 18c
1730 to 2145 19c
2145 to next day at 16c setback
Now I know it's not been particularly cold (apart from the odd morning at 5c) but find ours is getting up to the set temperatures in our 1900s built house.
The boiler sometimes does hit set point (48c) +5 then wait to cool to the hysteris. Is this harming the boiler? Should it be set higher so it hits the room temp before hitting the boiler set point temp?
I have balanced all the rads at around 12c but when looking at the boiler return (it actually shows the temp), it is only about 8c from a flow of 48c
The pump speed is on 2, speed 1 didn't seem to get the water around fast enough and speed 3 was too quick, and reached the set point as it does now (but earlier).
Maybe I've done all the rad/room calculations right (pure luck if so as certain sites give different powers needed) and the rad output where the room stat is gives out a max 2.2 Kw whereas with the current temp only about 800 watts is need, hence the low flow temp.
Thought please, especially about the boiler hitting set point +
Thanks

 

[Johntheo5]

The boiler will exceed the setpoint temperature if its minimum output is greater than the heat demand, when it reaches SP+5C the burner trips but the circ pump stays running, the burner will not refire until the anti cycle time has elapsed, providing the flowtemp is < SP-5C. It won't harm the boiler

 

[Johnytuono]

The boiler will exceed the setpoint temperature if its minimum output is greater than the heat demand, when it reaches SP+5C the burner trips but the circ pump stays running, the burner will not refire until the anti cycle time has elapsed, providing the flowtemp is < SP-5C. It won't harm the boiler

I notice in the menus that the anti cycle time is set at 3 mins, should this be longer with it being a lower flow temp? If so, what would you suggest please?
Thanks again

 

[Johntheo5]

The anticycle time should be (just) long enough to ensure that the flowtemp doesn't exceed SP+5C after the burner refires, otherwise it may take a number of cycles to achieve this, all boilers fire up at ~ 60% of their max output which is why they don't refire as soon as the flowtemp is < SP-5C, and yes, the lower the SP the greater the anticycle time, some boilers like Vaillant will automatically change the anticycle time with changing flowtemp SP. To give a example (Vaillant) which has a default setting of "20 minutes" will give a actual anticycle time of ~ 10 minutes at your SP temp of 48C, if the SP temp was increased to 65C then the anticycle time decreases to 4.5mins and if the SP temp was reduced to say 35C then the anticycle time increases to 15mins, I attached it below, for info.

 

[Johnytuono]

Thanks again John
Yes I was looking at that chart before, and wondered if I could go off that (as a starting point) for my WB.
So, if I change the anti cycle time to, say, 10 mins, and it doesn't get up to temp then I reduce the anti cycle time? or do I increase the flow temp instead? What I'm saying is, if it doesn't reach room stat temp, would it be because of a too long anti cycle time or too low a flow temp?
Sorry for so many questions

 

[Johntheo5]

You can use that as a guide.

If it doesn't reach room SP temp then that will generally only happen due to multiple cycles, if the boiler gets away after one recycle then the room should reach SP temp, so you would increase the anticycle time in this case. Obviously if you have the flowtemp SP set ridiculously low then the burner may only run for a minute or so which will result in a slow rise in room temp.

 

[Johntheo5]

Worcester Bosch Si Compact 30 Kw
7 rads + towel rad
Roughly 8-9 kw rad output
Ok, I keep hearing all the talk of trying lower flow temperatures and seeing the advice about trying different lower temperatures to see if your system would heat up ok with them.
So, I thought I'd try at 48c and work my way up.
Running Nest our schedule is
0715 to 1730 18c
1730 to 2145 19c
2145 to next day at 16c setback
Now I know it's not been particularly cold (apart from the odd morning at 5c) but find ours is getting up to the set temperatures in our 1900s built house.
The boiler sometimes does hit set point (48c) +5 then wait to cool to the hysteris. Is this harming the boiler? Should it be set higher so it hits the room temp before hitting the boiler set point temp?
I have balanced all the rads at around 12c but when looking at the boiler return (it actually shows the temp), it is only about 8c from a flow of 48c
The pump speed is on 2, speed 1 didn't seem to get the water around fast enough and speed 3 was too quick, and reached the set point as it does now (but earlier).
Maybe I've done all the rad/room calculations right (pure luck if so as certain sites give different powers needed) and the rad output where the room stat is gives out a max 2.2 Kw whereas with the current temp only about 800 watts is need, hence the low flow temp.
Thought please, especially about the boiler hitting set point +
Thanks

I have a nice little spreadsheet that I put together years ago when ASHPs became the greatest thing since the invention of the wheel but it gives a good feel for the minimum flowtemps required for different outside air temps, OATs, I will post it later, it may, or may not, be of interest.

 

[Johnytuono]

I have a nice little spreadsheet that I put together years ago when ASHPs became the greatest thing since the invention of the wheel but it gives a good feel for the minimum flowtemps required for different outside air temps, OATs, I will post it later, it may, or may not, be of interest.

Yes, please post it, would be interesting to look at.
I still can't get my head around the anti cycling times theory though. In my mind, a high flow temp (eg 70c) would need a longer anti cycle time (to cool down after hitting flow+5) than a low one (50c), which will cool down quite low anyway with it being already at a lower temp. I know the guide from Valiant goes against my thinking ie the higher flow the lower the anti cycle time.
Also re-reading what you put, once it reaches the flow SP +5, the idea is to reach the room stat SP ideally just before it has to recycle again? Have I got that right?
Also, do you know if Nest (like Honeywells' stat) have built in anti cycle times?
Thanks again

 

[Johntheo5]

Yes, please post it, would be interesting to look at.
I still can't get my head around the anti cycling times theory though. In my mind, a high flow temp (eg 70c) would need a longer anti cycle time (to cool down after hitting flow+5) than a low one (50c), which will cool down quite low anyway with it being already at a lower temp. I know the guide from Valiant goes against my thinking ie the higher flow the lower the anti cycle time.
Also re-reading what you put, once it reaches the flow SP +5, the idea is to reach the room stat SP ideally just before it has to recycle again? Have I got that right?
Also, do you know if Nest (like Honeywells' stat) have built in anti cycle times?
Thanks again

The rads will still be emitting heat when the burner trips and the hotter the flow & return tepms are, the greater the heat output of the rads so the will cool down much faster in any given time and flowrate and even if the flowtemperature at the end of the recycle period is the same as that of the lower flowtemperature at the end of its recycle period, the dT required to get it back up to its SP will be much higher so the less recycle time required.
"once it reaches the flow SP +5, the idea is to reach the room stat SP ideally just before it has to recycle again? Have I got that right?" Well, maybe ideally yes, but even if it still fires say for another 4 or 5 minutes before cycling again then it will eventually reach the SP temperature, remember oil fired boilers like mine cycle continuously (no modulation) for a long time before the SP is reached and boiler cut out.

 

[Johntheo5]

Worcester Bosch Si Compact 30 Kw
7 rads + towel rad
Roughly 8-9 kw rad output
Ok, I keep hearing all the talk of trying lower flow temperatures and seeing the advice about trying different lower temperatures to see if your system would heat up ok with them.
So, I thought I'd try at 48c and work my way up.
Running Nest our schedule is
0715 to 1730 18c
1730 to 2145 19c
2145 to next day at 16c setback
Now I know it's not been particularly cold (apart from the odd morning at 5c) but find ours is getting up to the set temperatures in our 1900s built house.
The boiler sometimes does hit set point (48c) +5 then wait to cool to the hysteris. Is this harming the boiler? Should it be set higher so it hits the room temp before hitting the boiler set point temp?
I have balanced all the rads at around 12c but when looking at the boiler return (it actually shows the temp), it is only about 8c from a flow of 48c
The pump speed is on 2, speed 1 didn't seem to get the water around fast enough and speed 3 was too quick, and reached the set point as it does now (but earlier).
Maybe I've done all the rad/room calculations right (pure luck if so as certain sites give different powers needed) and the rad output where the room stat is gives out a max 2.2 Kw whereas with the current temp only about 800 watts is need, hence the low flow temp.
Thought please, especially about the boiler hitting set point +
Thanks

OK, as you probably know (T50) rads are sized based on the difference between the required room temperature (20C is generally used) and the outside air temperature, OAT, a minimum OAT of -5C might often be used, there may be a standard but I don't know what it is, anyhow I do most of my calcs based on that, you will see from the spreadsheet that my calcs show that based on your 2200W rad and outputting 800W, 36.36% of its T50 rating will satisfy a room temperature of 20C with a OAT of 10.91C and will require a flowtemp of 44.8C, a OAT of 5C will require 60% rad output, 1320W, with a flowtemp of 56.8C, a OAT of 0C will require 80% rad output, 1760W with a flowtemp of 56.8C and (obviously) a OAT of -5C will require 100% output, 2200W with a flowtemp of 75C.

You can play with your own calcs, maybe use "Copy of calcs" sheet, the only think you need to do is to change the flowtemp to give the same flowrate as that at 100% output, 1.43LPM since the flowrate generally does or is not changed (automatically in the boiler) with flowtemp change but the flow/return temp dT does change.

EDIT: Just changed file to Rev1 but identical apart from highlights

 

[Johnytuono]

Thanks John for the chart. and info
Surprisingly, my calculations weren't too bad, roughly I was thinking, inside stat temp 19 c
OAT 10c ...flow 50c
OAT 5c...flow 60c
OAT 0c and below ...flow 65c to 70c ish
Like I say, the anti cycle WAS confusing me a little but now got my head around it a lot more, thanks to your explanation.

EDIT: John. for some reason the download won't work ( the previous Valiant one did)

 

[Johntheo5]

Have just PMd it to you but here it is again, below. (.ZIP)

 

[Johnytuono]

Thanks John, when I clicked on it again it worked.
I worked it out that my room would need 1.9 Kw @-1c OAT and room stat at 20c btw. So I can adjust accordingly and can manually weather compensate each week (or month).

 

[Johntheo5]

Yes, close enough, I get 1.85kw with flow/return of 67.93C/59.5C & flowrate 1.43LPM.

Of course your rads may be oversized or undersized but at least it gives a good feel for it.

 

[Johnytuono]

Looking at my flow/returns though, I'm averaging 10 to 12c at the rads but only about 7c at the boiler (according to the menu system), so my lpm through the rads are much lower. Is the 7c at the boiler because of the lower flow and OAT?