Central heating pump sizing

[Reddwarf4ever]

Looks like there is a 60 minute time out to do editing, which is a pity.

UPDATE
1- I have set the boiler to 18KW, first time ever....its modulating ok, but have had to reduce the target temperature to 68oC to keep the inlet below 55oC.....so quite a low DT
2-occasionally the pump declares ‘LO FLOW’ then shortly after shuts off, restarts with the same message, then after a few minutes reverts to the previous readings I.e. 0.7m3/Hr etc....set it to heating only for now to establish that works ok.

not sure if the higher flow; which it looks like I have, has dislodged some debris and causing the pump problems.....or it’s something else, all rads now very hot, even one in a room which previously struggled to get warm. Ideally want to heat the hot water in the day, together with the heating, as on hot water only, I don’t think I will get a low enough inlet temperature for condensing.... going to try again tomorrow....also if I try to heat the hot water on its own, not sure if the DT between the immersion cylinder and boiler outlet is enough

thanks for the replies......very confused about the pump setting options., is CP3 the middle icon of the three ..?.....happy to have a lower flow if it helps the DT....will look at the charts again...

still worried about the LOW FLOW message, although hasn’t happened since a few hours ago....🤞

also was considering increasing two lounge rads from single to double, won’t increase the flow obviously bit will emit extra heat and help the boiler DT, any thoughts ?

 

[John.g]

The pump shouldn't be displaying that message except its some form of protection, suggest changing to CP3 for now, you can see the different mode symbols on top and the 3 vertical bars are the settings for each mode.

 

[Reddwarf4ever]

Ok I set the pump to D3, the boiler is now at 11KW ( although it’s modulating there is no indicator of actual KW ( Gas ) being burnt.....dropped outlet temperature to 68oC and return @57oC just checked outside and it’s Not condensing.....can’t drop,outlet any more, don’t think it will manage Hot Water at such a low temperature.....should I try D1 ? What is the theory about having a lower flow Vs inlet temperature ? Is it simply it has more residence time in the rads ? Any thoughts on upgrading 2 rads 100cm x 50cm to double fin ? Can’t see it will hurt 🤔

been told power flush devices can be hired, will have to drain down in the summer if I fit new rads, could do it then......don’t want to do it if not needed though

P.S. I forgot to say I accidentally left hot water & heating on together, although this should be acceptable to do, have now left it on heating only....

 

[ShaunCorbs]

is the system hot already or cold ?

 

[John.g]

What do you mean by D3 on the pump, is it constant pressure as shown in the diagram, and can you read off the flowrate, the head and the watts, can then calculate the boiler output.

And yes your old pump was kaput or full of sludge as it should have been circulating 0.66 M3/hr (11 LPM) at a head of 4.9M based on your new pump readings above.

 

[Reddwarf4ever]

What do you mean by D3 on the pump, is it constant pressure as shown in the diagram, and can you read off the flowrate, the head and the watts, can then calculate the boiler output.

And yes your old pump was kaput or full of sludge as it should have been circulating 0.66 M3/hr (11 LPM) at a head of 4.9M based on your new pump readings above.

Sorry

meant CP3, trying CP2 now.....will get readings ASAP

 

[John.g]

"What is the theory about having a lower flow Vs inlet temperature ? Is it simply it has more residence time in the rads ? "
More sometimes equals less.
Yes, but you also need to keep the mean radiator temperature as near design temperature as possible, above you have flow/return temps of 68/57, mean of 62.5C which results in the rad emitting 81% of its rated output based on a room temperature of 20C, if you are happy with this but require a return temperature of say 50C but also require the same mean rad temperature of 62.5C then there is only one way of achieving this and that is to raise the boiler/rad flow temperature to 75C and keep reducing the flowrate until the return temp is 50C, this still gives a mean rad temp of 62.5C. and no problems in heating your DHW cylinder.

 

[Reddwarf4ever]

"What is the theory about having a lower flow Vs inlet temperature ? Is it simply it has more residence time in the rads ? "
More sometimes equals less.
Yes, but you also need to keep the mean radiator temperature as near design temperature as possible, above you have flow/return temps of 68/57, mean of 62.5C which results in the rad emitting 81% of its rated output based on a room temperature of 20C, if you are happy with this but require a return temperature of say 50C but also require the same mean rad temperature of 62.5C then there is only one way of achieving this and that is to raise the boiler/rad flow temperature to 75C and keep reducing the flowrate until the return temp is 50C, this still gives a mean rad temp of 62.5C. and no problems in heating your DHW cylinder.

Thanks, will experiment with your suggestions tomorrow...defo looks like this pump is doing a much better job than the old one......If CP1 doesn’t give a low enough head to achieve a good DT, is there another setting to try ?
1- does anyone know if the heat recovery is a constant, as the inlet temperature falls ? Or as I assume it’s a curve, giving less heat recovery as the temperature falls, think the majority must be achieved at around 50-55oC ?
2-I read it’s a good idea to increase rad sizes when changing to a condensing boiler to assist the low return temperature, so is my idea of changing the 2 rads in my lounge to double fin, a good way to go ?
3- is it normal to heat the hot water the same time as having the heating on ? Because the heat removed from the immersion cylinder will be no where near what the rads remove, and having both would ‘ share’ the flow....
4-.I assume in the summer months with hot water only, condensing mode would not easily be achieved

 

[Reddwarf4ever]

is the system hot already or cold ?

HOT 🔥

 

[John.g]

I would increase the flow temp to 75C then go to CP2 and write down the boiler deltaT and the pump flow,head & watts.
Then ditto for CP1, PP3 & PP2.
Remember though that if the head is too low then some rads will run colder than others and require balancing, for the moment don't bother with balancing.
I run my boiler at 75C with TRVs on 8 of the 10 rads and I've often seen 40C return temps, I have never balanced the rads, I run on PP3 which, with my system, gives ~ 12LPM @ 3.4M head. I would expect your system to run at [email protected] on CP1, 8.5LPM@3M on CP2, [email protected] on CP3. 9 [email protected] on PP3, & [email protected] on PP2.

Q1. If you mean boiler heat recovery = efficiency then the lower the return temp the greater the efficiency due to enhanced condensing effect.

Q2. I have used a factor of X2 on any replacement rads to allow for weather compensation etc if my present non condensing boiler ever gives up the ghost.

Q3. A lot of boilers now use DHW priority where, when DHW is required, the boiler switches to HW heating and automatically increases the boiler SP temp to 80C which gives very fast heating times but little or no condensing benefit and then switches back to CH after the 30/45 mins or so required.
But if you are not too worried about the length of time required to heat the cylinder then you can install a balancing gate valve on the cylinder return and by throttling it in to say 1/4 turn open you can easily get 50C for a lot of the heating period even with a flow temp of 75C.

Q4. As above.

The effect of return temps on condensing benefit is clearly shown below, not any huge benefits at 50/55C but below 50C, not bad at all.

Happy testing.

 

[John.g]

Condensing Effects on boiler efficiency.

 

[Reddwarf4ever]

Thanks that graph is exactly what I was looking for, even asked in a scientific forum, but got no where 🤯
trying various options with the boiler

CP1 Boiler target 68oC 8KW limit inlet 46oC
8.6-10W
1.8-2.0m
0.4l/h

CP2 Boiler target 68oC 8KW limit inlet 46oC
13.2-17W
2.6-3.0m
0.5-0.7l/h
not sure why the fluctuation, found if impress the Mode button once quickly, it cycles through the values, the manual says pressing it changes the mode, but only when it’s displaying the mode ( multipurpose buttons not helpful ) I previously held the button in until it displayed a rotating ‘arrow’ then pressed once, but although it display the readings, it had gone through a degas so values had changed, easier now I know.

One problem IMO apart from the pump, is the valiant boiler Eco tec plus 418, it does not modulate very well, often continuing to hard fire as it reaches the target temperature and thus overshoots, made worse by the fact it cuts out when only 3oC over temperature. Takes a long while going through the tripping restarting before it settles down again. Sometimes I drop the KW limit to 5KW and ‘ nurse ‘ it back up by slowly increasing the KW.....IMO I should t have to do this, & if I can do this then the boiler should. I wonder if the boiler is actually oversized, which again antagonises the problem. if it were sized as say 12 KW instead of 18 KW, wouldn’t it be able to cope with lower flows ? If that is the case then it will always be a struggle.....

I have 7 radiators which could be made double fin.....wonder how much effect that would have on the delta T ???

 

[Reddwarf4ever]

Ok on CP1... pump 9W 1.8m 0.5l/h fairly stable
boiler 68oC target & actual, inlet 55oC

So am just on the condensing temperature & it’s slowly going up.
i have 12 rads total
All 100x50cm
downstairs
2 double ( one of which only gets really hot when on CP3 in extension room)
2 single
1 towel
upstairs
2 towel
5 single

Detached brick house built 1987, 3 bed, game room, store room, lounge, kitchen, cloakroom, bathroom and shower room....usually at 23oC, wonder what size boiler I would need for this, need to compare with the 18KW I have.

wonder if, as per my previous post, the boiler is over sized, assuming a smaller one can handle a lower flow, which would help the DT.....all I can think of doing at this point is increasing rad sizes.....or living with an inefficient heating 🤯

 

[ShaunCorbs]

As you can adjust the output of the boiler work out your rad kw and add them all together that’s what you set your boiler out put to

 

[John.g]

I don't see any reason whatsoever to get a smaller boiler, you already have a 5.5kw (minimum output?) boiler, you seem to have some control problem when first firing up, either not modulating down rapidly enough after ignition (unlikely, with the boiler demand above of 10.23 kw on CP1 and 15.34kw on CP2, OR it is modulating down OK after start up but is then increasing the firing rate too fast to enable it to ramp down when the SP is reached, not helped by the 3 deg hysteresis.
You might carry out a simple test, once the boiler has settled down at its target temperature of 68C?, change the target temperature rapidly to say 73/75c and see if the burner cuts out on high temp (target+3C, in your case), if it does. then a control problem, IMO, or it may cut out on deltaT so make a note of whichever, if satisfactory, reduce the target to 68c and again watch its reaction.

 

[Reddwarf4ever]

As you can adjust the output of the boiler work out your rad kw and add them all together that’s what you set your boiler out put to

I can’t actually adjust the output, only limit the maximum.....can’t forever keep adjusting it, which is what I am doing now.....have to find a website to help me calculate the rad KW consumption....unless you know of one ?

 

[Reddwarf4ever]

I don't see any reason whatsoever to get a smaller boiler, you already have a 5.5kw (minimum output?) boiler, you seem to have some control problem when first firing up, either not modulating down rapidly enough after ignition (unlikely, with the boiler demand above of 10.23 kw on CP1 and 15.34kw on CP2, OR it is modulating down OK after start up but is then increasing the firing rate too fast to enable it to ramp down when the SP is reached, not helped by the 3 deg hysteresis.
You might carry out a simple test, once the boiler has settled down at its target temperature of 68C?, change the target temperature rapidly to say 73/75c and see if the burner cuts out on high temp (target+3C, in your case), if it does. then a control problem, IMO, or it may cut out on deltaT so make a note of whichever, if satisfactory, reduce the target to 68c and again watch its reaction.

Will do as you suggest.....it’s all driving me mental......had really hoped the new pump would solve my problems......I turned on hot water, while the boiler was firing steadily, the firing increased slightly, due to the cylinder demand.....left it alone, but after a while it started cutting out on over temperature.....and went into its usual cycle.....the cylinder temperature was increasing, had hoped it would be ok.....now turned off the hot water and the boiler is firing ok....it’s at 7KW 68oC inlet 43oC, bit only because it has been cycling for an hour or more....so for some reason it doesn’t like heating and hot water on together.....I can’t restrict the inlet flow to the cylinder as if on HW only, the boiler would have, I assume an insufficient flow......

the boiler does have an option for a separate HW control, just needs a valiant module, can’t find any info on that....have emailed vailiant....so if I can get some sort of stability in the day with the heating ( miracles do happen ) then can allow the HW to heat overnight with the higher temperature set for HW.....

thanks for all the continued support.......my Wife says my life has been taken over trying to get this boiler sorted...

😎😎😎

 

[ShaunCorbs]

I can’t actually adjust the output, only limit the maximum..can’t forever keep adjusting it, which is what I am doing now..have to find a website to help me calculate the rad KW consumption....unless you know of one ?

screwfix book find your existing rads roughly they list the kw output on there website

and yes you need to set the maximum to this value

I would stagger hot water times roughly an hour earlier than heating in the morning

 

[Reddwarf4ever]

I don't see any reason whatsoever to get a smaller boiler, you already have a 5.5kw (minimum output?) boiler, you seem to have some control problem when first firing up, either not modulating down rapidly enough after ignition (unlikely, with the boiler demand above of 10.23 kw on CP1 and 15.34kw on CP2, OR it is modulating down OK after start up but is then increasing the firing rate too fast to enable it to ramp down when the SP is reached, not helped by the 3 deg hysteresis.
You might carry out a simple test, once the boiler has settled down at its target temperature of 68C?, change the target temperature rapidly to say 73/75c and see if the burner cuts out on high temp (target+3C, in your case), if it does. then a control problem, IMO, or it may cut out on deltaT so make a note of whichever, if satisfactory, reduce the target to 68c and again watch its reaction.

Well I increased the KW to 15 or the boiler wouldn’t be able to increase firing, set output to 75oC climbed to 78oC and cut out......reduced KW to 7 as before changed outlet to 68oC and it cut out at 71oC.....even at only 7KW.....😩 waiting for it to ‘ settle down ‘ again

 

[Reddwarf4ever]

screwfix book find your existing rads roughly they list the kw output on there website

and yes you need to set the maximum to this value

I would stagger hot water times roughly an hour earlier than heating in the morning

Thanks will have a look.....going to run hot water 4am to 7am.....when the heating is off.....but will need to change outlet temperature to 75oC for the night and back to 68oC for the day....

 

[ShaunCorbs]

Well I increased the KW to 15 or the boiler wouldn’t be able to increase firing, set output to 75oC climbed to 78oC and cut out...reduced KW to 7 as before changed outlet to 68oC and it cut out at 71oC..even at only 7KW.***.😩 waiting for it to ‘ settle down ‘ again

this is normal if the system is hot

 

[Reddwarf4ever]

Well as expected the boiler ‘settled down’ again and is currently set at 6KW outlet temperature 68oC inlet 52-53oC......going to leave this alone and see how it goes tomorrow, will see if it heats up the hot water tonight, but suspect I will need to change the output temperature to around 75oC.....

don’t think it’s raising the house temperature much, but comfortable, hopefully long term if I change all the single rads to double, it will give the boiler more to work on.

 

[ShaunCorbs]

The output temp doesn’t have that great effect on room temp unless your running the room temp at your flow temp

 

[Reddwarf4ever]

The output temp doesn’t have that great effect on room temp unless your running the room temp at your flow temp

Sorry don’t understand that.......

 

[John.g]

I can’t actually adjust the output, only limit the maximum..can’t forever keep adjusting it, which is what I am doing now..have to find a website to help me calculate the rad KW consumption....unless you know of one ?

Well as expected the boiler ‘settled down’ again and is currently set at 6KW outlet temperature 68oC inlet 52-53oC...going to leave this alone and see how it goes tomorrow, will see if it heats up the hot water tonight, but suspect I will need to change the output temperature to around 75oC..

don’t think it’s raising the house temperature much, but comfortable, hopefully long term if I change all the single rads to double, it will give the boiler more to work on.

You said in one post that you have to run on CP3 to heat up all the rads and would suggest that you run in this mode for the time being at any rate.
What mode is giving you the above numbers?.

There is clearly something wrong with the boiler as you shouldn't have to keep chasing the output up/down to get the boiler to run, IMO you should be able to leave it at max output, I can understand setting the CH output to say 20kw on a 36kw combi with a heating load of 18kw, otherwise not.
You might consider getting a fixed price (if they provide this type of service) from vaillant to fix this boiler.