Central heating pump sizing

[John.g]

Re "condensing", it would be interesting if you could break the condensate line and measure it (if any), from my basic calc you should get around 0.16 Ltr/kw of gas burned if fully condensing, so at minimum output of 5.2Kw ~ 0.8 litres of water/hour. If you can run on return target temp control then you will be able to see the exact effect the return temp has on the condensate rate, or adjust the target flow temp when boiler firing harder to reduce the return temp but not below the minimum boiler output.

 

[Reddwarf4ever]

Not easy to break the condensate line, as fully insulated and glued fittings.
during the recent cold snap, the boiler worked ok all day, pretty much at its minimum of 5KW, when the weather became milder, it actually satisfied the room stat several times in the day, flow temperature still set at 64oC and return around 55oC. Going to increase two rads this summer to double fin & as high an output as I can find. Hot water working well, set room stat to 23oC a couple of times a day for an hour, so hopefully the hot water will heat efficiently The boiler still cuts out at +3oC above target and it does fire up a bit too vigorously, often overshooting......but learning to live with these limitations. Wish the radiators had been designed around a D 40oC wouldn’t have so many problems achieving a boiler DT of 20oC & a lower return temperature.

just phoned a ‘recommended‘ Gas safe engineer. Had no idea if the 3oC trip or firing rate from startup could be changed, thought my idea about increasing radiator sizes is wrong suggested I get valiant out as they should be able to fix my problems. But I am not convinced that I wouldn’t just be wasting my money, if I get told either “it can’t be fixed “ or “ that’s going to be expensive “, a quick look at the valiant website gives a ‘one off service‘ as £285, apparently as my boiler is 11 years old it can no longer have a contract cover ( Max 8 years old ) even if for the £285 my problems were fixed, it was would takes years to payback this with any ‘possible’ fuel savings. Keep emailing Vailant with the questions can the 3oC cut out be adjusted or fixed to 5oC and can the fire up rate on start up be adjusted, ‘ reduced’, maybe it is the P.I.D. Controller.....maybe should just wait till it packs up & replace with a non valiant boiler.....one that does more what I need......wish I knew a friendly gas safe engineer with a good knowledge of valiant boilers to give me some answers

 

[John.g]

Welcome back.

You still have a lot of control over your return temperatures but the boiler may not be happy.
Taking the above.....flow/return/deltaT, 23C room temp and a TRV...
64/55/9 if you increased the flow temp to 70C then the exact same rad output will be achieved.....70/48/22 so you are now condensing nicely with a deltaT of 22C .
If you increased the flow temp to 75C then the exact same output will again be achieved ....75/44/31, better condensing but at the boiler deltaT limit of 30C. (I run my boiler all the time at ~ 70/75 and see these return temps of 43/46C regularly). I'm sure I mentioned these numbers previously??.

Now, if you doubled the size of the rads to give the same output as above then, for the same flow temp of 64C......flow/return/deltaT 64/24/40, deltaT 10C greater than boiler limit of 30C.
60/28/32 excellent condensing but marginal deltat for boiler
55/33/22 again outstanding condensing and only 22 deltaT.

SO, to sum up, with existing rads (output) then with a flow temp of 70C you will have a return of 48C and a deltaT of 22C
If you double the rad size, you can have a flow temp of 55C with a return of 33C and a deltaT of 22C.

 

[Reddwarf4ever]

Welcome back.

You still have a lot of control over your return temperatures but the boiler may not be happy.
Taking the above.***.flow/return/deltaT, 23C room temp and a TRV...
64/55/9 if you increased the flow temp to 70C then the exact same rad output will be achieved.***.70/48/22 so you are now condensing nicely with a deltaT of 22C .
If you increased the flow temp to 75C then the exact same output will again be achieved ***.75/44/31, better condensing but at the boiler deltaT limit of 30C. (I run my boiler all the time at ~ 70/75 and see these return temps of 43/46C regularly). I'm sure I mentioned these numbers previously??.

Now, if you doubled the size of the rads to give the same output as above then, for the same flow temp of 64C.***..flow/return/deltaT 64/24/40, deltaT 10C greater than boiler limit of 30C.
60/28/32 excellent condensing but marginal deltat for boiler
55/33/22 again outstanding condensing and only 22 deltaT.

SO, to sum up, with existing rads (output) then with a flow temp of 70C you will have a return of 48C and a deltaT of 22C
If you double the rad size, you can have a flow temp of 55C with a return of 33C and a deltaT of 22C.

 

[Reddwarf4ever]

So basically what you are saying is there is no need for me to keep driving the flow temperature down I realised that radiator BTU released would suffer, but assumed ‘wrongly it seems’ that that was the way to go. I have read elsewhere ‘ but shouldn’t believe everything on the internet’ that keeping the flow temperature as low as possible was the best solution to ensuring a condenser boiler is in condensing mode.

reading your post, I conclude that raising the flow temperature would increase the DT across the radiators, and not adversely affect the return temperature.

will certainly give this a try, ( SO, to sum up, with existing rads (output) then with a flow temp of 70C you will have a return of 48C and a deltaT of 22C ) if successful, will result in a faster house warm up, a more efficient boiler & allowing the boiler to cycle frequently as it’s meant to, plus no need to replace the lounge radiators.

will update soon..... set flow temperature to 72oC.....

 

[John.g]

Yes, as long as the rads are controlled by TRVs, if not controlled by TRVs then reducing the flow temp is the only way to reduce the return temp.

 

[Reddwarf4ever]

Yes, as long as the rads are controlled by TRVs, if not controlled by TRVs then reducing the flow temp is the only way to reduce the return temp.

if a TRV shut back, wouldn’t the heat released from the associated radiator fall ?, or would the flow increase through the remaining radiators, if not wouldn‘t this result in an increased return temperature To the boiler ? If the TRV was fully open, then this would be the same as not having a TRV ???

8/12 have TRVs

 

[John.g]

Yes, the rad output decreases with TRV throttling, that is the whole point of a TRV, it maintains a room temperature by controlliing the flow through the rad and changing its mean temperature which changes its output.
Yes, the flow through the remaining rads will increase leading to increased output and room temp tends to rise so the TRVs on these rads will throttle in more to return the room temperature to normal.
Yes, having a TRV fully open is exactly the same as having no TRV.
Remember my comments above are based on the assumption that TRVs are used throughout and its pointless increasing the flow temp to 70C as the higher return from the non TRVd rads will mix with the lower returns from the TRV rads leading to a increased boiler return temp.

If you are controlling the return temps on the non TRV rads by reducing the flow temp then this is fine and if you want to maintain the same output but with a reduced return temp then you will have to throttle the lockshield valves to give a return of 48C with a flow temp of 70C or a return of 44C with a flow temp of 75C, as you are aware, in all cases including your own of 64C/55C that the rad(s) output is reduced to 65% of its rated output.

 

[Reddwarf4ever]

Set the boiler flow temperature to 72oC and the return is 62oC, so clearly not suitable for my system.....I don’t have TRVs on the lounge radiators, because that’s what I have been told, you never fit TRVs to all rads, unless that philosophy has changed.

so back to 64oC for now...

 

[John.g]

Throttle the lock shield valves until you get 46/48C , remember its recommended anyway to throttle to get a 20C deltaT with a condensing boiler.

 

[John.g]

Forgot that you have a "smart" circ pump so a much easier method of achieving your output of 5.3kw with reduced return temp.
Your present settings are flow/return/deltaT/Kw, 64C/59C/9C.
If you increase the flow temp to 67C, change from CC3 to CP3, observe the return temp, then change to CP2 and finally CP1, if the pump curves are as I think they are then you should/might get 67C/50C/17C. Downside is you would probably have to return to CC3 to enable the boiler to refire after shutdown. Maybe worth a try?.

 

[Reddwarf4ever]

Forgot that you have a "smart" circ pump so a much easier method of achieving your output of 5.3kw with reduced return temp.
Your present settings are flow/return/deltaT/Kw, 64C/59C/9C.
If you increase the flow temp to 67C, change from CC3 to CP3, observe the return temp, then change to CP2 and finally CP1, if the pump curves are as I think they are then you should/might get 67C/50C/17C. Downside is you would probably have to return to CC3 to enable the boiler to refire after shutdown. Maybe worth a try?.

Will certainly give that a try, Friday ....running 62C/52-55C/7-10C/5-6KW working well, heating cycles 2-3 times a day when the room stat is satisfied, Hot water heated at same time as heating is on, so not a problem so far. With the two lounge radiators increased from 2784 to 5439 BTUs ( based on 50oC DT, [although I realise mine are 40oC DT]) Each, hopefully will be enough to improve things next winter.

just wish Vailant would actually give me an answer about the +3oC cut out Vs +5oC, which it’s meant to be.

 

[king of pipes]

These maybe of interest to you gives a bit more info on the pump settings. Kop

 

[Reddwarf4ever]

Haven’t had a chance to try your ideas out yet, on my list.
But........

I Compared February 2020 average temperature, 7.44oC with February 2021, 7.75oC
Gas use in Feb 2020 was 71.16 units and Feb 2021 71.0 Units

On the face of it, doesn’t seem a lot of point running my boiler at 62-64oC instead of 75oC. Was considering upgrading two lounge radiators to double fin to increase heat output on each by 30%, but with almost zero efficiency gain, seems it may not be a viable idea. Will try Johns suggestions. Wish these condensing boilers had built in efficiency calculators, would just need a condensate flow meter to achieve this I think. Or even a light to say in condensing mode.....be a selling point maybe assuming of course customers understand what it means.

will also look at the pump you tube video(s) but the post only contained an image not links, but will look for them thanks.

looking at this chart, my boiler is around 87% efficient if I increases the size of my two lounge rads to double fin, I am not sure if I will achieve even a 45oC return temperatur, even then the efficiency will be 90% which is only 3% more than I have now. To get the best from a condensing boiler IMO, it needs to be installed the same time as the rads etc. To size everything to get the most out of the condensing boiler. Not so easy, as things stand now.

the graph shows 3 flow rates, so if I read it correctly, using the lower flow rate ( in Red ) would achieve a better efficiency

graph obtained from here

Monitoring energy efficiency of condensing boilers via hybrid first-principle modelling and estimation

The operating principle of condensing boilers is based on exploiting heat from flue gases to pre-heat cold water at the inlet of the boiler: by conden…

www.sciencedirect.com

most in depth explanation / discussion on condensing boilers I have ever seen

 

[ShaunCorbs]

Need to keep it condensing else it will break the boiler in a year or two as the heat exchanger will get blocked as there’s nothing to move the waste