Optimum setting for boiler flow when heating DHW to 60c

[scoobydo123]

All,

If hot water is stored at 60c this is measured with a probe at an outlet. What should you be looking at setting the boiler to? Would you expect a cylinder to recover without a single cycle if the boiler can modulate down to 5kw and the coil is rated at 19kw? I have had to set flow to 78 to get round the issue with micro firing on vaillant but this then has a negative effect on running in condensing mode for heating. With microfiring the cylinder took an age to be fully satisfied if running at more moderate settings. Would like to know what you would set at if microfiring wasn't present to ensure efficient heating of cylinder. Cylinder is around 4M away on insulated pipework.

 

[dcawkwell]

I have my vokera boiler set to 70 degrees and it cycles 2 or 3 times before the cylinder reaches temperature. This is inevitable as the temperature approaches the temperature.

I suggest you run the boiler at 61 or 62 degrees for an hour or so.
Then turn the cylinder thermostat up/down until you hear it click then turn it back slightly to set the cylinder thermostat correctly.
It may be set a little high hence the boiler is cycling a bit to get to
temperature.

 

[scoobydo123]

The stored water is at 60c as I have tested it with a probe so I am certain I am not storing water too high. What rated coil is in your cylinder? My issue is that I have a vaillant boiler that has badly written software that causes the boiler to start up at 22kw so it over shoots target flow temp within 6 seconds so never really puts that much more heat into the cylinder with a 6 second burn. However I can get round it by setting the flow temp in the mid 75's it just about restarts and stays burning for 45 seconds at which point it will then modulate to the required amount but his has a negative impact on efficiency as I dont need 75c flow in my radiators so it doesn't stay condensing.

 

[dcawkwell]

The stored water is at 60c as I have tested it with a probe so I am certain I am not storing water too high. What rated coil is in your cylinder? My issue is that I have a vaillant boiler that has badly written software that causes the boiler to start up at 22kw so it over shoots target flow temp within 6 seconds so never really puts that much more heat into the cylinder with a 6 second burn. However I can get round it by setting the flow temp in the mid 75's it just about restarts and stays burning for 45 seconds at which point it will then modulate to the required amount but his has a negative impact on efficiency as I dont need 75c flow in my radiators so it doesn't stay condensing.

I think my tank has a coil in the 15-20kw range.
Your boiler has a number of settings I would start by reducing
the output power from 22kw. It might briefly start up in that
mode but will drop back straight away. My vokera (35kw system boiler) starts up at well above minimum power but this isn't a problem. The flow temperature drops maybe 5 or 10 degrees then fires up the temperature picks up to the 70 degrees and the you can hear it modulate down and down until it finally drops out again. Sounds like you haven't got much circulation going on if the temperature goes up so quick it then has to cut out. 2 port valve not opening properly?

 

[scoobydo123]

Unfortunately Vaillant 4xx fire at 80% of max rate for the first 45 seconds so in my case 22kw. At a flow temp of 70 and with the water stored in the system almost up to temp (anti cycle time is only 6.5 minutes) remember that the boiler has cycled so return temp is near flow at 70 and you cannot loose enough latent heat to be able to take 22kw for 45 seconds. Note circulation is good as the first burn is nice and long but when the inevitable cycle happens I am not sure any system could take this level of output.

Flow is measured at 19.6L p/m within 1 Lpm of min spec (yes I know it is under) cant see less than another litre of water allowing the burn to extend 39 seconds!

 

[dcawkwell]

Unfortunately Vaillant 4xx fire at 80% of max rate for the first 45 seconds so in my case 22kw. At a flow temp of 70 and with the water stored in the system almost up to temp (anti cycle time is only 6.5 minutes) remember that the boiler has cycled so return temp is near flow at 70 and you cannot loose enough latent heat to be able to take 22kw for 45 seconds. Note circulation is good as the first burn is nice and long but when the inevitable cycle happens I am not sure any system could take this level of output.

Flow is measured at 19.6L p/m within 1 Lpm of min spec (yes I know it is under) cant see less than another litre of water allowing the burn to extend 39 seconds!

Agreed. What idiot thought a burn of 45 seconds was a good idea. Is there nothing in the d settings that can change this? If it is like the glow worms I thought that there were a whole host of d settings that could be changed.
I'll see if I can see anything in the manual.

 

[scoobydo123]

Thanks for your offer of looking but there is nothing in there that can change the behaviour enough to totally cure the issue. Have a look at the other threads about the 430f. There is one setting that does alter the start up but don't want to mention it here for fear of getting shot down by some. It is a fudge and the 45 second is set in stone as far as I am aware. Very similar to GW's but they start low and work up the exact opposite.

The idiot was someone in product design in France I believe who used PCB and software design that is not suitable for the resistive HE in the 4xx range from other models IMHO.

 

[dcawkwell]

Thanks for your offer of looking but there is nothing in there that can change the behaviour enough to totally cure the issue. Have a look at the other threads about the 430f. There is one setting that does alter the start up but don't want to mention it here for fear of getting shot down by some. It is a fudge and the 45 second is set in stone as far as I am aware. Very similar to GW's but they start low and work up the exact opposite.

The idiot was someone in product design in France I believe who used PCB and software design that is not suitable for the resistive HE in the 4xx range from other models IMHO.

Hmm Won't be buying any Vaillant system boilers then.
Yes the glow worm can change to a low start and move up. But don't see it
for the vaillant.

 

[David937]

The 600/800/900 series have a different start up sequence. Really you need the flow temperature about 5ºc-10ºc above what you have your cylinder set to; so about 65ºc-70ºc. You can't get much lower than that, if you had Vaillant controls, you could set it up so the boiler can run flat out for the cylinder and revert to a compensated temperature for the heating.


Ps.scoobydo123, I think you have seen the thread with some information about the updated PCB for the 400 series. If it's true, it seems very good.

 

[scoobydo123]

Known issue but not many installers are on-site long enough to see it happen. Main reason for the thread is to see if it is to be expected that a cylinder will be satisfied on first call for heat, yours doesn't, is it the one in your profile ie cylinder close to boiler who installed yours? On your cycle how long do you get a burn for approximately? Your boiler does not modulate as low as the vaillant but I need to get through 45 seconds of pain due to the software oversight.

 

[David937]

Known issue but not many installers are on-site long enough to see it happen. Main reason for the thread is to see if it is to be expected that a cylinder will be satisfied on first call for heat, yours doesn't, is it the one in your profile ie cylinder close to boiler who installed yours? On your cycle how long do you get a burn for approximately? Your boiler does not modulate as low as the vaillant but I need to get through 45 seconds of pain due to the software oversight.

Mine is a completely different set up; 2 small storage tanks at the back, but it doesn't start at 45 seconds of full burn, it uses a different system to heat the tanks too so it would be unfair to compare this but as far as I know it doesn't cycle when reheating the (small) tanks, only takes 2 minutes anyway. On heating mode it ignites at high rate then quickly modulates right down to 12kw (although the new version goes down to 6kw), then it will stay at 12kw for a while and ramp up the output if needed (this is dependent on the return temperature I believe). It's not given me any trouble, 12kw is still quite high though so the lowest I can manage on heating with little cycling is 45ºc so my VRC430f is set to use a minimum flow temp of 45ºc.

 

[scoobydo123]

I purchased the advance controls but have never fitted as I couldn't see how it was going to work given the poor start up sequence when a request for low target temps is issued. As you say with this I can set different flow temps and get round the issue but would have opened up a whole world of other pain it is installed in a rented property that I am the landlord so couldn't afford to have them moaning that the heating wasn't working properly and spending hours trying to get it to work.

Yes there is good news on the horizon it seems, I have been on at them for ages but they failed to admit to the issue. I reported to them as an installer and got no help only that they never had heard of the issue and it must be an installation/sizing issue. I then found out that I actually took out an extended warranty as a home owner and they paid 2 visits. First line guy changed PCB and NTC's as he could see the issue and said install was fine, he then reported back to 2nd line who came out to witness and said they all do it. I had it at 68 but at this temp major cycling issues happen as with 22kw going in there is no way after the first burn it can keep flow temps low enough not to be caught in the endless microfiring loop.