Auto bypass setting

[Nostrum]

There sounds like an issue other than the auto bypass here, Fezster. Do you have microbore pipework to radiators? How has the boiler and heat loss been sized?

I would imagine the pipework sizing is the problem from what you have mentioned on here. The auto bypass will raise the return temp so it's best to slowly wind it down over a period of days and see how you get on.

 

[happyflyer]

Fezster,

First of all lets try and establish something about your boiler, the data you gave appear to indicate that you cannot drop the boiler flow less than 0.45 ls so this is the max and min flow rolled into in, in other words the boiler manufactures doesn't no want any reduction in boiler flow at any time, I hope we have this right.

You could try this, open all your heat emitters, take of all control valves so that you have the full flow rate through the system, I am assuming the system is balanced properly, now open the ABV fully so that the spring pressure is released, this will cause most or some of the water to go through the bypass and starve the system side, with the boiler running slowly close the ABV each time you close it a bit go and test the F/R on each of the emitters, when you are happy that all the emitters are at design temperature, then you in balance , there may well be still water going around the ABV you will never know, I have said before a flow meter can be used but, the roof came in on me the last time so I won't go there, I doubt if anyone really knows what's going on around the ABV area in any domestic system, if you think there is a science to setting up a ABV, then think again, they work to the nearest brick, look if you get it wrong you will know soon enough, when the boiler starts dancing.

Its a bit like Nostrum said in the last post, take your time with it!!

 

[Ermintrude]

Turn partial load off of auto and rate it for the smallest zone?

 

[fezster]

Not aggressive. Merely a dash of comedy at the fact a simple problem could have been solved easily a long time ago as opposed to making a mountain out of a molehill.

You did not mention s53 is your first post, so set the bypass to give you 20delta T to prevent it and move on with life. Forget about flow rates and instruction books. The boiler will modulate if it needs to. Also forget the theory about using more gas. You will have a functional system.

You have here a sensible, safe, free answer for the system you have fitted. It may be a slight compromise but life is. The only way you can achieve perfection is by spending alot of money chopping out all the valves and fitting a low loss header with a pump on the boiler giving constant flow rate and separate pumps on each zone.

Thanks seems like a sensible answer. Im not planning on touching the system further, but am still curious as to how one would setup a system from scratch to cater for multiple zones (without a low loss header).

 

[fezster]

I doubt if anyone really knows what's going on around the ABV area in any domestic system, if you think there is a science to setting up a ABV, then think again, they work to the nearest brick, look if you get it wrong you will know soon enough, when the boiler starts dancing.

Very honest answer - and I've begun to learn that a lot to do with heating systems are educated guesses, and some compromises, on the whole. Not that there's anything wrong with that if it means the system does what it's supposed to.

There sounds like an issue other than the auto bypass here, Fezster. Do you have microbore pipework to radiators? How has the boiler and heat loss been sized?

No, I have approximately 1m 28mm to the zone valves, another 4m of 28mm from each zone valve reducing to 22mm for each zone along the length of the house, then 15mm to individual radiators, with some radiators sharing 15mm runs.

I'm fairly confident that the pipe work is up to the heat load, as with both zones open, and the bypass closed, I get around about 20 degrees delta T (even a little less). I used the idhee whole house calculator to arrive at the 34KW figure for heat loss. Rads are actually all oversized and total approximately 45KW.

 

[fezster]

Turn partial load off of auto and rate it for the smallest zone?

I did do this at one stage, but found the system was very slow to heat up with both upstairs and downstairs calling for heat.

 

[albatross]

but am still curious as to how one would setup a system from scratch to cater for multiple zones (without a low loss header).

Short answer is you can't do it. You balance a system perfectly on full load but it will go to pot as soon as trv's and zone valves close. One reason variable speed pumps, modulating boilers and auto bypasses were invented to cope with modern systems that have a constantly changing flow rate.

 

[TBServices]

If your rads are oversized, by a similar percentage for each, you should be able to run your system at a lower temp to get the design temp for the rooms.

You can range rate the boiler down to stop it firing too high.

 

[happyflyer]

Modern boilers have a very sophisticated way of measuring the rate of temperature rise different between the Flow and Return,, they will let you know when you have not got the correct flow with an error code. They no longer dance off the wall.

 

[Ermintrude]

Set partial load to 12kw, comfort off. Set flow at 63.

Your system is overshooting , it's like giving a 3 year old 4 cans of coke and a kg of smarties. No way he's sitting down quietly and his teeth won't last.

Far better to allow a small and controlled amount in

Boiler temp is measured by a pid control mechanism. Integration and diff calc used to forecast set point on gas valve that's why it modulates up and down seemingly by magic

 

[chris watkins]

You should have fitted a MAGNA1 25-60 or 80 not the UPS, as it does not conform to the EU pump energy saving directive which came in this time last year but then had you employed a proper heating engineer who understood large systems they would have known that.
Installing an energy saving speed controlled circulator would have gone some way to saving the energy now being wasted through not installed low loss header along with an incorrectly set ABV which means the boiler is rarely in condensing mode.

Throw in & stir the mix:boxing_smiley:

 

[doitmyself]

the data you gave appear to indicate that you cannot drop the boiler flow less than 0.45 ls so this is the max and min flow rolled into in, in other words the boiler manufacturer doesn't want any reduction in boiler flow at any time.

I'm having difficulty with accepting this.

A minimum flow at max output is understandable as it determines the temperature differential across the heat exchanger. A lower flow rate would mean the differential will exceed the spec of the hex manufacturer. But if the flow rate is maintained when the boiler modulates down, the differential will also reduce. If the differential is 20C at max output and the boiler has a 4:1 modulation ratio, the differential will be 5C at minimum output.

Whether this is a good thing or not, I don't know. Does any one have an answer?

A variable speed pump muddies the water even further as the boiler has no direct control over the speed, which would seem to be a logical requirement.

 

[Ollieg]

The problem with Vaillant 438 is even if you range rate the boiler down to 12 kw it will still fire at 60percent of its max ( 60% of 38kw) and then ramp up!!! Yes UP before it come down to 12kw. S53 is a soab. Your primary's sound ok. I would just set the bypass to keep your boiler working. Forget a small amount of let by when one zone is closed.

 

[doitmyself]

The problem with Vaillant 438 is even if you range rate the boiler down to 12 kw it will still fire at 60percent of its max ( 60% of 38kw) and then ramp up!!! Yes UP before it come down to 12kw.

The latest version of the PCB is supposed to cure this problem; but the OP already knows this and has the new board installed. We don't know if it has had any effect.

 

[fezster]

The latest version of the PCB is supposed to cure this problem; but the OP already knows this and has the new board installed. We don't know if it has had any effect.

I wouldn't say cured, but does seem to have helped, as the "ramping up" seems to be more controlled. What I can say is that a lower d0, the boiler behaves differently on initial start up, so Im not sure 60% of 38KW is correct. Perhaps x% of d0.

You should have fitted a MAGNA1 25-60 or 80 not the UPS

MAGNA1 was over £400. I decided to buy the UPS 25-80 as it was considerably cheaper, and in a brand new condition. Fitted myself.

Set partial load to 12kw

Interestingly, I didnt notice the word "partial" before. This would indicate you set it to the load of a single (or smallest) zone. However, as said, setting this too low results in ridiculously long heat up times. Not sure if the boiler modulates up or not.