Auto bypass setting

[fezster]

I have recently fitted a grundfos ups 25-80 and want to ensure my auto bypass is set correctly, but looking for some technical advice on how it should function on a system with more than ome heating zone.

This is on a system with a vaillant 438, two heating zones (approx 26kw d/s, 18kw u/s), and a h/w zone. My ABV is fitted after the pump, before the 2 port zone valves.

Looking at the instructions, I have worked out that with the boiler range rated to 34kw (the heat loss on my house using the whole house method), my boiler needs a minimum flow of 1462l/h. The 25-80 provides 6.75m head at that flow rate, and the honeywell du144 instructions show a setting of just below 0.2 bar (1.75 on the dial).

Now my understanding is that with the whole system running, the bypass should not be open. As TRV's shut down, the bypass begins to open to maintain minimum flow. This seems to work as expected - however, as my heating is divided into two zones, effectively, half the system is closed with only one zone in operation. This means the bypass is open even under normal operation. This does not seem to impact the ability for the rads to get hot, but I'm wondering if a) this is how it should be set on a two zone system, and b) is this wasting energy?

Thanks.

 

[Nostrum]

The bypass will only be open if the zone is restrictive and not allowing minimum flow rate to pass? I'd be surprised if this were the case unless a lot of radiators were off?

 

[kozak1968]

With the auto bypass valve win come instructions or call the manufacturer if not sure.

 

[fezster]

Thanks for the replies, but I think you've both missed the main point in my question. I've read the bypass instructions, and set the bypass accordingly. My question relates to a 2-zone heating system. I.e. even under "normal operation", the CH circuit will be restricted because one of the zones may be closed. This means that the bypass is open in this circumstance. How much energy is being wasted with the bypass operating in this way almost all of the time ?

 

[albatross]

I use engineers knowledge to set an auto bypass. Open it wide, close zone valves and have pump running then close the bypass while listening to the pump and water noise, you get to a point where noise/pitch increases and the pump is straining, go back a half turn or so to stop the noise and your bypass is set. It's main purpose is to allow the pump a path to push round in pump overrun conditions to stop boiler overheating. It's second purpose it to reduce noise/maintain flow rates as trvs and zones shut which it will do partially if you set it as i said.

 

[Nostrum]

Just reading back, your minimum flow rate seems high, as the boiler modulates down the minimum flow rate will decrease also.

Your bypass is set for flow rate at effectively full rate which is why it's going to be open for longer periods.

 

[albatross]

And it's not wasting energy as the warm water goes back in the boiler to be recirculated and the boiler heats up and shuts off on its thermostat.

 

[fezster]

And it's not wasting energy as the warm water goes back in the boiler to be recirculated and the boiler heats up and shuts off on its thermostat.

Thanks all for replies. Albatross - am I right in saying that having the bypass open in this way is not (significantly at least) detrimental, in terms of gas usage ?

Just reading back, your minimum flow rate seems high, as the boiler modulates down the minimum flow rate will decrease also.

Your bypass is set for flow rate at effectively full rate which is why it's going to be open for longer periods.

Nostrum, completely agree. To give some background. I fitted the 25-80 because I was unable to run my boiler at any more than about 26KW (previously a 26-50 fitted). I can now run the boiler at the capacity I need to, i.e. 34KW. However, if I adjust the bypass to open only when ALL zone valves are closed (a setting of around 5 on the honeywell), I find that with only a single zone open, on initial fire up, the boiler is pumping out too much heat, and the delta T (d40 and d41) gets perilously close to 30 degrees (and hence an S53). Setting the bypass to the MI instructions, as it is now, gives me a nice delta T of around 20 with any one zone open or both zones open.

My concern (as you rightly state) is that the boiler modulates down after the first minute or so, and at that point, having the bypass open would not be necessary, as the heat being output could be catered for by the single zone that is output. But as the pump is at a fixed speed, the bypass remains open continuously, and I suspect, the boiler is using more gas than is necessary. This is what I need clarification on.

If having multiple zones means the bypass must be set in this way as a suitable compromise, that's fine. Just want to be sure there's nothing else that can be done to optimize.

Many thanks.

 

[happyflyer]

I have recently fitted a grundfos ups 25-80 and want to ensure my auto bypass is set correctly, but looking for some technical advice on how it should function on a system with more than ome heating zone.

This is on a system with a vaillant 438, two heating zones (approx 26kw d/s, 18kw u/s), and a h/w zone. My ABV is fitted after the pump, before the 2 port zone valves.

Looking at the instructions, I have worked out that with the boiler range rated to 34kw (the heat loss on my house using the whole house method), my boiler needs a minimum flow of 1462l/h. The 25-80 provides 6.75m head at that flow rate, and the honeywell du144 instructions show a setting of just below 0.2 bar (1.75 on the dial).

Now my understanding is that with the whole system running, the bypass should not be open. As TRV's shut down, the bypass begins to open to maintain minimum flow. This seems to work as expected - however, as my heating is divided into two zones, effectively, half the system is closed with only one zone in operation. This means the bypass is open even under normal operation. This does not seem to impact the ability for the rads to get hot, but I'm wondering if a) this is how it should be set on a two zone system, and b) is this wasting energy?

Thanks.

Save me digging where do you get 1462 l/hr as the minimum, that looks like the maximum output rate 14621/3600 = 0.4 ls X 84 = 34 kw is my maths up the pole, or yours

forgive me if I am wrong its my age

 

[GasmanxxxR1]

I use engineers knowledge to set an auto bypass. Open it wide, close zone valves and have pump running then close the bypass while listening to the pump and water noise, you get to a point where noise/pitch increases and the pump is straining, go back a half turn or so to stop the noise and your bypass is set. It's main purpose is to allow the pump a path to push round in pump overrun conditions to stop boiler overheating. It's second purpose it to reduce noise/maintain flow rates as trvs and zones shut which it will do partially if you set it as i said.

When I went on the Honeywell control course this is exactly how they told us to do it, its always been my preferred way. Chuck the instructions out the window and do this

 

[fezster]

Save me digging where do you get 1462 l/hr as the minimum, that looks like the maximum output rate 14621/3600 = 0.4 ls X 84 = 34 kw is my maths up the pole, or yours

forgive me if I am wrong its my age

Yes, I think you're right. My mistake. I was using the graph in the MI's. However, the problem seems to stem from the boiler firing at full (or a percentage of full) power for the first minute or so.

My posts are slightly delayed as they need to be approved, it seems. However, as said in other post, with both zones open, delta T is around 20 and bypass is closed. With only a single zone open, boiler doesn't modulate down fast enough, so unless bypass opens, delta T rises close to 30. The bigger pump was to alleviate s53, and it has done so, but only with both zones open, or bypass open with single zone.

Please let me know if im missing something.

 

[Ermintrude]

When I went on the Honeywell control course this is exactly how they told us to do it, its always been my preferred way. Chuck the instructions out the window and do this

Plus one on pitch. That's how I set lots of valves.

 

[albatross]

Just like a lady, push her till she squeals then easy off a bit! hahaha

 

[fezster]

Save me digging where do you get 1462 l/hr as the minimum, that looks like the maximum output rate 14621/3600 = 0.4 ls X 84 = 34 kw is my maths up the pole, or yours

forgive me if I am wrong its my age

Correction to my earlier mail.

Page 8 of the installation and service manual shows the boiler specification. This states the *minimum* output at 38kw as 27.2 l/min.

 

[lame plumber]

I have recently fitted a grundfos ups 25-80 and want to ensure my auto bypass is set correctly, but looking for some technical advice on how it should function on a system with more than ome heating zone.

This is on a system with a vaillant 438, two heating zones (approx 26kw d/s, 18kw u/s), and a h/w zone. My ABV is fitted after the pump, before the 2 port zone valves.

Looking at the instructions, I have worked out that with the boiler range rated to 34kw (the heat loss on my house using the whole house method), my boiler needs a minimum flow of 1462l/h. The 25-80 provides 6.75m head at that flow rate, and the honeywell du144 instructions show a setting of just below 0.2 bar (1.75 on the dial).

Now my understanding is that with the whole system running, the bypass should not be open. As TRV's shut down, the bypass begins to open to maintain minimum flow. This seems to work as expected - however, as my heating is divided into two zones, effectively, half the system is closed with only one zone in operation. This means the bypass is open even under normal operation. This does not seem to impact the ability for the rads to get hot, but I'm wondering if a) this is how it should be set on a two zone system, and b) is this wasting energy?

Thanks.

you seem to forget one point and that your boiler modulates as required to heat the water needed to warm your place, best to let a plumber set it up as advised and stop worrying about this and that and maybe.

 

[happyflyer]

Correction to my earlier mail.

Page 8 of the installation and service manual shows the boiler specification. This states the *minimum* output at 38kw as 27.2 l/min.

I found the page and you are right minimum flow is 0.45 ls which is the heat release at 20 Deg C 0.45ls x 4.2cp x 20degc = 38Kw, which in real terms
you must not drop the boiler flow rate at all

 

[fezster]

you seem to forget one point and that your boiler modulates as required to heat the water needed to warm your place, best to let a plumber set it up as advised and stop worrying about this and that and maybe.

I have tried. Unfortunately, I've been very unlucky in that none of the heating engineers who came out were able to fully resolve my problems. I had many suggestions and spent quite a fair bit of money in trying various solutions, but in the end I decided I'd try to understand the problem myself and then hopefully be able to make a good judgment on what the solution should be. I'm now happy with how the system is working, and it's more a matter of intrigue as to the bypass being open in the way it is with a single zone in operation (and whether it'll impact my heating bills significantly!).

Pump was sized according to index circuit calcs with the help of another forums' member, so it should now be sufficient.

Setting of bypass seems to vary according to who you ask, and usually very differently to the instructions. Interestingly, most suggestions on setting it seem to focus on the safety feature of allowing pump overrun, whereas the instructions (honeywell at least) give equal importance to providing necessary minimum flow through boiler.

I found the page and you are right minimum flow is 0.45 ls which is the heat release at 20 Deg C 0.45ls x 4.2cp x 20degc = 38Kw, which in real terms
you must not drop the boiler flow rate at all

happyflyer - at 34KW, i calcualted this to be 0.406 l/s (which is the 1462 l/h figure I quoted earlier in the thread).

 

[albatross]

Are you really still twittering on about this? An auto bypass has never had so much attention. Nobody fitted them on domestic systems 10 years ago. I've told you the answer in post number 5 and it's been backed up by 4 other engineers. I think you need a hobby.

 

[fezster]

Are you really still twittering on about this? An auto bypass has never had so much attention. Nobody fitted them on domestic systems 10 years ago. I've told you the answer in post number 5 and it's been backed up by 4 other engineers. I think you need a hobby.

Are you always so aggressive?

If I had the answer, I'd have said thank you and been on my way. As it happens, your answer may well work in some, or even most, situations, but here, I've told you that closing the bypass so that it opensoonly when all other zones are closed (as you described) results in a large delta T, and sometimes s53. This ties in with the MIs stating a minimum flow rate through the boiler, which will never be satisfied with a single zone on first fire, and vaillants do not modulate down immediately.

If I've missed how post no. 5 addresses all of the above, my apologies.

 

[albatross]

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.

 

[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