Where to connect an ABV

[Reshweb]

I am considering the installation of an ABV to address a short cycling issue on an S plan system. Looking at the recommended position for the valve it says it has to be directly after the pump on the flow and connected to the return to the boiler.
However looking at the schematic, which I understand doesn't necessarily equate to the actual physical connections, it shows the valve on the return to the boiler after all radiators but before the hot tank return. Is the relationship with the hot tank return critical ??

Also it says not to install it near to bends, any thoughts on the minimum distance from a bend ?

Many thanks
Graham

 

[chris watkins]

I am considering the installation of an ABV to address a short cycling issue on an S plan system. Looking at the recommended position for the valve it says it has to be directly after the pump on the flow and connected to the return to the boiler.
However looking at the schematic, which I understand doesn't necessarily equate to the actual physical connections, it shows the valve on the return to the boiler after all radiators but before the hot tank return. Is the relationship with the hot tank return critical ??
No, as it will not be open if the HW is heating.

Also it says not to install it near to bends, any thoughts on the minimum distance from a bend ?
Min of 4 or 5 pipe diameters away should be adequate to stop turbulence.

See answers in red to your quoted questions Graham.
Are you happy setting it up ?

 

[Reshweb]

Thanks for the response Chris.
I am not sure what your response actually says though.
Are you saying it isn't critical whether the h/w return is before or after the ABV ?

 

[chris watkins]

Thanks for the response Chris.
I am not sure what your response actually says though.
Are you saying it isn't critical whether the h/w return is before or after the ABV ?

The by-pass connections are not critical, so long as you tee off the flow after the pump & tee into the return after any zone valves, so there is a clear circuit formed it will be fine.
What is your boiler ? Are you you installing an ABV to provide a pathway when the boiler over runs the pump to dissipate heat?
or is it being used to maintain minimum flow rates through system?

 

[Reshweb]

Ok thanks Chris.
The boiler I have is an LPG Worcester Bosch Greenstar CDi30 conventional.
I am in the middle of a major renovation and have fitted individual room stats and 2 port valves along with a mains pressure unvented h/w cylinder.
When I connected the radiator up in the most recent room I found it wasn't getting very hot. After some investigation, a chat on here and a call to Worcester it was discovered that the boiler is going into its overheat cycle and turning off for 3 minutes.
Currently there are still a number of radiators yet to be zoned so these effectively act as a bypass at the moment.
When the job is finished and all zones are in place I will need a bypass as the boiler has pump overrun.
However I also want to ensure a minimum flow rate when only one zone is calling for heat. At the moment, even with these bypass radiators, the boiler is going into its overheat cycle; even when the hot tank is calling for heat.
When all rooms are calling the boiler moderates ok without overheating !!

 

[chris watkins]

Sounds like you have a problem if it is doing it with the 22mm HW circuit open the un-vented coil should take most of what the boiler can produce even on full burn. Blockage ?? Who installed to cylinder etc. How big is the system ?

I would suggest using CCT's

 

[Reshweb]

What are CCT's Chris.

I was surprised that it overheated with the h/w calling for heat. It's run in 22mm throughout to the cylinder. I installed the cylinder as I am certified for unvented.
I wondered if the boiler isn't moderating correctly. It did take 18 minutes of the h/w calling before the boiler shut down. I checked the cylinder to make sure it was still calling for heat and the valve was open.
It's a 5 bedroom bungalow with 18 radiators/towel rails.

 

[chris watkins]

From what you describe I would suggest hydraulic separation is a must. The cheapest & easiest way to achieve this is using a Close Coupled Tee's arraignment.
I had the same issue on a Worcester installed in large London house.
There are a few threads already on here so have a search on LLH & CCT's.
If you need any more info let me know.

 

[chris watkins]

Here is the one I did last year & it worked a treat.

Main UPS 25/80 to the left out of shot in the side loft.
Alpha wired directly from the boiler provides constant flow through boiler. SWL turns on system pump.

 

[Reshweb]

Thanks Chris I will search around. So you had two pumps then ?
What does SWL stand for that you mention turns on the system pump.
Thanks for all your help - seems I may have to change all my layouts.
Unfortunately the pump is remote from the boiler and the pipework has been tweaked by a previous owner when extensions were added.
Graham

 

[chris watkins]

Switch Live (orange) from zone valves used to signal boiler to fire.

 

[Reshweb]

Ok got it.
What's the purpose of the two pumps. I have looked on the forum and elsewhere for info on CCT and LLH and it looks very complicated and would mean a major re piping of my complete system.
Can you give me an overview of how these work and how it solves a problem with multi-zone installations.
I was wondering about the fact that the boiler seems to overheat (eventually) when hot water alone is calling. I saw on another post someone mentioning balancing the H/W differential. I have never heard of this before but I suppose it makes sense to ensure the flow rate through the coil is slow enough to allow the heat to be extracted into the stored water ?
If the cylinder isn't taking enough heat out of the boiler because of the flow rate this might explain why it eventually overheats ?

 

[fezster]

Are you sure your boiler is set to a sufficiently high temperature to allow your cylinder to satisfy the stat? If your cylinder is set to 60, your boiler should be set at least 10 degrees higher than that (maybe slightly more - otherwise the boiler will short cycle as the cylinder approaches target temp).

 

[Reshweb]

Hi Fezster thanks for the reply.
In fact tonight I have found that the flow into the cylinder is lower than the tank stat is set to so continues to call for heat but never gets enough to turn the signal off to the boiler.
The boiler flow chart shows an over temperature loop (5 deg higher than required) that turns the gas off for 3 minutes.
This would appear to be acceptable as a normal operation ( no fault codes reported) of the boiler but I don't understand why it goes over temperature.
Is it influenced by the return temperature to the boiler ?
Does this cause the heat exchanger to go over temperature ?

 

[chris watkins]

In fact tonight I have found that the flow into the cylinder is lower than the tank stat is set to so continues to call for heat but never gets enough to turn the signal off to the boiler.
The boiler flow chart shows an over temperature loop (5 deg higher than required) that turns the gas off for 3 minutes.
This would appear to be acceptable as a normal operation ( no fault codes reported) of the boiler but I don't understand why it goes over temperature.
Is it influenced by the return temperature to the boiler ?
Does this cause the heat exchanger to go over temperature ?

In sort if the boiler doesn't have a high enough flow rate through it it can't make the modulation decisions, it reads the increasing temperatures as a satisfied system & shuts down. Most boilers have an anti-cycling routine which hold them from firing again for a period of time so you will find that rads & cylinders will not reach temperature.

Hydraulic separation gives the boiler the flow rates it requires respective of what the system requirements are.

What's the purpose of the two pumps.

Small (Alpha in the photo) boiler pump provides the minimum flow rate around the boiler via those two close tees connecting the boiler flow & return to the system flow & return loop (35mm) under the boiler. The other pump circulate water around the system & can deal with the ever changing pressure changes as your multi zones open & close.

 

[Reshweb]

Thanks for that Chris it's starting to make sense now.
So the Alpha pump creates a flow essentially just around the boiler via the close T's and maintains the minimum flow rate. How does a short loop like that manage to clear the minimum heat output of the boiler. Surely the flow and return will end up at a very similar temperature. Or is it the addition of the heat loss through the rest of the system that controls the temperature ?
You say the system pump is off to the left, where does the flow and return to/from the radiators connect into your pipework in the photo.
Is the radiator flow essentially connected to the right of your close T's and the return on the LH side ?

 

[chris watkins]

ou say the system pump is off to the left, where does the flow and return to/from the radiators connect into your pipework in the photo.
Is the radiator flow essentially connected to the right of your close T's and the return on the LH side ?

Yes

 

[Reshweb]

Great - got it
I would still like to clarify what can cause the boiler to over shoot its' desired temperature by not modulating correctly ?
My boiler has a flow/return differential requirement of 20deg,
if the return temp is too high can this cause the over temperature ?
Tripping out when the h/w is calling (as its' incoming temp doesn't meet its' stat setting) could give rise to the above as the return temp from the cylinder is much the same as the incoming and the same as the stored water temp.

 

[fezster]

A boiler can only modulate down to its minimum (found in the manual). In the case of your cylinder, if the boiler temp is not high enough to raise the temp of the cylinder any longer, the return temp slowly creeps up until it gets close to the flow temp. This means very little heat is being extracted and the boiler can't modulate down any further, so cycles.

 

[Reshweb]

Thanks Fezster that confirms my suspicions.

 

[Reshweb]

Question for Chris regarding the CCT solution.
Do I take it that a CCT solution does not need a bypass ?

 

[chris watkins]

Question for Chris regarding the CCT solution.
Do I take it that a CCT solution does not need a bypass ?

Yes, if the boiler needed an external one for pump overrun clearly it would no longer be required as water would circulate between the tee's.
You would need to look at the main pump & circuits to see if anything was required on that side but it is doubtful as the pump will simple stop when the last zone valve closes (if as we did wire it from the SWL) If you are using a variable speed it should be able to deal with all the zones closing or opening with out too much noise from TRV's & the like.

 

[Reshweb]

Thanks Chris
Regarding the pump on the CCT circuit what sort of spec should I be looking for.
Also how critical is the spacing between the T's ?

 

[chris watkins]

A small 5M head pump set on fixed min speed should be fine for your WB 30 but you would normally check what the MI require.
T spacing is important no more than a couple of diameters apart at the most & keep bends & other fittings away both up & down stream to stop turbulence.
P.S. Where am I sending the bill for all this design work Graham ??

 

[Reshweb]

I can send you a cheque Chris.
Looking at the best practice low loss header article mentioned in one of the posts it suggests the primary pump should be on the return but your photo showed it apparently on the flow (F drawn on the pipe).
Does this matter ?
And one "hopefully" final question :-
In this CCT arrangement and with only one zone radiator calling for heat, the minimum flow rate will be maintained but the system may still not be able to dissipate the minimum heat output is it fair to assume that the boiler will still over heat and go into its overheat cycle ?

 

[chris watkins]

I was looking at the position of the expansion vessel (neutral point) and the pump to ensure that the system was under positive pressure as far as possible.
Not sure why you would want it on the return.

The boiler will not be over heating, it will be reacting to the fact that the return water temperature is rising & it needs to modulate down to a low burn rate. Once it reaches the lowest setting & if the temperature continues to rise, as there is little system load, then it must turn the burner off until the temp fall & it can turn back on.
The point as previously stated is that it always has a good flow rate to base its this on.

 

[Reshweb]

Thanks for your help Chris