Boiler bypass pipework – correct design?

[StephenL]

A plumber recently told me my bypass pipework is poorly designed as it’s offering a low path of resistance v the motorised valve T junctions, and therefore is passing water constantly. I have noticed that, some minutes after the CH starts up, this bypass pipe gets hot along its entire length up to the point at which it meets the Return pipe to the boiler which is cooler.

The rads & DHW cylinder work well (they get hot quickly and there is no boiler cycling).
The boiler, DHW cylinder and MVs are all located in the basement.

The attached picture shows 3 MVs ['CH top 3 floors' (28mm In/Out), DHW cylinder (22mm In/Out) and 'CH Basement' (22mm In/15mm Out)], then the bypass which begins at the bottom of this short run. Below this you can see the gas pipe to the boiler and the filling loop with pressure gauge.

The bypass has a fixed valve set slightly open + an ABV set to 0.5.

The plumber thought the bypass should T off between the top & middle MVs (ie, 'CH top 3 floors' MV and DHW MV), but there is not enough pipe length to do this.

Is he right about the poor design?
What would be a better design?
If this is complex to achieve, is it worth it V the potential efficiency improvements?

 

[Chuck]

First thing is to check the boiler manufacturer's installation instructions. Do they say an external bypass is required? If so, are there any specifications that need to be adhered to, such as a minimum volume for the loop. Next thing we'll want to know is what setting/mode is the pump using.

How was the setting of "0.5" chosen for the ABV? If the ABV passing to a significant extent during normal heating operation it either needs adjusting (likely) or is faulty (less likely).

You don't need much flow through a short CH pipe for it to feel hot to the touch. Also, one expects the bypass section to heat up when the last zone valve closes.

What you shouldn't see happening is the bypass water detectably increasing the return temperature when the system is operating normally, i.e. with the radiators on at least one zone emitting heat.

The efficiency of the system will be impaired to some extent if the bypass water is increasing the return temperature significantly during normal operation.

 

[StephenL]

First thing is to check the boiler manufacturer's installation instructions. Do they say an external bypass is required? If so, are there any specifications that need to be adhered to, such as a minimum volume for the loop. Next thing we'll want to know is what setting/mode is the pump using.

How was the setting of "0.5" chosen for the ABV? If the ABV passing to a significant extent during normal heating operation it either needs adjusting (likely) or is faulty (less likely).

You don't need much flow through a short CH pipe for it to feel hot to the touch. Also, one expects the bypass section to heat up when the last zone valve closes.

What you shouldn't see happening is the bypass water detectably increasing the return temperature when the system is operating normally, i.e. with the radiators on at least one zone emitting heat.

The efficiency of the system will be impaired to some extent if the bypass water is increasing the return temperature significantly during normal operation.

 

[StephenL]

The boiler is a 9 month old Vaillant EcoFit Pure 430, heat only boiler. It does specify an external bypass with an ABV and does require a minimum flow, but Vaillant’s technical department was not able to supply me with this value, so upon the installation of the ABV 1 month ago I started by setting it to 0.6 which triggered an S.53 boiler status pause "water deficiency (flow/return spread too large)" and I then gradually reduced it to 0.4 which avoided the S53. Then, a few days ago, a plumber balanced my 15 rads (which have 12 TRVs) and set the ABV to 0.6 without triggering an S53 (I wrote 0.5 in error in my first posting). As I understand it, the higher the pressure setting the lower the bypass flow.

The pump mode is set to Constant Pressure 1 which has worked well for several months following a separate thread discussion on this forum where CP was recommended with my weather compensating Vaillant controls.

The bypass pipe is hot while the 2 CH MVs are open.

How can one tell if the bypass water is detectably increasing the return temperature when the system is operating normally, i.e. with the radiators on at least one zone emitting heat?

 

[ShaunCorbs]

Easy way to set them they shouldn’t operate when under normal operating eg port valve open but should operate when the port valve shuts etc

 

[Chuck]

How can one tell if the bypass water is detectably increasing the return temperature when the system is operating normally, i.e. with the radiators on at least one zone emitting heat?

Pipe thermometer(s) a few cm before and after the tee-junction where the bypass joins the the return.

Standard, i.e. pressure-activated, ABVs don't work too well if they are installed within a short loop employing a constant pressure pump. This is because, in this case, the pressure across the ABV doesn't vary much in response to changes in demand from the zones. Such arrangements can work, sort-of, but the adjustment of the ABV will be very sensitive.

 

[StephenL]

Pipe thermometer(s) a few cm before and after the tee-junction where the bypass joins the the return.

Standard, i.e. pressure-activated, ABVs don't work too well if they are installed within a short loop employing a constant pressure pump. This is because, in this case, the pressure across the ABV doesn't vary much in response to changes in demand from the zones. Such arrangements can work, sort-of, but the adjustment of the ABV will be very sensitive.

Further information:-
With the system cold and the DHW cylinder starting to call for heat (it’s a DHW priority system, so only the DHW MV is open), the entire length of the bypass pipe instantly gets hot. I also note the temperature of the Return pipe (seen in the photo with the Omega Filter in line) is hotter to the touch above the T (where the bypass joins the Return) V further down below the Filter. So, the bypass water appears to immediately contribute to the temperature of the Return.

I did the same test from cold, but this time with the bypass fixed valve shut, resulting in the bypass pipe remaining cold and there being no temperature difference before & after the T where the bypass joins the Return.

Is this bypass pipework poorly designed (in the wrong place relative to the 3 MVs), and what would be a better design?

 

[fowlerboi]

Have you got a gate/shielded valve between the MV and the cylinder?

 

[StephenL]

Have you got a gate/shielded valve between the MV and the cylinder?

Yes, there is a gate valve on both the Flow & Return sides of the cylinder.

I have done the same test as described above, starting with the system cold, for my CH. All TRVs set to maximum. Again the bypass pipe immediately gets warm, then hot, along its entire length, and heats up at a similar rate to the CH Flow pipework, so it is always letting water through. In the first 10 minutes the entire bypass pipe is much warmer than the cool Return pipe, right up to the T where it joins the Return pipe.

 

[fowlerboi]

keep turning the bypass up until it doesn't do that. Then once you've got it so its not red hot the other side of the bypass, turn off your demand and let pump overrun work, it should now get hot very quickly.

If it does that I wouldn't look much more into it. If that doesn't work then replace the bypass and re-configure the pipe layout. No point wasting time refilling and draining for the price of a new bypass, just do 2 birds one stone.