Bypass Valve still flowing when fully wound down

[ChrisNason]

I have a conventional (Loft Feed Tank and S Plan) system with a Vaillant 438 Boiler. I'm struggling to get flow to the final radiator. Most of the others have the adjustment valve virtually screwed down. The (new) boiler does not fire that often and probably only fires for 30 seconds.

I'm thinking that the last (new) radiator has not got good flow.

However, I've tried this evening to see if the ABV is adjusted correctly. I thought I was onto something when I found that its pipes were hot. It was set at 0.4 Bar. My calculations suggested it should be set to 0.38 (the Vaillant requires 27.2 l/min). I've tried it at many settings and now have it above 0.5 (0.6? but unmarked). It still flowing at the same temperature as the heating.

Do you think its a combination of all the lockshields being wound mostly down and a faulty final radiator?

Any thoughts on how I check the flow through the final radiator?

By the way. The system is mostly new but just some of the original pipes. This final (vertical) radiator replaced a large old panel that used to be the warmest on the old system. The pipes have been power flushed but nothing much was found in the system. It has a Fernox filter.

 

[D_Hailsham]

I have ... a Vaillant 438 Boiler. ... The (new) boiler does not fire that often and probably only fires for 30 seconds.

That's a very big boiler. Do you have a very very big house with poor insulation? How was the boiler size determined?

 

[ChrisNason]

House is not that big but it is very poorly insulated. Mainly old period windows that let in huge drafts. It has 8 radiators. The Vaillant engineer has adjusted it down with the electronic controls. (I understand from this process that it is also possible to adjust a smaller boiler up!).

The lack of flow to the final radiator is quite galling having spent all this money.

By the way, the pump is a Grundfos 15/60

 

[D_Hailsham]

House is not that big but it is very poorly insulated. Mainly old period windows that let in huge drafts. It has 8 radiators.

Are you sure it's a 438? There's no way that you would need that size boiler if you only have eight radiators.

It might be a worthwhile exercise to find out what size boiler you really need by using Boiler Size Calculator.

(NB Front to back walls: mid-terrace=0; semi/end terrace=1; detached=2)

Then use Stelrad Elite Catalogue to find out the output of your radiators (assumes they are modern flat steel).

Please post the results

The Vaillant engineer has adjusted it down with the electronic controls. (I understand from this process that it is also possible to adjust a smaller boiler up!).

This does beg the question, why did you have such a large boiler installed?

If the boiler is only running for 30 secs it hasn't been adjusted down enough. The results of the above calculations will tell you the correct setting.

The lack of flow to the final radiator is quite galling having spent all this money.

The lack of flow is due to to the system only running for 30 seconds before shutting down. A system needs to be running almost continuously while balancing.

 

[ChrisNason]

But I have the boiler now.

Perhaps I've not made my problem clear. It's the boiler that is only firing for short periods. The pump is continuous while the thermostat is calling. I'm trying to work out why the pump is not delivering a good flow to the final radiator. Is it being undermined by the bypass valve?

 

[D_Hailsham]

But I have the boiler now.

I appreciate that.

Perhaps I've not made my problem clear. It's the boiler that is only firing for short periods. The pump is continuous while the thermostat is calling. I'm trying to work out why the pump is not delivering a good flow to the final radiator. Is it being undermined by the bypass valve?

I understand exactly what you problem is; the root cause is an oversized boiler. The water is being heated faster than your radiators can dissipate the heat, so the temperature rises very quickly, which means the boiler's internal thermostat turns the boiler off. The pump continues working to keep the water circulating so it will cool down. When the water has cooled enough, the boiler will relight; and so the cycle continues.

You will never be able to balance the system or adjust the bypass until you get a proper continuous flow at the correct temperature.

What did the installer lower the heat output of the boiler to - it's parameter d.0?

 

[ChrisNason]

OK - Correction

Looking at my notes it's a 428 (not 438) and I know this is still over specced.

The Vaillant engineer (not the installer) backed it off to the eqivalent of a 418 (he said).

I'm hoping that changes your conclusion

 

[ChrisNason]

ps...

I see there is know way of balancing the flow to the hot water cylinder. shutting the water's Honeywell valve does improve the flow to the final radiator. Should it have been fitted with a lock shield when it was renewed?

 

[D_Hailsham]

Looking at my notes it's a 428 (not 438) and I know this is still over specced.

That requires a minimum flow rate of 20.3l/min, not 27.2.

The Vaillant engineer (not the installer) backed it off to the equivalent of a 418 (he said).

Probably still too high for eight rads. Don't forget that you only need the full output when it is freezing outside; most of the year you will need less than that. The boiler will automatically adjust the output down to 5.7kW. Below that level it has to turn the boiler on and off to produce the correct amount of heat (just like old fashioned boilers). This could also be what is happening.

I see there is know way of balancing the flow to the hot water cylinder. shutting the water's Honeywell valve does improve the flow to the final radiator. Should it have been fitted with a lock shield when it was renewed?
Yes. There should be a valve on the return (bottom pipe) from the HW cylinder so the cylinder can be balanced.

There are several version of the 15/60.

Is it the original pump or a new one?
What is the P/N of the pump (top right)?
What speed is the pump set to?

Which ABV do you have?

Read How to balance a CH system

 

[ChrisNason]

Thanks for your continued input.

The pump is a Super Selectric part nr 96281473.

I struggle to identify the ABV. It's a 22mm connected with reducers to a 15mm bypass that's sited just before the heating Honeywell and dumps back into the heating return about 30cm before the the T from the incoming water circuit. The ABV is a right angle unit with a white adjuster covered with a clear plastic lock cone. You remove the lock cone to adjust then replace it to lock the white serations to the tap nut.

I might try a photo to help.

 

[Agile]

Are both valves on the last rad fully open?

Have you tried increasing the setting on the ABV?

You should really have a lockshield on the return from the cylinder heating coil.

Tony

 

[mickyg]

is the abv a honeywell? black body with red cap? if it is im having loads of problems with them passing. Stopped using them. Honeywell stuff is going down hill it seems

 

[ChrisNason]

Hopefully a picture of my ABV

ABV

• ChrisNason
• 17 Nov 2010
• 1

 

[mickyg]

hmm dont recognise it. Doesnt sound right that the boilers shutting down though. Might be worth finding out the Return temp when it shuts down

 

[ChrisNason]

Agile...
I've screwed the ABV down to its maximum which I gather is about 0.6 but it's still flowing.

Pump is set to 3