Fitting a usually open motorised valve to bypass

[Chameleonclaude]

Hello,

I' thinking of connecting a usually open motorised valve to my bypass. The idea is to stop the hot flow passing into the return while the central heating system is on which would affect the modulating of the boiler and lower the performance of the HW system.

Am I a genius or a complete plonker, do tell!

Claude

 

[stem]

I did it about 8 years ago because the automatic by-pass valve I had always 'let by' even on the highest pressure setting and when all TRV's were open.

My boiler manual gives a minimum flowrate that has to be maintained. (10 year old non-condensing, non-system type) If yours has the same requirement, you would need to make sure this was still satisfied if the by-pass motorised valve was closed because the heating was on, and the system TRV's were also closed. Usually a rad or two without TRV's would suffice.

 

[Chameleonclaude]

Thanks Stem, that was the most perfect reponse.

Great to hear that someone else has done it before. All my rads down stairs are on constant open so should be okay there.

Dy you think it makes much difference to bills, efficiency etc etc

 

[Chameleonclaude]

Also my boiler has a requirement of 25L per minute whilst in overun which is currently served by a 15mm bypass with a taconova flow setter installed. Would IYO it be necessary to change the bypass to 22mm or would a 15 to 22 increaser and reducer within the 15mm pipe surfice?

 

[Agile]

I dont see what you are expecting to achieve.

If the rads are properly balanced and the auto bypass set correctly then it will not pass anything during normal operation ( until the TRVs are mostly closed ).

Stem seems to have had a problem ABV to manage.

Tony

 

[stem]

Dy you think it makes much difference to bills, efficiency etc etc

I didn't notice any change in the bills to be honest. The radiators warmed up more quickly, but the boiler fired for longer periods too.

 

[Chameleonclaude]

In principle i get what your saying, but i cannot find a setting on the bypass that doesn't let any water through and then open sufficiently when overrunning.

The issue is probably further complicated by the pump being one of the fandango ones that reduces flow speed based on pressure at the pump head (can't remember what they're called!)

What i want to achieve is a sytem in which the return temperaturre is always 20 degrees lower thsn the flow as is balanced at the rads. i think bypass (which doesn't close completely) must increase the return temperature which would cause the boiler to modulate reducing the heat output at the rads!

What i really wanted to do is find a clever way of sending the bypass around the HW cylinder but being as that my HW valve is on the exit side of the cylinder this make it more complicated!

 

[Agile]

So only now do you tell us you have a smart pump!

The reality is that those pumps are not compatible with an auto bypass.

The ABV relies on an increase in pump pressure as the TRVs close.

The smart pump aims to maintain a constant pressure as the TRVs close!

Mutually incompatible!

Achieving a 20 C differential is virtually impossible in practical applications.

Tony

 

[Chameleonclaude]

Oh yeah that's what there called!

Well i think im going to try one out. I think that i will move the HW valve to the feed of the cylinder and fit the bypass normally open valve to that feed (at a T-Junction of course). I guess ill have to put a one way valve in to ensure the HW flow doesn't travel up the bypass at times when the CH is off.

I'm really grateful to you guys for such brilliant input. This site and all who contribute to it rocks! I love it, I love it, I love it.

 

[D_Hailsham]

Grundfos's recommended solution is a Taconova flowsetter in series with an ABV.

See Bypass Commissioning Alpha 2, last paragraph.

 

[Agile]

Grundfoss agree with me.

But 2.4 li/min is less than Vaillant want as min flow rate in the boiler!

Tony

 

[Chameleonclaude]

Having read the grunfos literature (thanks for link) it seems my reasoning is valid and the recommendations about bypass settings can't in reality ever compare to shutting the bypass down during boiler run time.

I have the alpha 2 l which I don't think has the same brains as the alpha2 and for my particular system proportional mode is the only one that works. The intelligent setting seems to slow the pump down too much.

 

[D_Hailsham]

I have just found that you asked the same question last year - Balancing and Bypass issues

The solution (ABV and Setter in series) won't change, so why not implement it?

 

[Chameleonclaude]

That's a good point! Let me explain.

Last year i was struggling with flow temparatures not getting high enough. At the end of my episode of enquiry (the thread of last year) i brought an engineer in who told me that my gas valve in my boiler was faulty. Since then the whole system has performed much better.

In truth i forgot the extent of research at the time and the wisdom shared by all of you good people. Your right i should implement it! I already have the taconova setter so it's just a matter of fitting the abv. Do you think the abv should be before or after the flow setter?

I guess an abv will be cheaper than another motorized valve and probably less lightly to need repair or replacement.

 

[D_Hailsham]

That's a good point! Let me explain.

Thanks for the explanation.

Logically the ABV should be before the Setter, i.e on the flow side. The ABV will be closed until some of the TRVs close or the pump pressure increases. When the ABV opens, the Setter then prevents the flow rate through the bypass rising to high.

An ABV will be much cheaper than a motorized valve and will have no ongoing maintenance costs.