mystery valve?

[Crystal]

Yes it is a by-pass valve, but in my opinion it has nothing to do with TRVs but was installed to trim the boiler return temperature (although this was seldom done correctly, if at all)

 

[meldrew's_mate]

On a typical system (if there is such a thing) start with this 15mm "mystery valve" about 1 turn open. Turn the CH only on and shut down all the rads, then the temp differential across the boiler flow and return pipes should not exceed 15C (or manufacturer's figure), but you'll have to be quick before the boiler stat turns it all off. Open the valve more to reduce the differential and vice versa. Then leave it set and remove the handle to deter fiddlers (and all other musicians )
Set the 22mm cylinder "balancing" valve to about 1.5 turns and remove its handle too.

Hope this helps

 

[chrishutt]

it is a by-pass valve, but in my opinion it has nothing to do with TRVs but was installed to trim the boiler return temperature

That's a new one on me. Can you explain that please?

 

[corgiman]

chris that is what is known in the trade as bullshit

lmao

 

[Crystal]

chrishutt, corgiman has cut me deeply.I need time to recompose myself. In the meantime please read melddrew's-mate's post.

 

[corgiman]

pulling your leg crystal

 

[chrishutt]

that is what is known in the trade as bulls**t

Maybe, but I'd still like to hear what Crystal has to say about it. He/she is certainly quick to express views on other threads (just called me a pedant so may well be talking sense! ).

A few posts crossed there - yes, I've re-read MM's post, but I don't understand the point of that at all. Can someone explain?

 

[Crystal]

chrishutt,

I have recovered now, but may be scarred for life.

I am sure that you are aware that boiler manufacturers recommended flow/return temperatures and differential. These are different for the old cast iron boilers and the modern condensers for obvious reasons.

Generally boiler return temperatures below about 55 degrees c results in significant condensation within the combustion chamber and corrosion of cast-iron units.

If you think about it, if there is no by-pass, the water will flow from the boiler , through the radiators and return at whatever temperature happens to result. There is no ability to adjust the delta T and the only control available is from the boiler stat.

It is my contention that the by-pass shown in the photograph is an attempt to crudely adjust the delta T.

A modern system would incorporate a blending valve for more refined control.

These issues are of much more concern on large industrial boiler units where sophisticated controls are utilised.

If you have further interest, I suggest that you google "back end protection"

Then of course it could all be what corgiman called it

I should have put an emotithing after pedant

 

[croydoncorgi]

I don't think that too big delta-T on conventional systems to the point that condensation would ever be a major problem.... That's not to say that delta-T is not important, though. Noise in the boiler, thermal stress due to differing temperatures in the heat exchanger, etc. are all important effects too. Fix these and condensation and back-end rot become non-issues. Same rules apply in a conventional boiler as in a condenser: unless the Return temperature is less than 56 degrees or so, condensation is impossible, so no liquid water, so no back-end rot. On a conventional setup, with Flow temperatures approaching 80 degrees, you'd need to go some to get a delta-T of nearly 30 degrees.

i've never really gone into WHY managing delta-T is not usually a problem on domestic systems. I guess it must be due to low thermal inertia: not much circulating water, relatively small heat losses into the small room volumes, and high flow rates all mean that the Return temperature tends to stay high.

 

[chrishutt]

Generally boiler return temperatures below about 55 degrees c results in significant condensation within the combustion chamber and corrosion of cast-iron units.

I understand this point and accept that a by-pass could be a crude device to avoid condensation due to too low return temps. I have recently noted corrosion on a newish CI heat exchanger arising from condensation.

However I have never heard of this given as a reason for by-passes in domestic installations, only the need to ensure a minimum flow through boiler heat exchanger to avoid over heating, particularly in conjunction with a pump overrun. In such cases a by-pass would be vital if 2 port valves were used and/or if most/all rads had TRVs.

If avoiding condensation were really so important, it would make more sense to use a pipe stat on the return and a motorised/mixing valve to ensure that enough flow was diverted to the return to maintain temp. above 55C.

The kind of fixed by-pass in this example would not fulfill this function very well, unless it was passing quite a high proportion of the flow in which case the system would not be very efficient in normal operation.

 

[Crystal]

chrishutt,

I'm not defending the system, just saying that I believe that was the logic behind it (and as cryodoncorgi says to minimise thermal stress etc.) I remain open to contradiction; I like to learn.

Your point re. pump over-run and heat dissipation is well made and is accommodated by the by-pass as you say(although the effects are much more significant on modern low water content boilers)


croydoncorgi,

You have expressed my thoughts better than I could

 

[DP]

In present case bypass (during pump run- if present) is effected by three port valve, so byvass gate valve is not necessary

I would be using the gate valve on the flow to attain 11 degree differential. Having no flow control here can effect heating zone as (during CH and HW demand) primary water will favour shorter route via the cylinder coil.

 

[noonespecial]

Yes you are right ACOperson. it will work in the same manner as a single pipe radiator system