On 15/12/2017 22:34, Johnny B Good wrote:

I've been following the recent thread on "Heating system motorised valve
questions" started by John Smith, with some interest. Although his setup
is obviously based on the classic S plan using a couple of two port
valves in place of the classic Y plan's use of a Honeywell V4073A 3 port
mid position valve, John Rumm's post referencing the DIY wiki on the
various CH system control plans caught my attention with the 'Y' plan
plumbing schematic.

It's quite obviously an extremely simplified schematic, devoid of the
typical niceties of flow balancing valves and bypass pipework with flow
restriction valves which lead me to taking a closer look at my own fully

Indeed, for the purposes of that article it only seeks to show how the
zone values function in the system.

However it does in a way highlight that we possibly don't have a good
diagram in their of the canonical vented DHW system. Perhaps someone
ought to draw one ;-)

We do have one for a heat bank:

http://wiki.diyfaq.org.uk/index.php/DIY_Heat_Bank


pumped Y plan system to compare against the various plumbing arrangements
shown in the "Ideal Mexico Super CF.65, 75, 80, 100, & 125 Conventional
Flue Gas Boilers Installation & Servicing guide", published November 1983
[1] which had been left by the local Gas Central Heating firm we'd used.

There were a few interesting 'departures' from the fully pumped system
plumbing arrangements for the boiler connections shown in that
installation and servicing guide which raised a few questions I'd like to
present to the cadre of Central Heating Experts that frequent this NG.

The wiki article tends to stick to the examples in the Honeywell
"standard" docs.

[snip]

Compared to the Y Plan plumbing circuit shown in the wiki http://
wiki.diyfaq.org.uk/index.php/File:Y-Plan-Water.gif there is a gate valve
on the flow side plumbing between the mid position valve and the upper
heat exchanger coil port on the hot water tank, obviously there to
balance the flow when calling for both heat and HW.

Yup quite commonly done since otherwise the cylinder's HE could starve
the rads of flow due to it being a very low resistance. (making it
behave more like a W plan system). This would be fine with a modern fast
recovery cylinder than can swallow the full output of the boiler, but
not good a traditional cylinder that will max out at say 5kW transfer
rate. Then you just leg loads of boiler cycling, and no heating for as
long as it takes the cylinder to slowly lumber its way up to its set point.

However, in addition,
there is also a valved shunt (15mm pipe) tapped into the pump outlet to
the AB port of the 3 port valve 28mm pipe and the HW H/E coil return
which seems a little excessive of pump protection since the ground floor
shower room (adjacent to the utility room) has a heated towel rail
(previously a small radiator) with no TRV fitted to provide the required
safety shunt.

Probably a belt and braces... also to protect against future changes to
the bypass rad or the addition of any blocking elements (TRVs etc)

I'm not sure whether this departure from the guide (after some 30 odd
years of service) is important.

The fact that its been working (mostly) trouble free for 30 years would
indicate not ;-)

I can't see why there would have been any
problems with such an arrangement even though I did have to push a length
of pyro down the feed pipe from the attic to unblock it about a year or
three after it had been installed. This was a one off problem that's
never repeated in the subsequent 30 odd years so this plumbing variation
does not appear to be of any consequence, at least not in my case.

So, my questions a

Is my current header tank feed and expansion pipe arrangement something
to be concerned about?

No. The only real concerns are systems where the vent gets scaled and
completely blocked, or where significant air is induced into the system
on a regular basis (either by "suction" on the vent, or by pumping over.
Then that will lead to massive corrosion problems, and lots of
"sludging" up.

and, is there any good reason not to close the bypass shunt between the
pump out flow and the H/E return?

Only the old adage "if it ain't broke, don't fix it!"

All in all, including two or three doses of Fernox MB1, I doubt I've
spent more than 350 quid in repairs/servicing over the past 35 years or
so since the system was installed so I can't complain. Looking at others'
experience with "Modern Energy Efficient" Condensing Boiler systems, I've
saved far more on expensive repair costs than any savings in gas
consumption ever could.

It does rather depend on circumstances... on systems with very high gas
usage there is more upside to a modern system. I ripped and replaced an
ancient Ideal Mexico RS based system with a very poor vented DHW system
(really not well suited to the property at all) about 5 years ago. I
went to town with it and did fully weather compensated heating, split
into separate zones, unvented DHW etc. Even ignoring that the house is
now way more comfortable, and the DHW system is like a veritable heated
fire hose in performance as valid justifications for the hassle and
expense, its now also pretty much paid for itself in reduced energy
costs[1]. In that time its maintenance costs have been a couple of top
ups with Sentinel X100.

[1] When you consider the old system was probably throwing 35p of every
quids worth of energy I bought it straight out the flue...




--
Cheers,

John.

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