Gravity hot water temperature control

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I have an old Potterton boiler with gravity hot water and pumped heating. The system still works very well and seems reasonably efficient (£25 monthly for a 4 bed detached). However, during the colder months, I have far too much hot water at too high a temperature. Would it be possible to add a thermostatically controlled valve to the return from the hot water cylinder (thus maintaining the vent on the flow pipe) in order to control the hot water temperature. I have a 3-way motorised valve which I could use by blanking one of the inlets, and then controlling it from a cylinder stat. Would I also need a narrow bore by-pass.
Terry D.
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monthly
cylinder
You can fit a normal DHW blending valve to the cylinder draw-off pipe, not blocking the open vent. hot in one port from the cylinder, cold in the other. the outlet is blended to whatever you set it to. They are in Screwfix or http://www.plumbworld.co.uk
Better still replace the old cast-iron clunker with a modern condensing boiler with a fully pumped system. Your £25 per month will be down to aprox £15 per months, saving £120 per year.
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heating.
too
stat.
aprox
At a cost of at least £1k (unless you DIY it, but that's back to the thread we had before... ;)
£120 a year saving, you'd need nearly 10 years to recoup that.
D
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a
hot
by
not
This is DIY. A Ravenheat condensing boiler is £399.
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IMM wrote:

Does a condensing boiler necessarily have to be wall mounted with a balanced flue? My old Potterton is nicely hidden away in a cupboard with no access to an outside wall and I want to keep it that way. If I hadn't wasted £12 a month for 20 years on a useless British Gas service contract, I could now replace my heating system twice over. Sorry, but I'm still really not convinced about the apparent advantages of condensing boilers, or the ability of installers & service engineers. BG quoted me over £2000 when my system was knocking I subsequently sorted it myself for £25. At an alleged saving of £120 a year, it would take 16 years to recoup my investment. As I'm 60 now, I don't think I'll bother unless I have to. I'm sitting here now at a nice comfortable 22 deg.C with a can of lager at 15 deg. :-) Cast iron rules-OK.
I would be very surprised if any owners of four bed detached houses are getting away with only £15 monthly for gas heating. Let's have a survey: 1. Type of house (detached, semi etc). 2. No. of beds. 3. Size of house (small. med or large, or sq metres). 3. Do you also cook by gas? 4. How much do you pay monthly. 5) Who is your gas supplier?
Terry D.
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All domestic versions I have see, yes.

Kestons can be fitted anywhere with the flue only being cheap plastic drain pipe.

BG are not good at times.

Boilers are amongst the worst offenders when it comes to energy wasters in your home. Boilers alone account for up to a third of all domestic CO2 emissions. If you don’t think your boiler should get away with it any longer, then read on. As the current lifespan of a boiler is 10-15 years, making the wrong decision about what boiler to have really could mean a life sentence of wasting money and time and damaging the environment.
A heating system that uses a high efficiency condensing boiler and with the correct heating controls can save as much as 40% on your fuel bills. It really is time to shop your old boiler.
<<<
see: http://www.saveenergy.co.uk/boilers /

Your boiler will not last that long.

Cast iron is very inefficient. Unless your house is heavily insulated, or you turn rads off in many rooms, or you have a small house I can't see you spending £300 on gas per year.

That's what I pay, maybe less. I have a condensing boiler.
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And nor will the new one, from the very data you quote from! What's the point in recommending someone fit a boiler expected to last 10-15 years when the payback period in their situation is 16 years?
Replace you boiler when it *needs* replacing. Otherwise you're just contributing to pollution in the manufacture, transport and fitting of an unneccessary replacement.
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wrote in message news:<bnpi44> > >

in
years,
life
the
It is NOT 16 years. Do your sums inc capital costs of replacing the old clunker. Did you read the web site?

That is a moot point.
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"Terry D" wrote | I would be very surprised if any owners of four bed detached | houses are getting away with only £15 monthly for gas heating. | Let's have a survey: | 1. Type of house (detached, semi etc).
Flat
| 2. No. of beds.
1.
| 3. Size of house
27 sq metres
|3. Do you also cook by gas?
No.
| 4. How much do you pay monthly.
Last four bills, recent first, all before adding VAT
July - Oct 740 kWh @ 2.659 p/kWh = £19.68 - 3.56 prompt payment discount on previous bill
April - July 543 kWh @ 2.659 p/kWh = £14.44 - 6.36 ppdopb
February - April 1059 kWh @ various rates (price change) = £25.83 - 7.20 ppdopb
October 2002 - February 2003 2029 kWh @ various rates = £46.12 - 4.37 ppdopb
| 5. Who is your gas supplier?
Scottish Gas.
Owain
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monthly
cylinder
Just done exactly this.
Fitted a 2 port spring return zone valve in the return line from the hot water cylinder, thus leaving the vent unrestricted. Fitted a cylinder thermostat. Modified the wiring as C plan.
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Be very careful. A gravity fed (not just circulated) primary system requires an unrestricted feed line too, as well as an unrestricted vent. It is now common practice (to avoid pumping over) to have the vent straight after the boiler flow and then the feed. If steam is coming from the boiler flow up the vent, the water can't enter the boiler by that path, but instead needs an UNRESTRICTED path down the return. You may find that with an old cast iron setup, that can happen as the flow line is on the return line next to the boiler. You MUST check, though.
See the following system. If a zone valve is inserted at position 'Z', the the feed line can't find its way to the boiler return inlet. The boiler flow outlet is not accessible due to high pressure steam going through that section up to the vent. You can use this system sometimes, but this would require the boiler to have all the safety features required for sealed operation or combined feed/vent systems (i.e. user resetable cutouts etc).
--- / \ | | | tank |-------| | +----+--+ | | | feed > | | < vent +--+ | | | ---- | | / \ | | | /-----Z--+-++ | \ | | | / | +------+ | \--|---|Modern| |HWC | |Boiler| +----+ | | +------+
If the system is old style feed/vent, then there isn't a problem because the feed is further on. See below (where you can see that feed water can find its way safely to the boiler):
--- / \ | | | |-------| | +----+--+ | | | feed > | | < vent +--+ | | | ---- | | / \ | | | /-----Z----++ | \ | | | | / | | +------+ | \--|-----+-| Old | | | |Boiler| +----+ | | +------+
This vent/feed arrangement wouldn't work with a modern boiler, as the pressure difference across the boiler is large. This would cause a similar pressure difference between feed and vent, causing water to be pumped through it.
Christian.
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requires
the
flow
In the above, the pump must be between the zone valve (Z) and the feed & vent pipes. The feed must be between the vent pipe and the pump, eother on then p[ressure or suction side.

the
But below there is a possibility of pump over. On a modern boiler there is no need for a direct return path to the boiler for the cold feed. Modern boilers have double protection stats: run and high limit. The probability of both failing at the same time is very slim. That is why one pipe system are permitted - just one 22mm pipe for the F&E tank.

It would work, depending where the pump is located.
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(large snip)
I'm getting confused as to what you disagreed with. BTW, no pump was shown, as the system is unpumped. However, in a pumped system, the pump would in all cases between the vent (or fill point, if appropriate) and the zone valve.
In any case, the main issue is to ensure that the filling arrangements are suitable for the boiler, if the boiler doesn't have the safety features required for combined fill/vent operation. As the boiler runs a gravity hot water system, it is likely to lack such features. If the vent/fill arrangements are like the top system and a zone valve is installed, the system could be unsafe.
Christian.
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shown,
You are on about modern and old. Few modern boilers allow gravity these days. Those that do will not be around for long as legislation will phase them out.

With a pump, it should be as I explained.
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But the system in question is: "an old Potterton boiler with gravity hot water and pumped heating".
I'm just saying that the feed/vent arrangements must be examined to be safe if a zone valve is installed. In particular, the feed to the system must have clear pipework with no valves all the way to the boiler return (and not crossing or combining with the vent pipework) unless the boiler is a "modern" type with overheat cutout.
Christian.
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Christian McArdle wrote:

[SNIP]
[snip] OT - but am I missing something as whenever anyone posts ascii diagrams they look like complete gobbledegook to me? Shouldn't they be in a fixed font like courier ?
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Yes. It is up to your newsreader to choose the font. The font selection is not part of the message, which is just simple unformatted text. Find your newsreader's settings and change all the fonts to Courier, even the supposed "variable width" ones.
Christian.
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supposed
Or copy all the text into Word and set courier there.
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is
your
Or even easier, copy into Notepad. Already in a fixed width font, so no need to change it, and loads heaps quicker than Word.
D
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requires
Christian.
Good point. The valve is fitted in the airing cupboard in the return line as it leaves the HW cylinder in the airing cupboard before it tees into the feed/return line which then runs on to the boiler. Thus the gravity return is closed off by the valve but the feed and vent is still open.
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