2 port Vs 3 port central heating valves

I am just about to embark on upgrading my old oil fired gravity hot water / pumped central heating system to a gas fired fully pumped system and am unsure what arrangement of valves to use. What are the advantages / disadvantages of using a 3 port valve instead of two 2 port valves to control a fully pumped hot water and (single zone ) central heating system?
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Go for a 3-way throw over valve with an end switch (3 port) and a quick recovery cylinder (part L is not quick recovery). The cylinder will take "all" of the boilers output, reheating in a matter of minutes, and be cheaper to run. The system will be a "priority" system. These valves are generally to order, but come fast. A few have the end-switch, like Drayton and others. The end switch makes it easy to wire up.
The now common 3-port mid-position valve can be temperamental. If going this route, which I suggest you do not. Use two 2-port valves, unless space is a problem.
A quick recovery cylinder can be downsized. So a normal 114 litre cylinder can 80 litres. The 80 litre Telford Typhoon from Travis Perkins is approx 100. Albion and Range do them too
http://www.albion-online.co.uk
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IMM,
Thanks for the information. By throw-over valve, do you mean a diverter valve? I assume that by using this arrangement all hot water from the boiler will be diverted to the cylinder when demanded. If several baths, washing machine and shower are used in succession (probably unlikely I know), would that mean the radiators may not be supplied for some time?
On the subject of quick recovery cylinders, what is Part L? I am also considering an unvented cylinder and a sealed system boiler. Which unvented cylinders would you recommend as quick recovery?
Eric

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Heat banks: http://www.heatweb.com Also from Range cylinders under "thermal storage" on their web site. Albion don't do heat banks.
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Thanks Bill, I'm coming to the same conclusion as the lack of reliability of the 3 port valves seems to be a widespread opinion.
Eric

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You mean a 3-port mid-position valve. A 3-port diverter is the most reliable option. One reliable simple head. With two valves you double the problem. Diverters and 2 -ports tend to use the same heads.
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EMC wrote:

IMHO using 2 (or more) 2 port valves is better in every respect than a 3 port valve except that the 3 port (either mid-position or diverter variants) is marginally lighter on material costs.
If you decide to go the 3 port way - be strongly advised that the diveter valve variant (either but not both CH and HW) - has been found wanting by many.
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This new to me. A 3 port diverter is as simple as a two port. They use the same heads.
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I had diverter valves of this type in my system from when the house was built until I ripped it out with the rest of the abortionate plumbing that they had done in the airing cupboard.
There were four replacements over a 15 year period which I don't think is impressive. Typically, either the mechanism would stick in the base or a leak would develop.
Using 2 port versions and having the boiler do the priority control seems to be a much better solution.
.andy
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Sounds like a poor make and compounded by a poorly installed system. They can be fitted on the cooler return pipe which enhances longevity.
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there were then I think a Danfoss and two Honeywells, with the latter two lasting about 4 years each. I wouldn't call Danfoss and Honeywell poor makes as such.
The fitting position was on the return in one of the recommended positions so no clue there either.
I even went to the trouble of exercising the heating at least weekly during the summer months so that the valve wouldn't stick. The failures weren't at any particular time of year.
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wrote:

On a far simpler valve you have a failure rate far higher than the more troublesome mid-position 3 port valve. Sounds like the system was faulty somewhere.

Correct and sufficient inhibitor can lubricate valve mechanisms. A two pump casting Grundfoss Tee setup can be used instead of any 3-port valve, mid-position or diverter.
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There was nothing much else to it so I can't see that that would influence anything

I did all of that religiously

Interesting idea and I believe not much different in cost to a conventional set of pump and valve.

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Then you are just unlucky. Can someone be "that" unlucky using three different makes of valves?

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IMM wrote:

I don't mean to imply that there was a reliability issue. As you say the heads are the same as a two port unit. What I mean is that people are unhappy with a system that cannot simutaneously run the heating and the water! More than once I have been asked to 'fix' a system with a diverter valve that the owners perceived as broken, and as you know, the customer is always right.
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I assume they have old fashioned British Standard pig of a cylinder and not a quick recovery. If they had a quick recovery they would not complain as they would not notice the heating has been off for a few minutes or so.
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IMM wrote:

problem it would (at least for one customer) not have fixed it. The usage the cusotmer wanted to put the system to was simple: Off for days at a time then come home and switch both heating and HW on, a completely reasonable way to operate the system for his lifestyle. Even a 15 minute delay would really have been unacceptable.

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