De-strat pump in unvented horizontal cylinder?

[GB1309]

Apologies if this has been covered before but I searched on "Telford" and saw nothing. This relates to events 12 months ago when we had a problem but I never resolved it to my satisfaction so I thought I'd run it past the brains trust:

We have a horizontal Telford Tempest 150ltr being driven from a Viessman Vitodens 100 and it all works well (now). However the thermostat supplied by Telford was out by about ten degrees which which meant the output water temp (at the top of the tank) was hitting 80 degrees before the stat turned over and the boiler was having to work hard to get those last few degrees into the water down at the stat tube point.
(Insulation is great up there by the way as I built what amounts to a Kingspan shed in the loft area.)

My question relates to the answer I got when I called Telford concerning the failed stat. "Have you got a de-stratification pump fitted sir?" Was all he seemed to want to talk about. I gave up and bought a new stat for £20 of the net and it works fine.
My properly qualified installation/service engineer said he had not heard of the requirement for a de-strat pump and had never fitted one. Neither was any reference made to such a need in the Telford installation data.
I fully understand the way such a pump would work and the benefits of mixing water whilst heating. Are these pumps now commonly fitted (particularly to horizontals)? Am I missing a trick?

Cheers,

Graham.

 

[Riley]

Really shouldn't use non manufacturer parts with unvented cylinders will invalidate any warranty and potentially insurance claims if anything went wrong. Secondly you need to be G3 qualified to work on these cylinders

 

[townfanjon]

Graham stratification is far worse the taller the cylinder is i.e a 300 litre will suffer far greater than a 90 litre .
I think a d-strat pump on a 300 would be a good idea.
A 150 litre horizontal , I cant see it would make much difference .

 

[Dotty]

As alluded to, it must be a manufacturer specified part and installed by a g3 qualified plumber.

 

[GB1309]

Thanks for the advice and reassurances. I understand the height/volume issues.
This cylinder was put in by and is maintained by the same G3 qualified chap.
The stat I obtained was exactly the p/n of the original. (1/3 the price via Telford)
Point still remains that it was the Telford help desk that raised the de-strat issue although no information was supplied with the cylinder (12 months earlier) that would have indicated it might have been required.
I wondered if it was something they had come up with later to deal with reported problems.
Who knows. It works well now. I give it 3 x 30 minute cycles a day and it does a house of 4 persons with 2 bathrooms.
Cheers and happy new year,
Graham.