UFH Boiler Flow Temp -

18 May 2011
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United Kingdom
I had a new Vaillant Eco Tech Plus 630 boiler fitted in December 12 and the heating engineer set the flow temp at 60c from the boiler. I have mixing valves fitted to each UFH manifold.

I posted various queries about the flow temp / operating time etc here having had the boiler fitted see....


I received some excellent advice thank you all.

Last winter 2013/14 i decided to lower the boiler flow temp to 55c. I ran the system 24/7 and the house was usually comfortably warm - only on the coldest days did we use our open fire. I have just renewed my gas / electricity deal and my gas usage was a little over 25% lower over the past 12 months. I accept that we had a milder winter last year and this would have affected demand / gas usage. However I have lowered my utility costs from £130.00 pcm to £85.00 pcm for the coming year - new deal same supplier.

Spoke with my HE a couple of weeks ago when he serviced the boiler and he maintains that an increased flow temp of 66c (higher than the previous 60%) is the most efficient operating temperature for the boiler and the system, the DHW is supplied from the same flow. The system, which was fitted by someone else, has the DHW supplied from the same UFH flow from the boiler. The DHW is now much hotter!!!

I keep an eye on my gas usage and at the current 66c setting i am using consuming 80% of the gas I was using in the coldest period of last winter and it is very mild at the moment.

I dont have a weather compensating unit fitted to the boiler but I guess that it may be cheap if i manually adjust the flow temperature to reflect likely outside temperatures. I don't like going against the advice of the professional but...................
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Seems something wrong or missing!

Presumably you have a HW cylinder?

Does your boiler have a controller to give an increased boiler flow temp when reheating the cylinder?

Cylinder should be set at 60 C and the boiler should be at 70 C when reheating it!

Hi, yes I have a Megaflow cylinder.

I dont know if the boiler has a controller. They may be one. Likewise the HE could have up'ed the DHW temp on the Megaflow at the service and I didn't notice, hence the hotter DHW.

The temp of the DHW I can deal / live with, it is really the UFH boiler fed flow temp that i want to get right. As it appears the higher the flow the more gas used (there is a mixer valve). Obviously I would expect to use more energy as the water is being heated to a hotter temperature. However the additional energy used appears disproportionate and is it necessary.
You don't seem to know much about your system.

Are you quite sure that you have not left the electric immersion element switched on to electrically heat the cylinder?

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You don't seem to know much about your system.

Are you quite sure that you have not left the electric immersion element switched on to electrically heat the cylinder?


The immersion is off

this is how I see it. One flow from boiler feeds both UFH and DHW. Temp of both set at boiler. 24/7 the Sunvic valves on UFH manifold call for water. Honeywell valves fitted beyond the UFH call for DHW when timer programme is on.
Obviously a lower flow temperature will result in lower fuel costs. However you need to find the point at which you are still able to maintain a comfortable temperature within the property. If as you say you found that 55C was working quite well that then poses the problem of not being able to satisfy your cylinder stat if set at 60C which it should be to kill off any legionella. However your vaillant boiler is capable of running two flow temps, hence the two dials on the front of the boiler but it needs to be set up properly to do so.

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