UFH New Boiler

18 May 2011
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United Kingdom
Just had a new Vaillant Eco Tech Plus 630 boiler fitted and the heating engineer has set the flow temp at 60 degrees from the boiler. Have 3 manifolds, 10 loops, all with adjustable mixing valves to control the mixed underfloor and under screed zones. The old boiler didn't have a programmable flow temperature, set with an adjustable dial. Having searched various subjects on this forum could I have your opinions on:

1) It is more efficient to leave the UFH heating on 24 / 7. I have set it up to run from 04.00 to 20.00.
2) Can the flow temperature be decreased - does making the return temperature lower make the boiler more efficient
3) I have bled the manifolds twice and removed quite a bit of air - fine yesterday afterwards as the system was running at around 1.1 bar. However not running overnight, I assume the cooling water has dropped the pressure, as a result the boiler had switched itself off this morning, it was at 0.3 bar. Topped up the boiler a little and running again at 1.5 bar. Is it trial and error to reach the correct pressure
4) Any other tips.
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Have you got a separate, red expansion vessel? The one in the boiler will be too small for all that pipe work.

What programmer is fitted to the boiler?

No separate expansion vessel. However after installation we had it running all loops for 24hrs at 25 degrees room temp and coped with that ok.
No programmer fitted to boiler, programmer works from the UFH manifold.
If you are feeding the UFH directly from the boiler then thats not normally done in the UK. Normal in Italy and Europe though.

Feeding UFH at 60 C is way too high and would make the floor too hot to walk on with bare feet!

Some info missing!

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Sorry perhaps i haven't been clear in my initial post.

Water flows from my boiler at 60 C, it them reaches the manifold(s) where is it mixed with the cooler return water and circulated round the under floor pipes at a lower temperature.

Hopefully that helps
In that case you have a blending valve and a circulating pump!

If you only have UFH then you could probably reduce the boiler flow still further to say 55C or even 50C to increase efficiency.

That assumes your controls increase the boiler flow temperature when required to heat up the cylinder and has hot water priority.

Hi Tony

Thanks for the reply. Setting the flow temp lower was what i suspected was needed. I have lowered it a little to 56 C and will see how it goes. May well lower the temp a little more over the next few weeks will see how it goes.

I take it the lower the flow temperature back to the boiler the more efficient the condensing boiler will operate, so long as the return temperature is not so low that it takes too long to heat the property. Trial and error I guess.

My old Baxi Barcelona (don't laugh) had been left on 24 /7 and I suspect at a lower flow temperature, it always ran well - explosive ignition apart!!! and i think having it on 24 / 7 extended the life of the old boiler. However the new Vaillant seems to be "on" a lot more than the old Baxi. My heating engineer told me to run the new boiler 24 / 7 and not to worry about it. All I see is my bill going up. On the plus side I can not believe how quiet the Vaillant is compared to the Baxi.

Finally Tony what is your opinion on Dan's comment that the boiler is too small. Initially two heating engineers told me a Vaillant Eco Tech Plus 624 or 630 should be OK. Having found my initially heating plans from Unipipe, doubt I need 30kw. My original UFH needs 12.5kw and i have added a 54m2 extension. Hot water is from a Megaflow cylinder. The old Baxi had a red expansion cylinder inside the boiler but I haven't seen inside the Vaillant.

he was talking about the expansion vessel not the boiler. If yoj look at the UFH specs you will find the water volume..and this will enable a calculation to be undertaken to see.

The boiler needs to be set at a higher temperature than the target hot water temoerature, otherwise the boiler will be for ever heating the hot water cylinder
Leaving the boiler on 24/7 will use more heat that just using it when you need it.

Common fallacy even among heating engineers!

You will need about 18 kW for heating!

So who decided to fit a boiler nearly twice as powerful as you need?

That reduces boiler efficiency!

Tony that is unresearched twaddle...it depends on the usage pattern of the on off strategy...

if its a village hall used once a week of course it will be better to turn it off... but a house ocuupied 18 hours a day, I think not..

And UFH with a high thermal mass should have its heat input regularly topped up to overcome the thermal inertia...
Thanks for the recent replies. Looks like I should have gone for a 24kw replacement, not a lot i can do about that now!!

I have decided I will leave the boiler on 24/7 for the time being. A quick gas usage cost check for the past week works out at £3.36 a day. About 10% cheaper than the old Baxi Barcelona.

Next job, when I can afford it, to fit programmable room stats to all heating zones. Have had them fitted to the recent extension, but the old stats are not programmable. I reckon that will be preferable to turning the whole system off overnight...

Thanks for taking the time to help me
Swanny, if you are able to monitor the usage that accurately then you could compare having it on 24/7 with having it on only when required.

Being UFH that would normally mean turning it off at 2130 when rerquired to be cool by 2330 and turning on at 0500 when required to be warm by 0700.

Thats takes into account the probably two hours thermal inertia.

That should save about 30% of your heating costs!
Being UFH that would normally mean turning it off at 2130 when rerquired to be cool by 2330 and turning on at 0500 when required to be warm by 0700.

Thats takes into account the probably two hours thermal inertia.

That should save about 30% of your heating costs!

That's what my heating engineer also suggested, but added he would leave the boiler on 24/7 and fit individual room stats.

I will run it as it is for a while and check usage. Then turn it of between 21.00 and 04.00 and see what difference it makes.

Down rate the boiler if needed (d.00) - what does that mean.....

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