John Guest UFH room pack and Vaillant 837

Joined
16 Jan 2012
Messages
3
Reaction score
0
Country
United Kingdom
Hi All,
I'm hoping for some assistance here. We've recently had a Vaillant 837, rads and a UFH system installed as part of a refurb. The majority of the house is served by rads with an extension that has a single zone room pack from John Guest. Its been setup with 2 x zone valves so both the rads and the UFH have their own programmable room stats. The operation of the control side is fine with both being able to fire up the boiler as expected.

The issue arises when the UFH is on alone. The boiler fires up and very quickly goes into anti-cycling mode. I see the tempertaure display on the boiler rise rapidly when it fires up for up to 15 seconds and then it cuts out to anti-cycling when it hits around 60 degs C. I've altered d.0 to 12 (it was at factory default 28) and this has slowed down the time before it goes into anti-cycling but hasn't solved the problem. I also reduced the anti-cycling max time to 5 mins to see if that helped but it didn't.

My theory is that the UFH is not taking enough flow from the boiler (as the manifold mixes the UFH return cooler water with the incoming hot from the boiler to get the right temperature flowing into the UFH). If the return from the UFH is warm, then it seems to take very little from the boiler. I'm assuming here that the boiler internal bypass is firing which flows the hot straight back around and makes the boiler go into anti-cycling mode.

When in anti-cycling mode, the water temperature drops well below what the UFH is asking for and the floor cools down. The floor doesn't really get up to temp when this occurs.

Has anyone got any guidance on what I should check/investigate here or any other theories on what I can do to fix it. Is my setup correct or do I need something special to connect a boiler like this to the UFH?

I've currently got the zone valve for the rads set to manual so it is always open which allows 'excess' heated water to flow through the rads which don't have TRVs on them (1 x hallway and 2 x bathroom towel rails) which seems to stop the anti-cycling.

Also, as a QQ on the side, what temp should i have the UFH set at ? Its a 75mm screeded floor with stone tiles on top.

TIA
DM
 
Sponsored Links
if you have space (and money)you should install the vr 61 module with mixing valve and low loss header...
 
I am pleased to see you have seperate controls on your UFH and Rads. At least thats correct.

Your problem is you have a powerful 37 kW boiler and seem to expect it to be able to provide perhaps just 2 kW.

The reality is that your boiler will modulate down to about 12 kW and then go off when the set flow temp is reached.

Thats a feature of boilers that the only modulate to about a third of the maximum.

I cannot suggest anything else that you can do apart from accepting that the UFH is probably working fine but the only problem is that the boiler is seriously cycling.

Tony
 
Sponsored Links
"Is the UFH pump working correctly?" - Yes, I believe so, it comes on at the right times and the return gets warmer from the UFH circuits (2 circuits in 1 zone)

"if you have space (and money)you should install the vr 61 module with mixing valve and low loss header..."

Hmm, space is in short supply (and money for that matter) but if these bits do what I need can you explain a bit further?


Thanks
DM
 
Whilst a LLH may slightly improve the situation ( at a cost ) I dont see how its going to prevent cycling when a 37 kW boiler is supplying just 2 kW of heating to the UFH.

All I can see it doing is applying WC to the UFH but that is not the OP's stated objective which is slely to prevent the cycling ( which I see as inevitable ).

As the OP has zoned and controlled rads as well then they will compensate for any under temps when the room is occupied.

Please explain Alex?

Tony
 
Thanks for the replies.

Is it confirmed that the UFH only wants 2kw or so? Also, Agile stated that he sees the cycling as inevitable....is this the consensus?

Have I done something wrong with this setup? Surely people have similar setups working satisfactorily? Should I have done something different? If you were in my shoes now, what would you do to fix this or make it work better?

I'm assuming that if the cycling is seen as inevitable, the hotter I run the UFH means it takes more KW so reducing the amount of cycling? What temp can I run the floor at (JG manual says up to 58 degs C I think). As per the OP, its a 75mm screed floor and tiles on top.

Thanks so far all.

DM
 
One other option qould ge a buffer tank arrangement, but it will be expensive, time/money/space consuming.

Your cycling problem is as Tony said, and a bit late to be worrying about now. I would be surprised if your house needs 37kw for heat, and if it does then a combi is probably the wrong choice of boiler anyway.

Assuming of course it is a combi?
 
yes as the minimum modulation rate is about 9kw, you will need to be dissipating this when the boiler is firing to avoid cycling and stress on the pump...

that's why you would need both a low loss header and to weather comp the system...that means a certain forfeit of control, a gain in system efficiency and a more comfortable living environment.
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top