Discuss Boiler keeps short cycling with only UFH is on in the Water Underfloor Heating Advice area at PlumbersForums.net

Yes for the ufh manifold the pump isn’t rated for the heating pipework from the boiler etc

So you will need two or theee depending if it’s a system boiler/ combi or heat only
point 2 the manifold pump isn’t rated to pull from the pipework so you will need another one here

Point 3 yes
point 2 the manifold pump isn’t rated to pull from the pipework so you will need another one here

Point 3 yes
thanks @ShaunCorbs.

Just to add some context, I am looking to replace my heat-only boiler with a system boiler. I already have an external pump beside each of my zone valves - photos attached of zone valves marked 1 = HW, 2 = UFH, 3 = CH

The external pump is a UPS2 25-80 (photo attached). Any issues with this pump being used for the central heating?

From what you have said, the manifold pump isn’t rated to pull from the pipework so you will need another one here. Can you expand upon where another pump will need to be fitted?


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That pump is fine for the ch you will need a separate 15-5/60 pump for the ufh after the cct if the cct is above the boiler etc

One of the CCT (or LLH) advantages is that the primary circulating flow can be controlled to give a flow high enough to control the maximum dT which can exceed the boilers max allowable, generally ~ 25C, another advantage is that it helps to get the boiler away without exceeding the flow temp tripping point since all gas boilers fire up at ~ 65% of max output for up to 60 secs before modulation. The disadvantage is that the boiler return temperature increases with reduced demand.
For example if you set the primary flow to 14.3LPM to give a dT of 20C with a 20KW rad demand, the boiler dT will be 20C with flow/return temps of 65C/45C, a rad demand of 9kw will result in a dT of 9C, flow/returns of 65C/56C, a UFH demand of 9KW will result in the same dT of 9C with flow/return of 65C/56C. A rad demand of 2.5KW will give a dT of only (theoretically0 2.5C with flow/return of 65C/62.5C and the same for a UFH demand of 2.5KW.
Of course it will not prevent cycling if the minimum boiler output is greater than the heating demand.
Is a CCT/LLH required?.
The above boiler, a Vaillant 438, has a HEX with dP of 4.05M at a flow of 27.2LPM.
If a manual bypass was installed then opening it to give a 10.7LPM permanent by pass will result in a HEX dP of 3.5M (at 25LPM) which leaves 4.5M (8M UPS2 installed) remaining head to circulate 14.3LPM through the rads etc.
You will then end up with (boiler) flow/returns of rads only 65C/53.5C, dT 11.5C, UFH 9kw only of 65C/60C dT 5C, UFH 2.5KW 65C/63.5C, dT 2.5C..

If you could source a true constant curve pump then a ABV could be used to get much improved, lower boiler return temperature and greater boiler efficiency.
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@ShaunCorbs - thanks for this.

From the diagram you provided, would:
  1. the 'heat source' be the equivalent of the new system boiler and the 'primary circulator be the internal pump within the system boiler?
  2. where it shows the "secondary circulator along with the 'spring loaded check valve' - would this be for the CH or UFH?
  3. and for the "another secondary circuit" - would this be for the UFH or CH?
  4. from the two secondary circulator pumps shown on your diagram, where would I use my existing 25-80 pump?
  5. similarly, where you mention about the need for an additional 15-5/60 pump for the UFH, where would this be located?
If you were able to provide a quick sketch of how my system should be set up, this will be greatly appreciated.

At the end of the day, what I am trying to achieve is to be able to run the UFH on its own without the boiler continually cycling. When this happens, the room that contains the UFH hardly heats up so the way I get around this is to run the CH at the same time so that the boiler continually runs and the UFH room heats up.

I had a look at the latest Vaillent ecotec plus system 25kW boilers which have a modulation range of 1:10 ecoTEC Plus - https://www.vaillant.co.uk/for-installers/products/ecotec-plus-610-615-620-625-630-635-154880.html. I was wondering if this would reduce the boiler cycling with just the UFH on?
That would be the rads as it’s the hottest heat load

Then the ufh eg horizontal one

25-80 for the rads

15-60 for the ufh

Same as drawing horizontal set

You want the highest modulation possible eg lowest kw figure of ideally under 1kw if possible

Only just seen you sent me a only sorry will have a look later
sorry @ShaunCorbs but I don't fully understand your reply based on the diagram you provided.

From your diagram:
  • where it shows the "secondary circuit & circulator along with the 'spring loaded check valve' - is this the CH circuit and the secondary circulator being my existing 25-80 pump?
  • where it shows the "another secondary circuit" - is this the UFH circuit, and where the pump is shown, is this the additional 15-5/60 pump you refer to, for the UFH?
Will draw it out tonight for you
can do it two ways eg below


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many thanks @ShaunCorbs. On the Hydronic CCT, stupid question but what do the symbols (attached) denote? From your diagram last week, you showed the position of two CCTs, spring-loaded check valve and purging valve - where would these appear on the Hydronic CCT diagram?

On the System CCT diagram, I just see the 25-80 pump but do not see the 15-80 pump, nor the CCTs, check & purging valves - I they not needed?

Are there any advantages with the Hydronic CCT setup over the System CCT setup, and vice-versa?

Check valve / swing check

As the boiler is cct you only need one pump ideally a modulating pump is best and no there not m

Both the same

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