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Discuss UFH running temperature and how to gently warm screed in the Water Underfloor Heating Installations area at PlumbersForums.net

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Hi all, I've just had my wet UFH connected up and turned on for the first time in my new extension and I've got a couple of questions:
  1. The screed was laid 6 weeks ago and my floor finish tiles haven't been laid but I'm looking to get them laid in about a week, so would like to spend the next week gently drying the screed using the UFH. What is the best recommended method of doing this?
  2. I have read previously that wet UFH operates at a much lower temperature than conventional central heating but in my house I now have conventional central heating with rads in the main house and then wet UFH in the new extension. So is there something clever that goes on to ensure the UFH requests water at a lower temp from my boiler than the central heating? How do I check what water temperature my UFH is utilizing?
Thanks!
 

EvilDrPorkChop

Gas Engineer
Advent Win
Hi all, I've just had my wet UFH connected up and turned on for the first time in my new extension and I've got a couple of questions:
  1. The screed was laid 6 weeks ago and my floor finish tiles haven't been laid but I'm looking to get them laid in about a week, so would like to spend the next week gently drying the screed using the UFH. What is the best recommended method of doing this?
  2. I have read previously that wet UFH operates at a much lower temperature than conventional central heating but in my house I now have conventional central heating with rads in the main house and then wet UFH in the new extension. So is there something clever that goes on to ensure the UFH requests water at a lower temp from my boiler than the central heating? How do I check what water temperature my UFH is utilizing?
Thanks!
1 - Your screed will be dry by now, even without heat.
2 - Your UFH manifold will have a mixing valve that reduces the temperature so the floor will run at a set temperature - you can set the temperature on the mixing valve. It'd be best to start at around 40oc and I usually advise turn it up 5oc a day. If you have tiles you'd be able to run it on 60oc without issues.

Check with your tiler as they will probably want the floor off a few days before tiling.
 
1 - Your screed will be dry by now, even without heat.
2 - Your UFH manifold will have a mixing valve that reduces the temperature so the floor will run at a set temperature - you can set the temperature on the mixing valve. It'd be best to start at around 40oc and I usually advise turn it up 5oc a day. If you have tiles you'd be able to run it on 60oc without issues.

Check with your tiler as they will probably want the floor off a few days before tiling.
Thank you. Sorry complete noob here so there are two gauges on the manifold, one connected to a red valve and one to a blue valve. I'm assuming these are not the mixing valve but something else? Then there is a valve that looks a bit like those you get on radiators with min and max labels on it and blue and red arrows going in opposite directions. Is this the mixing valve?
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From memory screed dries at 1mm thickness per day if that's any help. Also you could test the dampness with a hydro meter.
Yes I've read that a lot. But that equates to 65 days best case for me although many people seem to say it goes up to 2 days per mm after 50mm of screed or something like that. I can't afford to wait that long hence I thought having left it to dry naturally for 45 or so days I could use the UFH now.
 
Yes I've read that a lot. But that equates to 65 days best case for me although many people seem to say it goes up to 2 days per mm after 50mm of screed or something like that. I can't afford to wait that long hence I thought having left it to dry naturally for 45 or so days I could use the UFH now.
Depends on the type of screed. You also need to make a distinction between two phases. The 'curing' or 'hydration' time, which is when the reactions that cause hardening occur; these require the presence of water. This is followed by the 'drying' time, which is when the water left in the 'pores' of the set screed evaporates.

You shouldn't rely on time alone to judge when it is safe to lay tiles on screed. The residual moisture content should be measured before the covering is applied. See, for example:


Screedscientist is a great site BTW. Definitely one to add to your reading list for when you are self-isolating.
 
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Depends on the type of screed. You also need to make a distinction between two phases. The 'curing' or 'hydration' time, which is when the reactions that cause hardening occur; these require the presence of water. This is followed by the 'drying' time, which is when the water left in the 'pores' of the set screed evaporates.

You shouldn't rely on time alone to judge when it is safe to lay tiles on screed. The residual moisture content should be measured before the covering is applied. See, for example:


Screedscientist is a great site BTW. Definitely one to add to your reading list for when you are self-isolating.
Thanks - useful read. I don't think my builder has the equipment required to do a moisture test of the screed. I know he should but reality is he doesn't so is there a DIY method to roughly determine whether it is safe to lay tiles?

Also, I did think I would get ditra matting laid on the screed first to protect against any moisture issues. Does that seem sensible?
 
Thanks - useful read. I don't think my builder has the equipment required to do a moisture test of the screed.
You could go old school and tape a piece of plastic sheeting over a couple areas and leave it for a few days then see if it's damp underneath the plastic. This simulates what's going to happen under your tiles. Not as good as a reliable percentage reading, but better than nothing.
 
You could go old school and tape a piece of plastic sheeting over a couple areas and leave it for a few days then see if it's damp underneath the plastic. This simulates what's going to happen under your tiles. Not as good as a reliable percentage reading, but better than nothing.
Will do. There was actually a roll of rockwool left on the screed in one particular place for the last few days and I just moved that and there are dark patches there where it seems rising moisture has got trapped under the rockwool at the surface. So I think that's kind of done the type of test you mention. Does that mean it's not ready for tiling yet?
 
. there are dark patches there where it seems rising moisture has got trapped under the rockwool at the surface. So I think that's kind of done the type of test you mention. Does that mean it's not ready for tiling yet?
I wouldn't risk it based on your description. In your position, I'd probably have the UFH on low (say 40°C flow) and consider renting a dehumidifier, which would allow me to monitor the rate of drying from how much condensate it collected each day.
 
I wouldn't risk it based on your description. In your position, I'd probably have the UFH on low (say 40°C flow) and consider renting a dehumidifier, which would allow me to monitor the rate of drying from how much condensate it collected each day.
OK so UFH on all the time at 40C flow? I actually already have a dehumidifier which we mainly use for drying clothes indoors so I can use that if it's not going to have any adverse effects on the screed.
 

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