Alex Carr

Hello Jamesessex - I'm not going to blind you with ashp science as I'm no expert in this area, but I want to say one general thing. We have a Vaillant ashp powering ufh in a basement floor and vertical rads upstairs plus heating for hot water. The rads heat up quickly and we are happy with their performance, but the ufh is by its nature slow and takes near half a day to entirely heat up our spacious basement (heatmiser thermostats in each basement room) - everything I've read says low and slow if you don't want to get nutted with heating bills. That appears to be how it is with ufh, low and slow but ultimately effective. I will add that our property is 200 years old and we did an extensive retro-fit with regard to insulation, so there is relatively little heat loss. The minutae of running an ashp economically and effectively is, in my humble opinion, a steep learning curve which requires expectations to be re-tailored.

Thanks for that information John, but it will take a bit of time to get my head around what you are saying as it is not entirely comprehensibe to me. The installation company have been no help at all in helping me understand how to achieve the optimum running of the system. (There's a significant learning curve which I'm only just taking tentative steps to negotiate.) I would like to study the contents of your reply and, if it's agreeable to you, ask you some Q's. We'll get our first month's bill in a day or so and we'll see how our untutored use of the system pays out - if I can re-calibrate the workings as you are suggest the second month's bill should reflect a more economical set-up.

Yes, have a buffer. Weather compensation? I may be wrong, but I presume you mean that when the outside temperature falls below a certain point the heating kicks in; yes, the system does this. Yes, timed running slots for heating. More Q's?

I won't mention the name of the company but installing and set-up of a Vaillant ASHP to provide heat for ufh downstairs, vertical Stelrad radiators upstairs plus domestic hot water, has been a saga I would not like to live through twice. Here's my query: I understand how to programme the UFH & DHW, and they seem to be working ok, but operation of the rads seem only controllable by valves fitted to the base of each rad (there are 4 + an unprogrammable thermostat in each of three rooms). I don't get the relationship of the rads to the rest of the ASHP system - do they operate solely on the basis of their valve settings? (1 -5; the temperature range of these settings is not obvious; I have to corelate the valve setting to the temp settings on the wall-mounted thermostats to see what level of heat the rads are generating. Currently the UFH doesn't turn on until 6 a.m. each day but the rads are going during the night, and I sure as hell don't want that - is there a way of getting control of these beasts above and beyond the valve control on each rad? Enlightenment would be hugely appreciated.

Everything I read said that 8:1 using sharp sand was the proportion to use. No deviation from that from anywhere

Dry biscuit screed - 8:1 w. sharp sand - laid 22 days ago. Leaving it to dry out for another 22/23 days on the basis of 1mm per day drying time. Then will lay 15mm engineered wood flooring across the battens embedded in the screed. I don't know what the texture of the screed should be like after this drying out period because right now it's crumbly and is not setting in the way, for example, a semi-dry 4:1+0.5 water based screed would. Am I right to worry that the screed will not harden; or am I just needlessly anxious? Some input on this subject from a more knowledgeable member would be really appreciated.

I'll have to read up a bit on that to help me decide whether or not to do it. Also I'll see what the tiler says.

Having laid the screed the flooring contractor is now out of the picture. It was my contractor who screwed down the battens, much to my surprise. He's sensible most of the time, but he did mess this up; still, sins are there to be forgiven. He has a lot of work to do still on various other things and he will open all doors and windows during the day, shut them when it's time to go home and will leave the dehumidifier on overnight. As I said in my original post, there are five rooms and a hallway which have been screeded. I will wait 50 days for a 45mm thick dry screed to lose it's moisture and will periodically check the RH with a meter. Before laying decoupling mats in the 3 tiles rooms, I will seal the screed - any tips or advise on that?

Yes, that's my plan. Dehumidifier left on and check the RH every few days.

I think patience is the thing. 1mm per day drying time + dehumidifier left on overnight when the contractor & his crew are not there. Dry out the battens & screed, lower the rel. humidity levels... and anything else I can think of. Correct: no MVHR. The property is the bottom two floors of a 200-year old, 4-storey Edinburgh Grade II listed building. A f*cking nightmare to renovate but my wife insisted she wanted to buy the place because it is spacious like no other place we saw and it has a 30m long, 8.5m wide garden.

I'm calculating 1mm per day. Also usue of a dehumidifier to reduce the rel. humidity. Anything that wil help.

I think not as there's no way of knowing how close the battens have been fitted to the ufh pipes; it would only need one slip with the saw to cause irreversable damage.

Thank you for that Nick. I confess I am a worrier and a believer in Murphy's Law, that anything that can go wrong will go wrong. Everything I've read says 1mm per day drying time, though 2mm sounds appealing.

Thanks for that. I suspect that is what I'll do.

1mm thick steel plates? What is the function of the plates? And then tiled over that?

Here's a mess I need advice on: wet ufh pipes running through 5 rooms & a hallway; engineered wood floor in two of the rooms & the hallway; the other 3 rooms tiled. Dry biscuit screed (8:1 w. sharp sand & battens laid @ 400mm centres to support the engineered wood floor) in the 2 rooms and the hallway; semi-dry screed (4:1 w. sharp sand) laid in the 3 rooms to be tiled. The semi-dry screed was about to be put down in the 3 rooms to be tiled: my contractor, usually so reliable, said he would lay down some battens (45 x 45mm) as levelling guides for the fellow doing the screeding. And then we'd lay a decoupling mat (current choice: Schluter Ditra Matting) over the screed when it had dried out (at the breakneck speed of 1mm per day) and then pull in the tiler. However, this is where the ordure hit the fan: I assumed the battens were temporary & were only to be used for levelling purposes and would be removed as the work progressed – very wrong. I went home because I was unwell and have been laid low with a virus for the past 10 days; my wife went to the job site (our home to be) after the screed had been laid and took some photos of the finished work, which is when I saw battens where they shouldn't have been. They had been heavily screwed into the sub-floor of the 3 to-be-tiled rooms. I called the contractor and said the battens would have to be removed as we couldn't have strips of softwood supporting a tiled floor as stresses could be created with wood shrinking and ridges forming between the battens and the screed, and so on, and so on. He said he would have a look at the situation Monday morning. I dragged myself out of my sickbed today, Sunday, and went along to have a look for myself. On the basis that I wouldn't ask the contractor to do what I wouldn't be prepared do myself (even at the age of 75) I tried to get one of the battens up, but that turned into a mess, with the still damp screed crumbling at the edges because it was adhesively clinging to the sides of the battens, and the ufh pipe getting partly exposed, and deep screw holes filled with screed muck that was impossible to remove. Needless to say the air turned blue Net result: it looks like the battens will have to stay: The $64,000 question: will the Schluter Ditra matting (3.5mm thick) provide a sufficiently permeable barrier to block the influence of potentially misbehaving softwood battens? I do realise that when the ufh is going, in the fullness of time the battens will dry out, but until then I don't know what to expect. Please, compassionate responses only as it was someone else who dropped me in this sh*t. Constructive opinion would be very much appreciated

Per sq.mt, what is the weight of 65mm screed?|

Hello Kelvin - Thanks for the info

What would be the minimum thickness of a traditional screed to support the tiles with the addition of a decoupling mat?

I have limitations on build-up space. But what would be the traditional screed consist of?

Because the biscuit screed is simply thermal mass, it has no structural integrity. The cement boards are there to convey heat from the screed to the tiles and also to ensure the floor can be walked on without the biscuit screed coming under pressure.

You're description of a biscuit screed is correct, which is why I've been looking for 12mm cement boards to cover it to carry the tiling. The floor joists have 70mm PIR insulation fitted between them and sit under 12mm OSB boards ; Visqueen covers the OSB; the UFH pipes are sitting on the visqueen and are waiting for the screed to cover them. My question was really about whether a 65mm semi-dry screed (4:1) would have sufficient structural integrity to allow the tiles to be laid on a decoupling mat glued to the 4:1 screed, and if it does could I do away with cement boards I had originally planned to lay over the 8:1 dry biscuit screed? If it does not, what ratio of sand to cement (builder's sand or sharp sand) + polypropolene fibres would do the trick? And how thick? However, the thickness could be tricky - as I have limited space to work with.

I've had some v. helpful advice today on this thread, but I have one question I can't yet find an answer to: With a 65mm semi-dry floor screed (sharp sand with a ratio of 4:1) with a decoupling mat on it and on the mat 10mm ceramic tiles, will the screed be strong enough not to misbehave what with the weight of bodies walking backwards and forwards on the tile floor? I read this earlier: "These membranes excel at absorbing stress, effectively neutralising tensions between the substrate and the tiles. By doing so, they prevent stress cracks from transferring through to the finished tiled surface, ensuring the structural integrity of the flooring." Any thoughts on whether a 65mm semi-dry screed can stand the strain?