Thick screed wobbles….

[G and J]

I was so certain in my plan:

 

1). Reasonably airtight and insulated house not needing huge amounts of heat even in cold weather.

2). ASHP running wet UFH at a lowish temperature to be happily energy efficient.  Single zone, weather compensation for heating and cooling, no thermostat.

3). Nice lot of underfloor insulation to put the UFH pipes onto.

4). Fat, circa 100mm screed to act as a big storage radiator.

 

Lovely.  
 

Or is it?

 

In the last week or so I’ve had my ear bent by three very experienced builder type people, all telling me I’ll really regret a fat screed.  The consistency of their arguments is astounding, they are all saying that we'll hate how long it takes to heat up, it’ll be cold and it’ll never get properly warm as all the heat gets eaten by the screed, it’ll be impossible to control and if I had even half a brain I’d go for a 50mm flowscreed instead.  
 

These three are all nice guys who really do mean well and have our best interests at heart, so I’m sure that they believe what they are telling me.

 

I tell myself that I understand the physics, the heat won’t be just lost unless I fail to insulate or achieve airtightness, that slow response for our set up is a good thing as we want a constant temperature, etc.    But faced with such certainty I’m struggling not to wobble.

 

Am I being dim?  Is a fat screed a good idea?  Are there downsides I’m not aware of?

[JohnMo]

I had plenty tell me airtight was a disaster, my house would be full of mould in 3 months. They were wrong.

 

Have they ever experienced a thick screed house in your case maybe not. First alarm bell is 

17 minutes ago, G and J said:

hate how long it takes to heat up

So they operate the system by thermostat not WC. Thick screed is glacial in response time from cold. But that is point of WC it drip feeds the floor. A well insulated house is also glacial in response so match each other well.

 

20 minutes ago, G and J said:

never get properly warm as all the heat gets eaten by the screed,

It absorbs lots of energy and that's the point in a lot of respects. It rides temperature swings pretty well. It's a low temperature heat source for the house, it self modulates well. Think screed high UFH is what they are referring to so very different.

[Bornagain]

29 minutes ago, G and J said:

I was so certain in my plan:

 

1). Reasonably airtight and insulated house not needing huge amounts of heat even in cold weather.

2). ASHP running wet UFH at a lowish temperature to be happily energy efficient.  Single zone, weather compensation for heating and cooling, no thermostat.

3). Nice lot of underfloor insulation to put the UFH pipes onto.

4). Fat, circa 100mm screed to act as a big storage radiator.

 

Lovely.  
 

Or is it?

 

In the last week or so I’ve had my ear bent by three very experienced builder type people, all telling me I’ll really regret a fat screed.  The consistency of their arguments is astounding, they are all saying that we'll hate how long it takes to heat up, it’ll be cold and it’ll never get properly warm as all the heat gets eaten by the screed, it’ll be impossible to control and if I had even half a brain I’d go for a 50mm flowscreed instead.  
 

These three are all nice guys who really do mean well and have our best interests at heart, so I’m sure that they believe what they are telling me.

 

I tell myself that I understand the physics, the heat won’t be just lost unless I fail to insulate or achieve airtightness, that slow response for our set up is a good thing as we want a constant temperature, etc.    But faced with such certainty I’m struggling not to wobble.

 

Am I being dim?  Is a fat screed a good idea?  Are there downsides I’m not aware of?

Ignore them.

 

Our house from the ground up consists of hardcore, blinding,damp proof membrane, 300mm of EPS, 100mm thick concrete slab then finally 170mm of screed which included UFH pies.

 

For all practical purposes our floor is a 270mm thick thermal store.

 

It takes a lot of energy to heat it up but it then takes ages to cool down, this allows us to heat it up using economy 7 electricity and the heat easily lasts through the day.

 

If we were building again I would do exactly the same.

 

The only downside is that it took a long time to cure and hence delays tiling at the time you are very keen to get into the house.

[nod]

Everybody has an opinion and that’s all they are Opinions 

I run a tiling business and would always prefer flo-Screed But yet I laid 75 mil semi dry Traditional screed in both of our builds and probably our next build also 

[marshian]

54 minutes ago, G and J said:

In the last week or so I’ve had my ear bent by three very experienced builder type people, all telling me I’ll really regret a fat screed.  The consistency of their arguments is astounding, they are all saying that we'll hate how long it takes to heat up, it’ll be cold and it’ll never get properly warm as all the heat gets eaten by the screed, it’ll be impossible to control and if I had even half a brain I’d go for a 50mm flowscreed instead.  

Your logic to me is totally sound - you want a nice big UFH slab with capacity to buffer temp swings outside 

if they are typically installing 50mm flowscreed in fairly modern builds with occupancy only outside working hours It would make sense - scheduled heating needs quick response in terms of warm up and a big chunk of screed is going to be much slower to respond. Perhaps that’s where they are coming from?

 

I know I’m doing low temp (currently 21 to 27 deg C depending on the OAT) with a gas boiler and rads (no UFH and ASHP) but low and slow 24/7 to meet the needs of the house (or heat loss) makes so much more sense to me and I’m using less energy than I was last year when I was running scheduled heating with 50 deg flow temps and only heating when the house was occupied - everything is at room temp.

 

last year when the heating went off the room temps rapidly fell this year I turned the heating off for 10 hours and the house lost just 1 deg C in the 10 hours

 

it will take at least that time again to recover unless I raise the WC curve but it’s still comfortable so I’ll let it sort itself out

 

 

[scottishjohn]

I maybe be out of date with boiler min temps --

but 21-27c  cannot be your boiler temp surely

self condensing biolers do not run that low --or certainly did not use to 

 please give me  more info on this set-up 

 

you heating thermal store with 50c and then regulating it from there ?

[JohnMo]

2 minutes ago, scottishjohn said:

I maybe be out of date with boiler min temps --

but 21-27c  cannot be your boiler temp surely

self condensing biolers do not run that low --or certainly did not use to 

 please give me  more info on this set-up 

 

you heating thermal store with 50c and then regulating it from there ?

Out of date by a decade or 2. Most of Europe has been running Weather Compensation or room compensation for years and the boilers sold there have to run weather compensation, so boiler flows down to 20 to 25 are mandatory. My Atag, did so once an outside sensor is attached. All Viessmann boilers do plus many more. The British boiler market is just way out of date. You need to be a combi boiler or priory hot water setup to allow low low temps for heating and high temps for DHW.

[Nick Laslett]

3 hours ago, G and J said:

In the last week or so I’ve had my ear bent by three very experienced builder type people, all telling me I’ll really regret a fat screed.  The consistency of their arguments is astounding, they are all saying that we'll hate how long it takes to heat up, it’ll be cold and it’ll never get properly warm as all the heat gets eaten by the screed, it’ll be impossible to control and if I had even half a brain I’d go for a 50mm flowscreed instead.  

@G and J, I’m actually expecting to put on my UFH tomorrow morning for the beginning of the heating season. Last week the house was still sitting at 20-21°. The ASHP will be left to do its thing and I won’t touch it till next spring. 
 

The temperature in these well insulated house is just very stable. 
 

Remember, your floor is your radiator. The heat is going into the house. You can’t lose the heat, it is impossible in a well insulated, good air tightness house. 
 

Your screed will only take a long time to heat up, if you let it get that cold. Otherwise you are just asking the ASHP to take the floor from 16° at the beginning of the heating season. Ground temp is on average 8° warmer than air temp. 
 

I will check my floor temps tomorrow before turning on the ASHP. I have 90mm upstairs and 150mm downstairs. 
 

The real challenge over the first two years is dialling in the weather comp curve. You will overshoot the ideal temperature and be opening the windows to cool the house down and be annoyed that 28° flow temp is overheating the house and you wish you had 150mm depth screed. 

Here is the temp curve chart from last week for upstairs. 
 

 

 

8am to 8pm temp change, no heating, no people, just empty rooms, 90mm concrete floor. 

 

Here is the last week. 
 

Edited yesterday at 15:26 by Nick Laslett

[Nick Laslett]

This is the chart for the 1996 rental house, with no cavity wall insulation, 6mm double glazing gap, open trickle vents. In the same village. 

 

 

Oil fired central heating on for 8 hours each day. 3 hours in morning, 5 in evening. Set to 22° on hallway controls.  
 

Look at the swings in temp, compared to the new house with no heating. 
 

In the rental house over night all the heat is losted. In the new build, the house just heats up with no input, gaining 1° in temp from solar gain. 
 

I’ve never really look at the rental house chart with much thought in the colder periods. I was much more focused on house cooling issues. The reality is that in the winter you can easily put on another layer, or fire up a heater. 
 

Anyway look at the rental house heat loss across the morning. From 6.00am to 9.00am the heating gets the house to target temp, then by 4pm it has lost nearly half of that heat. The evening heating session gets the house back to target temp, then it plummets overnight. 
 

 

Edited yesterday at 15:58 by Nick Laslett

[JohnMo]

Our is currently at a lowly 23, heat pump selected to heat, a while ago, as I didn't need cooling. Circulation pump is on, floor dropped to 20.1 overnight, but is currently at 20.8, soaking up the free sunshine. Heat pump actually ran about 5 days ago as overnight it dropped to 4 degs after a dull wet day. It just looks after itself as @Nick Laslett says.

[Nick Laslett]

43 minutes ago, Nick Laslett said:

 

 

The red section where the temp is below 20°C, I had the windows open to try and cool the house down. 

[Nickfromwales]

4 hours ago, G and J said:

I was so certain in my plan:

 

1). Reasonably airtight and insulated house not needing huge amounts of heat even in cold weather.

2). ASHP running wet UFH at a lowish temperature to be happily energy efficient.  Single zone, weather compensation for heating and cooling, no thermostat.

3). Nice lot of underfloor insulation to put the UFH pipes onto.

4). Fat, circa 100mm screed to act as a big storage radiator.

 

Lovely.  
 

Or is it?

 

In the last week or so I’ve had my ear bent by three very experienced builder type people, all telling me I’ll really regret a fat screed.  The consistency of their arguments is astounding, they are all saying that we'll hate how long it takes to heat up, it’ll be cold and it’ll never get properly warm as all the heat gets eaten by the screed, it’ll be impossible to control and if I had even half a brain I’d go for a 50mm flowscreed instead.  
 

These three are all nice guys who really do mean well and have our best interests at heart, so I’m sure that they believe what they are telling me.

 

I tell myself that I understand the physics, the heat won’t be just lost unless I fail to insulate or achieve airtightness, that slow response for our set up is a good thing as we want a constant temperature, etc.    But faced with such certainty I’m struggling not to wobble.

 

Am I being dim?  Is a fat screed a good idea?  Are there downsides I’m not aware of?

They’re simply stating a knee jerk, uninformed, uneducated “builders” response.

 

Stick to your guns and when built you can invite them around long after they’ve put their CH on, and you’ve not yet had to .

 

Then they’ll soon revert to praising your methodology and saying “it’s the future” lol. 😜 

[saveasteading]

4 hours ago, G and J said:

 Is a fat screed a good idea?

Minuses It costs more / the headroom may be significant / it takes longer to heat up if it has been off.

 

Pluses, it allows for more certainty of thickness on an uneven layer of PIR (which seems pretty much the standard), 

It will hold and release heat after the supply is off.

The heat store principle may allow the exploitation of cheaper power from solar, or overnight rates.

 

But it seems that, in a new house with ashp and ufh most people will turn the system on and leave it on.

 

Me? I'd go for 60mm on an accurate insulated layer. 

Your average builder may prefer to spend your money on thicker screed to overcome their tolerances.

 

 

 

 

[marshian]

1 hour ago, scottishjohn said:

I maybe be out of date with boiler min temps --

but 21-27c  cannot be your boiler temp surely

self condensing biolers do not run that low --or certainly did not use to 

 please give me  more info on this set-up 

 

you heating thermal store with 50c and then regulating it from there ?

 

No that’s the flow temps the boiler is running based on my WC curve - No thermal store, no volumiser or buffer just 135 litres of water spread across 13 rads in the CH circuit. Flow rate is 8.5 Litres per min.

 

example below

 

 

only time it goes to conventional gas boiler temps is when it’s recharging the HW tank then it ramps up to 68 Deg C says “to hell with condensing efficiency - lets get back to space heating”

 

Viessmann boiler 100W “heat only” 16kW with WC and HW demand box

 

I’m basically running it at ASHP temps for CH 

[SteamyTea]

4 hours ago, G and J said:

three very experienced builder type people

You have, in a way, answered your own question.

 

For a giggle, asked them about specific and volumetric heat capacity of air, water and concrete.

[JohnMo]

A 100m² floor at 100mm thick holds about 5 to 5.5kWh energy for each Deg you raise it's temp. If the floor is warmer than the room this energy will pass into the room at a ratio of the room to floor dT.

 

Run a 6kW heat pump for 4 hours, it will raise the floor temperature by just over 4 degs (100m2). It makes heat pump selection quite fluid, as long as you have around 20L of engaged water per min output kW of heat source you won't have any cycling issues. The floor is just a massive thermal store.

 

You just need to brave don't listen to the people that have never experienced a thick screed. Plenty of people on here have and wouldn't change it.

[Oz07]

6 hours ago, Bornagain said:

Ignore them.

 

Our house from the ground up consists of hardcore, blinding,damp proof membrane, 300mm of EPS, 100mm thick concrete slab then finally 170mm of screed which included UFH pies.

 

For all practical purposes our floor is a 270mm thick thermal store.

 

It takes a lot of energy to heat it up but it then takes ages to cool down, this allows us to heat it up using economy 7 electricity and the heat easily lasts through the day.

 

If we were building again I would do exactly the same.

 

The only downside is that it took a long time to cure and hence delays tiling at the time you are very keen to get into the house.

Why 170mm on top of 100mm slab. That just seems crazy to me, sure there is a reason?

Why don't people just put the ufh in the floor slab and have no screed with self compacting concrete? Do you have to increase the thickness to allow for the pipes interrupting the continuity?

[Oz07]

2 hours ago, Nickfromwales said:

They’re simply stating a knee jerk, uninformed, uneducated “builders” response.

 

Stick to your guns and when built you can invite them around long after they’ve put their CH on, and you’ve not yet had to .

 

Then they’ll soon revert to praising your methodology and saying “it’s the future” lol. 😜 

Im sure you're right but I think there's a few to many physics / heating boffins here who don't realise how clever they are. Us layman just want our boiling hot rads and open windows!

[JohnMo]

33 minutes ago, Oz07 said:

Im sure you're right but I think there's a few to many physics / heating boffins here who don't realise how clever they are. Us layman just want our boiling hot rads and open windows!

And a house full of the thermostats and short cycling boiler, that's using twice the energy it needs to. Then complain about the price of gas, while having a pint, down the pub. The way things used to be before I built a house.

[Bornagain]

42 minutes ago, Oz07 said:

Why 170mm on top of 100mm slab. That just seems crazy to me, sure there is a reason?

Why don't people just put the ufh in the floor slab and have no screed with self compacting concrete? Do you have to increase the thickness to allow for the pipes interrupting the continuity?

1) The intention was to have 100mm of screed, but complications with ceiling heights, door & window heights meant it was simpler to just have a thicker screed.

 

2) We did our build in 2011 and at that time the typical approach seemed to be to have the UFH pipes in the screed, so that’s what we did.

 

[marshian]

27 minutes ago, JohnMo said:

And a house full of the thermostats and short cycling boiler, that's using twice the energy it needs to. Then complain about the price of gas, while having a pint, down the pub. The way things used to be before I built a house.

 

Oh how well I remember those days - as if it was yesterday (was actually just 14 mths ago)

 

Full Wiser system with smart TRV's on every rad

Micromanaging schedules to individual room temps to control heat loss

Boiler had a 10kW min output and wouldn't cope with a flow temp below 45

Short cycled like a complete twat at anything below 50 Deg C flow temp (My record was 19 cycles in 1 hour)

When short cycling it chewed thro gas putting pretty much zero heat into the house and was fundamentally puking the heat out of the flue

House was either too hot after too long or too cold after spending way too long waiting for it to warm up

Ahhh happy days...............

 

 

 

 

 

 

 

 

PS They bloody weren't!!!!

[G and J]

Thank you guys, I was sure of my physics but we all have a wobble at times so it left me wondering what I hadn’t factored in. 
 

The power of bog standard is huge, it appears, and bog standard in Blighty for newish houses is a well insulated wind tunnel, methinks, and perhaps for such builds thin screeds have advantages.  

 

So if it works well I’ll tell everyone how clever I am.  And if it doesn’t I’ll blame buildhub.  Simples.  🙂 

 

(Neither statement in previous paragraph is true, but it made me smile.)

[Nickfromwales]

34 minutes ago, G and J said:

Thank you guys, I was sure of my physics but we all have a wobble at times so it left me wondering what I hadn’t factored in. 
 

The power of bog standard is huge, it appears, and bog standard in Blighty for newish houses is a well insulated wind tunnel, methinks, and perhaps for such builds thin screeds have advantages.  

 

So if it works well I’ll tell everyone how clever I am.  And if it doesn’t I’ll blame buildhub.  Simples.  🙂 

 

(Neither statement in previous paragraph is true, but it made me smile.)

Our pleasure, lol. 

[Nick Laslett]

Temperature updates. 
 

Upstairs room temp is currently 18.5°C

Upstairs floor temp is currently 15°C

 

Downstairs room temp is currently 19.9°C

Downstairs floor temp is currently 15°C

 

ASHP Water temp at Outlet & Inlet 17°C

ASHP Outside Unit Temp 18°C (It is a very sunny day today)

 

MET Office says temp is 17°C

 

It is still a bit too warm in the new build to put on the ASHP. The house will probably gain another degree across the day as it is so sunny and there is no wind.
 

We do not have especially high U-values, only double glazed. External doors are only 1.3 U-value. We are an ICF build. We don’t’ have our ACPH score yet, but hope to get below 1.0.

 

The point being, imho that the thickness of your UFH substrate is irrelevant with a well insulated, low ACPH house. 
 

I will try to update with the heat up time when I finally switch on the ASHP. 

Edited 9 hours ago by Nick Laslett

[saveasteading]

20 hours ago, Nick Laslett said:

where the temp is below 20°C, I had the windows open to try and cool the house down. 

20 really isn't hot.  Dress differently, close blinds or curtains. 

In the Mediterranean, people cope with much higher than that.

Commercial buildings in spain are not permitted to use cooling until 27°C.

I think we can adjust too.

I think I took this pic in a bank.