Late addition of UFH - have I the depth?

[EdSt]

Doing a kitchen extension and due to our lifestyle (both out at work all day and require warm house quickly of an evening) I’d ruled out UFH at planning stage. Partly also because the existing house space which we’ll break through to wouldn’t match as just has radiators. 
 

(current heating system is oil boiler which I’ve confirmed can run UFH as well as rest of house heating system) 

Was talking to bricky the other day and he got me thinking about it on basis that it’s relatively cheap to put in, gives a nice heat and wouldn’t matter about existing areas as air temp would balance and be nicer ambient temp. 
 

Different levels out the back of the house mean the extension is in block and beam (B&B) construction and block work shell is now up… I.e my levels are set. Current drawings/plan is sand blind on B&B, 125mm PIR, 100mm Concrete slab, 5-8mm self level, 8mm tile on 8mm bed. 
 

Question. Could I do sand blind, 150mm Kooltherm/PIR then 75mm Screed (instead of conc) as a structural floor finish before self level and tiles? 
 

Thanks in advance !

 

[crispy_wafer]

Seems a pretty routine buildup that many have used and incorporated UFH in before.

[Russell griffiths]

You don’t need sand blinding on beam n block. Why would you. 
you also need to get away from the thoughts of you have a cold house that you heat up quickly. 
forget that, you have a warm house that is warm all day, you don’t let it cool down and then blast it with heat to get it warm. 
make it comfortable and keep it like that. 
if you can’t get your head around that, or you think the house won’t work like that then put in radiators if you want a sudden burst of heat into the room. 
a 75mm screed acts like a big heat emitter, it releases heat nice and steady, you can’t ask it to suddenly chuck loads of heat into the room. 

[EdSt]

12 hours ago, crispy_wafer said:

Seems a pretty routine buildup that many have used and incorporated UFH in before.

Cool. Thanks for that. 

[EdSt]

9 hours ago, Russell griffiths said:

You don’t need sand blinding on beam n block. Why would you. 
you also need to get away from the thoughts of you have a cold house that you heat up quickly. 
forget that, you have a warm house that is warm all day, you don’t let it cool down and then blast it with heat to get it warm. 
make it comfortable and keep it like that. 
if you can’t get your head around that, or you think the house won’t work like that then put in radiators if you want a sudden burst of heat into the room. 
a 75mm screed acts like a big heat emitter, it releases heat nice and steady, you can’t ask it to suddenly chuck loads of heat into the room. 

 

[EdSt]

Yeah, thanks I knew that already but you‘ve conveyed it well, it’s a mentality shift. 👍. 

Edited December 6 by EdSt

[bmj1]

You've always got the option of swapping sand/cement screed (75mm) for liquid screed (40mm) when dealing with a tight buildup.

 

It's a much better product also (higher conductivity)

[G and J]

What are the downsides of a liquid screed?  Cost? Cracking?

[bmj1]

1 hour ago, G and J said:

What are the downsides of a liquid screed?  Cost? Cracking?

 

You'll need a specialist installer for it. Likely to be a little more expensive.

 

We used cemfloor, if you call them they can point you to a couple of local specialist installers. I would suggest to get a couple of quotes and go from there.

Edited December 8 by bmj1

[G and J]

1 hour ago, bmj1 said:

 

You'll need a specialist installer for it. Likely to be a little more expensive.

 

We used cemfloor, if you call them they can point you to a couple of local specialist installers. I would suggest to get a couple of quotes and go from there.

Good plan. If the quotes come in ok what’s the upsides of a liquid screed if one does have the depth for either?

[bmj1]

8 minutes ago, G and J said:

Good plan. If the quotes come in ok what’s the upsides of a liquid screed if one does have the depth for either?

 3x the conductivity means warmup in 15-20 mins not 2 hours

 

Will be cheaper to run as less heat loss below.

[G and J]

3 minutes ago, bmj1 said:

 3x the conductivity means warmup in 15-20 mins not 2 hours

 

Will be cheaper to run as less heat loss below.

I shall try and look up the specific heat capacity.  I like the idea of heating during bursts of cheap rate lecky, so that’s also important to me.  And thank you for the info.

[JohnMo]

2 hours ago, bmj1 said:

It's a much better product also (higher conductivity)

Read product websites with caution. Sand cement screed, isn't the same as concrete, the thermal conductivity changes dramatically with material make up. Same as some screeds are insulation. Big range of thermal conductivity in most material.

 

Various grades of concrete for example.

 

 

 

[JohnMo]

28 minutes ago, bmj1 said:

3x the conductivity means warmup in 15-20 mins not 2 hours

 

Will be cheaper to run as less heat loss below.

Thin screed of almost any type is fast warm-up. It also cools quicker as there is less heat capacity.

 

You going to have to explain the less heat loss below bit? Not sure that makes much sense.

[bmj1]

6 hours ago, JohnMo said:

Thin screed of almost any type is fast warm-up. It also cools quicker as there is less heat capacity.

 

You going to have to explain the less heat loss below bit? Not sure that makes much sense.

Sure. Heat doesn't rise when moving between solids.

 

So it will try and move both up into the house and down into the ground. 

 

The higher thermal conductivity of a liquid screed means much more heat captured/transmitted upwards into the house, instead of lost below.

[bmj1]

6 hours ago, JohnMo said:

Thin screed of almost any type is fast warm-up. It also cools quicker as there is less heat capacity.

 

You going to have to explain the less heat loss below bit? Not sure that makes much sense.

 

It's a double benefit. Screed is half the thickness and 3x the conductivity. 

 

So 6x faster warm up

[JohnMo]

3 hours ago, bmj1 said:

 

It's a double benefit. Screed is half the thickness and 3x the conductivity. 

 

So 6x faster warm up

Who really cares about fast warm ups, prefer a steady and stable temperature, which dips a little at night. Dumped radiators to get away from swings in temperature.

 

If you need fast warm up stay with radiator or install fan coils which are faster again.

[G and J]

11 hours ago, JohnMo said:

Read product websites with caution. Sand cement screed, isn't the same as concrete, the thermal conductivity changes dramatically with material make up. Same as some screeds are insulation. Big range of thermal conductivity in most material.

 

Various grades of concrete for example.

 

 

 

Crumbs.  How do I ensure I get a suitable material for my downstairs floor/UFH then?

 

All other things being equal-ish, given the choice I’d go for a material with a greater specific heat, accepting slowness of response to heat input in return for stable room temperature (and by implication floor temperature).

[JohnMo]

27 minutes ago, G and J said:

Crumbs.  How do I ensure I get a suitable material for my downstairs floor/UFH then?

 

All other things being equal-ish, given the choice I’d go for a material with a greater specific heat, accepting slowness of response to heat input in return for stable room temperature (and by implication floor temperature).

First is depth of screed. Then choose a screed. Look at product datasheets

 

Here is a bunch of different materials, some useful most not so.

 

https://help.iesve.com/ve2021/table_6_thermal_conductivity__specific_heat_capacity_and_density.htm

[ringi]

On 08/12/2024 at 21:42, JohnMo said:

Who really cares about fast warm ups, prefer a steady and stable temperature, which dips a little at night. Dumped radiators to get away from swings in temperature.

 

Weather compensation can have issue if it takes much more then say an hour for the surface floor temperature to respond to a change in flow temperature.

[ringi]

On 08/12/2024 at 10:55, bmj1 said:

 3x the conductivity means warmup in 15-20 mins not 2 hours

 

Will be cheaper to run as less heat loss below.

 

Also less difference between floor surface temperature and flow temperature so a better COP if heatpump (assuming no radiators) .

[JohnMo]

8 hours ago, ringi said:

Weather compensation can have issue if it takes much more then say an hour

That assumes your house almost no decrement delay. So you need to be careful with assumptions. In a well insulated house new or old house your weather compensation curve almost flat. Weather doesn't really change much in a hour, maybe drop or increase a couple of degrees.

 

Plus the output of a floor modulates (thick or thin screed) depending on the dT between floor and room. So when room temperature drops floor output increases.

 

You design your system for what you want, it's flexible, there are advantage and disadvantages to every system. No right or wrong answer. 

 

So testing done with our 100mm thick screed

 

I have used pure WC and not had an issue, except expensive electricity outside ToU cheap periods.  Batch charged the floor overnight and that works fine down to a just below zero (7 hrs charging and rest of the keeping house pretty stable), below that it needs a top up early evening. Running against a thermostat works, but you can overshoot and undershoot a lot with cheap thermostats and higher than ideal flow temperature. With thick screed you have to use 0.1 hysterisis thermostat.

 

Certainly with a thick screed you have no need for a buffer as the floor is a huge one.