Ideal screed would be semi dry ready mix screed with fibres well compacted and laid perfectly.

Simple,cheap, easy to repair, durable, non brittle, good feel underfoot, fibres help it stop cracking, goodly thermal mass, the list goes on

High tech -- horrid, plasticky, impossible to take up, expensive opps no sorry very expensive, specialist to lay, poor feel,brittle?

I ve never been able to afford high tech screed systems but have seen them elsewhere. Get prices then decide.

Sand cement screed is slightly flexible with fibres, It is nice to be able to get screed up without trashing all the pipes -- not possible with high tech -- even more crucial for repairs etc

under a laminate there will be little difference

Usually high tech screed goes in a lot thinner and so tends to feel hollow.

Posted By: PikeyWe will be having underfloor heating in our new bungalow, the pipes will be set in a 75mm thick screed over 150mm polystyrene insulation on a concrete slab. It will be run by an Air Source Heat Pump.

How are you sizing the heating system & the air source heat pump? If the house is not being left unoccupied for long periods, it makes sense to run the air source heat pump continuously or to use a thermal store & the thermal mass in the house to run the ASHP only during Economy 7 periods.

Posted By: PikeyIn particular, will one or the other be more responsive or make the system run more efficiently?

Putting the insulation below the slab & putting the underfloor heating pipes in the slab will greatly increase response times, but will allow you to make best use of the ASHP. It will also avoid the need for a screed, so the answer to your question would be "no screed at all".

David

I'm considering easy-screed from Ready-mix with fibres. Seems to be midway between hi and lo tech. Was put off anhydrite a bit by need to seal/prime the surface before tiling. I see thinner as an advantage, less material and less time to dry out. However reading Tony's comments re semi-dry sand and cement maybe that would (as name implies) dry out faster although it is thicker. Metal or poly fibres in the mix reduce cracking and aid heat distribution from UFH I'm told.

Posted By: PikeyAt the moment, we both work normal full time office jobs and have no children, so if we had a typical central heating system we'd have it on for an hour in the morning and then again for a few hours in the evening.

I would ask the heat pump supplier to estimate the warm-up times with different screeds. You're unlikely to have as much excess power available as a gas/oil boiler might provide. This combined with the heat capacity of the screed may lead to excessive warm-up times. You'll also need to be careful with your floor coverings.

What U values & airtightness are you aiming for?

David

With those U values you should be OK with an ASHP. I would go for more insulation in the roof, but that's going off-topic.

If I wanted to use intermittent heating then my main concern would be warm-up times. If the only issue is the thermal mass of the screed, you can always compensate by setting the heating to turn-on a couple of hours before you get home & turn-off a couple of hours before you go to bed. However, combine this with the wrong floor covering or an ASHP which has marginal capacity & the whole thing is likely to become unmanageable.

Has the ASHP been selected? Could you give us the model number, power rating & the surface area of the floor to be screeded? Will the ASHP heat any other areas? Will it also heat domestic hot water? How is the system to be managed/zoned? Have the heat pump company suggested a thermal store?

David

Its a 5kW Mitsubishi EcoDan unit, floor area is 60m2, it will also do the hot water yes. I dont think we've got room for a thermal store.

Very grateful for help so far

Posted By: PikeyIts a 5kW Mitsubishi EcoDan unit, floor area is 60m2, it will also do the hot water yes.

On the information available it's difficult to estimate the typical temperature drop while you're at work during the day. If we assume the temperature drops 10 degrees in mid-winter then taking screed data from the following site:

http://www.ecoscreed.co.uk/

An 50mm anhydrite screed will take around 2.35 hours to get back-up to temperature:

Floor area 60 m2
Screed thickness 50 mm
Screed mass 5550 kg
Screed specific heat capacity 762 J / kg.K
Delta T 10°C
Power 5000W
Warm-up time 2.35h

A 65mm sand & cement screed will take around 4.72 hours:

Floor area 60 m2
Screed thickness 65 mm
Screed mass 8580 kg
Screed specific heat capacity 991 J / kg.K
Delta T 10°C
Power 5000W
Warm-up time 4.72h

Remember this is just the time taken to get the screed back-up to temperature after a 10°C drop, it ignores the heat capacity of the room, heat lost during the heating process, the affect of any floor coverings, power lost due to the ASHP COP dropping in cold weather, power lost to radiators upstairs & power lost to domestic hot water heating. So the time to get the room air up to temperature will be considerably longer.

I would recommend using the anhydrite screed (with an appropriate separation membrane if tiling) or switching to 24 hour heating.

David

Posted By: Dantenz

No, it's not simple at all, and seeing as you pretend to know so much about it why ask a heat pump manufacturer to determine the heat up time for various screed types..it's not their area of expertise neither is it in their interest to imdemnify such a prediction.

Datenz, I found your response dismissive, unconstructive & rather insulting. I am not "pretending to know so much about it", I am merely sharing my time & my knowledge of 'O' level Physics in trying to help Jeff sort through the conflicting information he is receiving.

Jeff stated that the "company supplying our heat pump has sized it for us". The screed forms part of the heating system in the same way that a radiator would, so any attempt to size the system should take this into account.

There are too many ASHPs being installed today in inappropriate situations. The whole heating system needs to be designed together with the building fabric if we are to see the benefits. It makes sense for the company sizing the system to do this, but as I said earlier "If they're not willing to do it then I would recommend finding a heating services engineer who is willing to do it".

I am not saying that the heat pump company should indemnify the screed choice, but if they ignore its affect in dimensioning the system then Jeff should get professional advice from a heating services engineer.

David

Appreciate your calcs by the way

So running it 24/7 will cost less than having a faster responding system and turning it off/down while we're at work/sleeping?

Posted By: PikeySo running it 24/7 will cost less than having a faster responding system and turning it off/down while we're at work/sleeping?

Intermittent heating will save money overall because the amount of heat lost through the floor, walls & roof is proportional to the temperature difference. If the temperature in the room is lower then less heat will be lost. Running it 24x7 will always cost more, but given the response times, it may the only practical way to maintain the desired temperature when the house is occupied.

As Tony points out, when intermittent heating is turned-on the 2-4 hour delay before the screed reaches temperature can lead to the room temperature 'over-shooting' the desired temperature. This affect can be reduced or eliminated by using programmable thermostats which more subtley vary the temperature versus time, by embedding thermostats in the screed &/or by using weather compensation. Weather compensation varies the flow temperature from the ASHP, so that in warmer weather the flow temperature is lower, reducing the risk of over-shoot or over-heating.

Talk to the heat pump supplier about the control options. As a minimum he should be able to provide a programmable thermostat which allows the temperature to be set-back, to say 16°C, while you're asleep or at work, reducing the time taken to re-heat & reducing the risk of over-shoot.

David

Dont you want a screed to be a good thermal conductor though? Im not sure the reasoning and the maths on that Ecoscreed website is entirely sound!

Or maybe I dont fully understand

Posted By: DantenzPlease accept my apologies David, It was wrong of me to say what I did. I just get rather frustrated some times at what might seem to be by some, as a perfectly resonable simple question and yet the question is almost impossible to answer. I say this because I am often at the receiving end of such questions where the answers are not always black or white or even determinable.

No problem, I can understand the frustration. We need to find better ways of designing houses & heating systems which give people more certainty over finished performance. We can't load the whole problem on the builder or on the individual suppliers. In electronics we have the concept of a systems architect or systems engineer who is responsible for making sure the entire system of component parts performs as expected. I think we need something similar in building to ensure building services & building fabric perform together as expected. Following the Passivhaus approach is one way of achieving this, but this doesn't help if you're just having a heat pump fitted.

David