Pro's and cons of a heat pump

Hi Guys

A follow up on my sons house if that's ok. I know not strictly electrical but I value the amazing input on what seems almost any subject on this forum from some very knowledgeable experienced contributors.

It looks like we are going down the heat pump route. I will discuss this with the heating guys once we find someone as well.

We are hoping to be able to install under floor heating. I have lifted a few of the ground floor floorboards. There is approx a 2 foot void below.  The floor joists are only 4x2 suspended on dwarf walls.  

I am proposing to make a cradle below the joists by packing them down with maybe a 1 inch batten then counter battening across them below this so I can install 100mm cellotex between the joists supported by the battens leaving a 1 inch gap between top of cellotex and underside of finished floorboards which will probably be chipboard. Then run heating pipes in this gap. 

Looking on Youtube it appears to say fit aluminium plates or screed around the pipes to help spread the heat. I am a bit worried about the weight of screed on the floor joists although I could add extra support for them. It would be very thin and probably crack so don't really know how effective that would be. 

I also don't know if 100mm cellatex is thick enough to do we are trying to achieve.

I hadn't considered carpet underlay either until a builder friend mentioned it tonight but know we need something that lets the heat through rather than keeps the cold out will be better I think.

If anyone on here has any experience on any of the above, thinks it a crackpot idea  or has better ideas it would be appreciated. 

Many thanks

Gary

Hi Gary,

Firstly, I am not an expert in this area, so please do not consider anything I say to be advice ;-)

Not sure if it would be of any help to you, but Heatpunk allowed me to carry out a heat loss exercise on my place to see if my radiators are suitable for use at heat pump flow temperatures (I'm hoping this means I can now drop my flow temperature to about 50°C this winter). It's not a trivial exercise to use it, but it is very comprehensive and appears to give results that tally with my current gas use reasonably well. Best of all, it's free to use for non-commercial purposes.

Have you considered installing a moisture barrier? You don't want to end up with condensation / damp building up and causing the joists to rot. Having heat in there will obviously help.

keylevel said:

Have you considered installing a moisture barrier? You don't want to end up with condensation / damp building up and causing the joists to rot.

100%. Often, older houses have poor, or no sealing of the 'solum' (ground on which the building stands). My house had rot and damp under the floor boards because there was no solum seal.

We often forget the themal radiation from above that helps the ground moisture vaporise and then re-condenses onto the underside of the flooring. A simple plastic sheet, with 75mm turn ups, is sufficient to stop most of that ground water vapour.

You don't need the 50mm of concrete on top that many suggest. That's just the new build, belt & braces, fixes many potential problems approach.

The plastic sheet can be secured by nice boulders, bricks or small bags of concrete sand (i.e. to-hand weighting). You still need the air brick to be clear, and sleeved (minimal airflow up the cavity, not a gale!). Cold air hold very little actual moisture, even at 100%RH (RH quickly falls as the air warms!)

Thank you for this.  Are we saying a plastic sheet on the solid floor that that dwarf walls are sat on. Just weighed down to stop it lifting basically. There are plenty of air bricks from what I can see.  I will obviously seal around all the edges of the cellotex as best I can to prevent cold air coming up from below to prevent condensation as well. Making sure the air bricks can do their job. I haven't checked the height of them yet but know in my own house they were at joist height rather than below them.

Gary

aligarjon said:

I am proposing to make a cradle below the joists by packing them down with maybe a 1 inch batten then counter battening across them below this so I can install 100mm cellotex between the joists supported by the battens leaving a 1 inch gap between top of cellotex and underside of finished floorboards which will probably be chipboard. Then run heating pipes in this gap. 

That's pretty much what I did (only variation us that I have a cellar below the suspended timber floors, hence 7" joists and access from below). I found my (100+ year old) joists had twisted quite significantly in places, so getting a snug fit the the Celotex wasn't going to happen with a simple friction fit, so I cut the 150mm sheets about 20mm too narrow, then filled the 10mm-ish gaps between sheet and joist (and between sheets) with expanding foam (replace the short "straw" the foam cans come with with a 400mm-ish length of flexible plastic tube to make it easy to inject in any direction, including upwards, while keeping the can upside down - just make sure the tube is securely attached ... it does make an awful mess if it pops off at the wrong time (been there, done that, ruined the t-shirt). 

Mostly I just clipped the 15mm pipe to the side of the joists - in a few places I did add some aluminium heat "spreader plates" (in the front room where the outside wall area was large compared with the floor area, and the bathroom where perhaps higher room temperatures would be nice) - and it all seems to work very well. Whether the spreader plates made a significant difference or not I can't tell, but they certainly seemed to do no harm.

I would strongly suggest doing your heat loss calculations though - output from UFH in a time floor is limited (from memory from a long time ago from a back of an envelope calculation, I think I reckoned on about 30W per m2 (without spreader plates). That works fine for me but I have about 150mm of celotex in the walls and similar under the roof, together with good air-tightness and heat recovery ventilation ... so your experience may vary as they say.

I've not noticed any condensation problems even with a pretty stubbornly damp cellar below - I think in most cases where condensation would be likely the floor would be the warmest surface (even insulated, some heat still leaks downwards), so it tends to condense elsewhere.

   - Andy.

aligarjon said:

I am proposing to make a cradle below the joists by packing them down with maybe a 1 inch batten then counter battening across them below this so I can install 100mm cellotex between the joists supported by the battens leaving a 1 inch gap between top of cellotex and underside of finished floorboards which will probably be chipboard. Then run heating pipes in this gap. 

That's pretty much what I did (only variation us that I have a cellar below the suspended timber floors, hence 7" joists and access from below). I found my (100+ year old) joists had twisted quite significantly in places, so getting a snug fit the the Celotex wasn't going to happen with a simple friction fit, so I cut the 150mm sheets about 20mm too narrow, then filled the 10mm-ish gaps between sheet and joist (and between sheets) with expanding foam (replace the short "straw" the foam cans come with with a 400mm-ish length of flexible plastic tube to make it easy to inject in any direction, including upwards, while keeping the can upside down - just make sure the tube is securely attached ... it does make an awful mess if it pops off at the wrong time (been there, done that, ruined the t-shirt). 

Mostly I just clipped the 15mm pipe to the side of the joists - in a few places I did add some aluminium heat "spreader plates" (in the front room where the outside wall area was large compared with the floor area, and the bathroom where perhaps higher room temperatures would be nice) - and it all seems to work very well. Whether the spreader plates made a significant difference or not I can't tell, but they certainly seemed to do no harm.

I would strongly suggest doing your heat loss calculations though - output from UFH in a time floor is limited (from memory from a long time ago from a back of an envelope calculation, I think I reckoned on about 30W per m2 (without spreader plates). That works fine for me but I have about 150mm of celotex in the walls and similar under the roof, together with good air-tightness and heat recovery ventilation ... so your experience may vary as they say.

I've not noticed any condensation problems even with a pretty stubbornly damp cellar below - I think in most cases where condensation would be likely the floor would be the warmest surface (even insulated, some heat still leaks downwards), so it tends to condense elsewhere.

   - Andy.