Most efficient way to run underfloor heating

dllive

I should also add - it will take longer to heat a tank from cold than to have it constantly heated. (I think thats right isnt it matelodave ?). Thats the point of an ASHP - keep it always ticking-over.

matelodave

We only heat our hot water for a two hours a day - between 04:30 and 06:30 to 45degrees - its a 200l hot water tank and give us all the hot water we need. There's a sterilising function that switches on once a week to 60. IMO it doesn't really need it and certainly not that often but our controller will only do it once a week - once a month is probably more than sufficient, especially if you've got a non-vented hot water tank.

The heating is on 24hours a day (well the heat pump is) we have weather compensation set so that flow is around 42 degrees when it -5 ouside and 25 degrees flow when it's +20. I've also set the weather compensation offset to uplift by 5 dgrees between 0700 and 0900 to boost the reheat time a bit first thing in the morning. 

The programmable room stats are set to 19 dgrees during the day, lounge to 20 in the evening and everywhere has a set back to 18 degrees (except in the hall and spare room where its 17). The study is set to 20 degrees because that's where my wife does her thing most days and whinges - she has a digital thermometer in there and moans if it drops below 19.5.

Because we have underfloor heating and a low flow temperature the response time is very slow (about 1 degree in four hours)so we dont let the place get cold. It helps that we are at home all day virtually every day. If we go on holiday, we set the room stats back to around 14 degrees but use the holiday mode to restore the stat settings the day before we return, otherwise it can take 24-36 hour to reheat the place. It took the first winter to get it about right and then a couple of minor tweaks and now it just gets on with it.

IMO once you are reasonably comfortable with tweaking the settings then just try adjusting settings to see what affect it has (both on your comfort levels and in terms of power consumption). Only change one thing at a time, do small tweaks and give it a chance (a couple of days) to settle down to see if it was better or worse and keep a log of what you did and what the results were (dont forget to take account of the outside temp as well as it can have a quite dramatic effect on power consumption).

They really arent all that complicated but it does help if you can understand that they dont work like a normal boiler and they are usually a lot more controllable than a normal boiler with just on/off and a temperature adjustment.

seagull74

Just joined to de-lurk and thank all contributors for the info in this thread. Just moved into a new build with an Ecodan, no manual and was lost. Have a much better idea now. Will look at the videos and see if Ican tweak the settings.

Reed_Richards

I have not read every single post and this question may be answered somewhere in the middle but why is underfloor heating so slow to raise the room temperature?  Underfloor heating does the job eventually so it can't be lack of output, or at least lack of output in the steady state when the room is at the desired temperature.  The only explanation I can think of is that the floor has a very high thermal mass.  Is that it?   

shinytop

Isn't it because the water temp/delta T is lower?  The same way a boiler heats the house up quicker than a ASHP with radiators. If the floor is well insulated, it shouldn't be heating up should it?

Reed_Richards

shinytop said:

Isn't it because the water temp/delta T is lower?  The same way a boiler heats the house up quicker than a ASHP with radiators. If the floor is well insulated, it shouldn't be heating up should it?

Okay, first point.  A boiler DOES NOT heat a house with radiators up quicker than an ASHP except insofar as the boiler may be a bit quicker to heat the water in the central heating system up to temperature (I did a before and after calculation of that factor for my own system here:  https://forums.moneysavingexpert.com/discussion/6240076/i-bought-a-heat-pump/p6 ).  The reason it is not true is because you fit radiators to match the delta T that your heat source provides.  For a modern gas or oil boiler that would be a delta T of 50 C.  For a heat pump delta T would probably be about 25 C for a radiator.  

So suppose you have a room that needs a 500 W radiator.  if you have an oil boiler, as I did, you will get a radiator that gives you 500 W at delta T = 50 C (i.e. 75 C flow and 65 C return for a room temperature of 20 C).  If, as I did, you replace it with a heat pump you rip out that radiator and replace it with one that gives you 500 W for a 50 C flow and a 40 C return, as I did.  Watts are a rate of energy supply, Joules per second.  There is no such thing as a fast 500 W and a slow 500 W; both 500 W radiators will heat the room at the same rate.  Sorry about the rant, @shinytop, but this is a popular enough misconception with people who don't own an ASHP.

Second point, of course the floor heats up.  You cannot put your hand on the floor and feel the hot areas where the water pipes/tubes are beneath and the cold areas in between, you warm up the whole floor and that radiates heat from its entire (large) surface area.  Sure, you insulate as much as possible underneath the heating elements but you still have to heat the surrounding material and I guess this must have a large thermal mass and therefore take a long time.          

matelodave

Generally underfloor systems have a much lower temperature than radiators, usually only around 27-30 degrees so the delta T is only 7-10 degrees. Add to that many of them are embedded in the concrete slab which will take several hours to get up to temperature - one of the reasons you dont keep turning u/f heating on & off. Although my overlay system takes some time to heat.

Most people also underestimate the effect of floor coverings, tiles are pretty good at transferring the heat but wood, laminate and carpet are insulators and will slow it down even more (wood, laminate an vinyl gennerally have a limit of around 27-28 degrees). Even low tog carpet and underlay will have quite an effect. You also have to take into account the amount of furniture that you put into the room after all the calculations have been made - a big three piece suite, sideboard, wardrobes and double bed etc will all cover some of the heating area and reduce the output.

The lower deltaT will limit the amount of heat that the floor will dissipate so it's usually only around 50-75w/m2 possibly up to 100w/m2 where there are tiled floors and less if you run at really low flow temps or have thick carpet.

Based on a dissipation of 100w/m2 a 20m2 room may only get around 2kw = 48kwh a day whereas a 3kw radiator would deliver 48kwh in 16 hours and 2 x 2kw rads would manage it in 12.

shinytop

Reed_Richards said:

shinytop said:

Isn't it because the water temp/delta T is lower?  The same way a boiler heats the house up quicker than a ASHP with radiators. If the floor is well insulated, it shouldn't be heating up should it?

Okay, first point.  A boiler DOES NOT heat a house with radiators up quicker than an ASHP except insofar as the boiler may be a bit quicker to heat the water in the central heating system up to temperature (I did a before and after calculation of that factor for my own system here:  https://forums.moneysavingexpert.com/discussion/6240076/i-bought-a-heat-pump/p6 ).  The reason it is not true is because you fit radiators to match the delta T that your heat source provides.  For a modern gas or oil boiler that would be a delta T of 50 C.  For a heat pump delta T would probably be about 25 C for a radiator.  

So suppose you have a room that needs a 500 W radiator.  if you have an oil boiler, as I did, you will get a radiator that gives you 500 W at delta T = 50 C (i.e. 75 C flow and 65 C return for a room temperature of 20 C).  If, as I did, you replace it with a heat pump you rip out that radiator and replace it with one that gives you 500 W for a 50 C flow and a 40 C return, as I did.  Watts are a rate of energy supply, Joules per second.  There is no such thing as a fast 500 W and a slow 500 W; both 500 W radiators will heat the room at the same rate.  Sorry about the rant, @shinytop, but this is a popular enough misconception with people who don't own an ASHP.

Second point, of course the floor heats up.  You cannot put your hand on the floor and feel the hot areas where the water pipes/tubes are beneath and the cold areas in between, you warm up the whole floor and that radiates heat from its entire (large) surface area.  Sure, you insulate as much as possible underneath the heating elements but you still have to heat the surrounding material and I guess this must have a large thermal mass and therefore take a long time.          

I know about radiator Watts as I played around with various heat loss and radiator calcuators a lot.  I was just speculating delta t might have an effect since the almost universal advice is not to let an ASHP-heated house cool down too much as it takes longer to heat up.  Feel free to rant anytime though  .

I can understand ufh embedded in concrete floors heating up the concrete.  What about those that are overlaid or on suspended floors?  Don't you want the heat to go into the room rather than the floor?  Or isn't that how ufh works?

matelodave

Here you are chaps, have a shufti at this - https://www.engineeringtoolbox.com/radiation-heat-transfer-d_431.html

Once you'been slightly baffled keep scrolling down and there's a heat output calculator where you can fill in the emmisitivity coefficient of whatever is emmiting the heat (dont worry theres a link to a table so you can decide what material your emmiter is made of), you fill in the temp of the emitter and the surrounding air and the area in square meters and it'll give you a rough idea of how much heat gets transferred in watts. Probably not easy to find the area of a finned radiator, but a flat one shouldn't be difficult as would a concrete floor.

According to my calcs a concrete floor has a coefficient of 0.85, a temp of 30 degrees, if the room temp required is 20 degrees and its 20 m2 you'd get just over 1 kw - thats why it takes a long time to heat a room with underfloor heating.

Actually you get a bit more than that - here is some useful info if you want to understand u/f heating but as I said earlier the output is only around 70-100w depending on the type of u/f and what you put on top of it. Overlay systems are a bit more responsive than those embedded in the slab. page 50 gives you an idea of the heat output of differnt types at varying flow temperatures
 
https://www.colglo.co.uk/files/Polypipe Underfloor Heating Guide.pdf

Reed_Richards

My installer took details of floor area, ceiling height, window area, outside walls, wall insulation, loft insulation and fed this into a software package to calculate the heat output requirement for each room.  We then picked radiators to match or slightly exceed that requirement.  @matelodave you seem to be implying with underfloor heating it's just pot luck if you get enough heat or if high ceilings, too much furniture and thick carpets stop the room ever warming up.