Why UFH?

[Robert Clark]

Hi Guys

 

In a Passivhaus where heating is seldom required, why would you use UFH which is slow to heat up when slimline electric heaters would deliver heat quickly and maybe more controllably ?

[PeterW]

They heat the air not the fabric, and would be using peak energy at a 1:1 CoP. 
 

An ASHP heated slab can use off peak energy and a CoP of 3:1, so essentially can create a heat store at 1/6th of the energy requirements of panel heaters. 
 

Cooling is more of an issue these days, and a cooled slab is very effective for the purpose of controlling overheating. 

[Jeremy Harris]

+1 to the above. 

 

The vast majority of our heating (which isn't much) is overnight, during the E7 off-peak rate, and uses the ASHP (which seems to run at a COP of around 3.5 or so) to charge the slab up from the UFH.  The running cost is about 2.328p/kWh.  If we used electric panel heaters, then the cost would be over 6 times higher, at 15.729p/kWh.  There's also an environmental benefit with an ASHP, in that, because it uses less than 1/3rd of the electricity, there will be less than 1/3rd of the emissions etc from the power generation.

[epsilonGreedy]

1 hour ago, PeterW said:

They [electric panel radiator] heat the air not the fabric
 

 

Not doubting this statement, just trying to understand the science. Is this because a panel radiator is subject to more heat loss due to convection compared to a heated floor slab and also because it has a tiny area compared to a floor slab hence little area to irradiate IR heat from?

 

 

[jack]

2 hours ago, Robert Clark said:

In a Passivhaus where heating is seldom required, why would you use UFH which is slow to heat up when slimline electric heaters would deliver heat quickly and maybe more controllably ?

 

Especially where a PH has a high time constant (ie, takes a long time to change from whatever temperature it's reached), there's rarely a need to deliver heat quickly. The house tends to stay at around the same temperate with only small amounts of heat.

 

As an example of how thinking needs to change with a PH, our plumber was concerned that the long time it would take our "undersized" (5kW) ASHP to heat up our DHW cylinder would result in the house temperature falling uncomfortably in very cold weather. In reality, it takes a full day without heating to notice much of a change. Certainly nothing noticeable happens in the couple of hours it might take to heat the cylinder.

[scottishjohn]

i think he is really asking the question   so he can save money on build costs .

If it  were me --i would at least fit UFH piping in the floor  as its very cheap at build stage to do -, maybe you can balance cost of heat pump etc with more expensive running cost of simple panel heaters?

 fiiting pipes in floor at build stage leaves you options  later 

 

[Ed Davies]

39 minutes ago, epsilonGreedy said:

Not doubting this statement, just trying to understand the science. Is this because a panel radiator is subject to more heat loss due to convection compared to a heated floor slab and also because it has a tiny area compared to a floor slab hence little area to irradiate IR heat from?

 

Exactly. A post on my blog on the subject where I try to work it out from first principles: https://edavies.me.uk/2014/08/radiators/ TLDR, I think a heated floor emits heat about 50/50 split between thermal IR and convection/conduction. For a “radiator”, on the other hand, it's more like 1/6th by radiation and 5/6ths by convection/conduction. Multi-leaf radiators maybe even more by convection/conduction.

[Bitpipe]

8 minutes ago, scottishjohn said:

i think he is really asking the question   so he can save money on build costs .

If it  were me --i would at least fit UFH piping in the floor  as its very cheap at build stage to do -, maybe you can balance cost of heat pump etc with more expensive running cost of simple panel heaters?

 fiiting pipes in floor at build stage leaves you options  later 

 

 

Can also run spurs to rooms for future electric heating and leave them in wall if not needed.

 

 

[Jeremy Harris]

6 minutes ago, Bitpipe said:

 

Can also run spurs to rooms for future electric heating and leave them in wall if not needed.

 

 

 

 

We did this in the bedrooms, as I wasn't convinced that having no heating in the bedrooms would work  in practice.  In practice we've found the bedrooms never get cold, if anything they are too warm in hot weather, so we've not bothered fitting panel heaters to the spurs, but I did install air conditioning in our bedroom this summer.

[nod]

We have UFH to the ground floor

 Never use the bedroom and bathrooms radiators 

[nod]

In fairness we don’t use the UFH that often either 

It just shows how poorly insulated mass produced houses are 

Compared to what we are all building 

[joe90]

Likewise, UFH downstairs and just electric towel radiators in bathrooms (plus electric UFH In our en-suite). I didn’t run spurs to bedrooms as 13amp sockets are more than adequate for small heaters in a passive house IF required in very cold weather.

[Robert Clark]

Are you using an ASHP for hot heating a hot water tank too?

If you had an ASHP which was in cooling mode could it still be used to heat hot water?

[Jeremy Harris]

2 minutes ago, Robert Clark said:

Are you using an ASHP for hot heating a hot water tank too?

If you had an ASHP which was in cooling mode could it still be used to heat hot water?

 

 

I'm not, but others are.

 

Yes, you can both heat hot water and cool the floor with an ASHP, it just needs a motorised valve to close off the tank when the floor cooling is on,  plus a valve to open and close the UFH, with the hot water set to priority.  What happens is that if the tank calls for heat, if the floor cooling is on then the UFH valve closes, the tank valve opens and the ASHP is turned on in hot water mode.  When the tank is up to temperature, the ASHP turns off, the tank valve closes and the UFH valve opens.  If cooling is still needed the ASHP then turns on in cooling mode.

[Ed Davies]

It's a minor [¹] sadness that you can't pump heat out of the floor slab into the DHW tank.

 

[¹] Very minor given the small amount of cooling typically needed in the UK. In warmer climates, however…

[jack]

4 minutes ago, Ed Davies said:

It's a minor [¹] sadness that you can't pump heat out of the floor slab into the DHW tank.

 

[¹] Very minor given the small amount of cooling typically needed in the UK. In warmer climates, however…

 

I think that every time I walk past my ASHP when it's in cooling mode and I feel all that lovely heat being lost to the atmosphere.

 

Even less feasible is using cold air from heating to make refrigeration more efficient. Slightly more feasible might be to divert some of the cold air in summer to keep a larder outside the heating envelope cool.

[ProDave]

1 hour ago, Robert Clark said:

Are you using an ASHP for hot heating a hot water tank too?

If you had an ASHP which was in cooling mode could it still be used to heat hot water?

A further point of explanation, is unlike a boiler, a heat pump will only ever heat space heating or Domestic Hot Water one at a time, never both together.  This enables the heat pump to run at a low flow temperature for heating and only run at a higher temperature when heating DHW.

 

It is this separation of functions that enables it to switch from cooling mode to heating mode when DHW is called for.

[Nickfromwales]

Having a slab means you can charge with heat also means you can almost always load shift from off-peak electricity, ‘injecting’ heat into the slab once or twice a day. 
Even heating by direct electricity is palatable with such an effective heat store, subject to the house performing well of course. 

[joe90]

3 hours ago, jack said:

Slightly more feasible might be to divert some of the cold air in summer to keep a larder outside the heating envelope cool.

 

Jack, “great minds think alike”, I considered an outside veg store opposite my ASHP to utilise all that wasted “coolth”.

[PeterW]

40 minutes ago, Nickfromwales said:

Even heating by direct electricity is palatable with such an effective heat store, subject to the house performing well of course. 

@Nickfromwales is right here - for ultra low capital cost, and assuming you have a heat input that really is down in the low kW requirements then a Willis heater setup will work. At less than £100 to do, there is a real challenge to make a commercially installed ASHP pay at that point. @TerryE has this setup in his place. 

[Nickfromwales]

19 minutes ago, PeterW said:

@Nickfromwales is right here - for ultra low capital cost, and assuming you have a heat input that really is down in the low kW requirements then a Willis heater setup will work. At less than £100 to do, there is a real challenge to make a commercially installed ASHP pay at that point. @TerryE has this setup in his place. 

KISS at its absolute finest. 

[Nickfromwales]

I would still fit a heat pump just for cooling. Just great for killing two birds with one stone.
If you’re not in ‘need’ of cooling it is very hard to justify the capital expense, ( at a time when every penny counts ), and more of a concern when you factor in it’s inevitable maintenance, repair, and ultimately it’s replacement. 

[Robert Clark]

If we had an ASHP would it be feasible to use it to supply bathroom towel rails or would electric ones be better?

[Nickfromwales]

3 minutes ago, Robert Clark said:

If we had an ASHP would it be feasible to use it to supply bathroom towel rails or would electric ones be better?

For the amount of heat you’d need from them, if used in conjunction with wet UFH in the main slab, I’d stick to the sheer simplicity of electric towel rads + electric UFH. 
If you’re relying on these rads as your main auxiliary heat source, eg no UFH, then prob best to feed them from a HP as you won’t be able to load shift. 
Would be disproportionately expensive as a heating design though, imo. 

[Jeremy Harris]

7 minutes ago, Robert Clark said:

If we had an ASHP would it be feasible to use it to supply bathroom towel rails or would electric ones be better?

 

 

If run at typical UFH flow temperature then the towel rails wouldn't even feel warm to the touch, I think, plus you may well find, as we have, that in the heating season the UFH only comes on for an hour or two every couple of days.  I opted to fit electric towel rails, wired to a circuit that includes a time switch,  so that circuit comes on for a couple of hours each morning and evening.  Seems to work well.