200 watts to keep my study warm? Please check my maths.

[epsilonGreedy]

I am thinking ahead to what electricity capacity my garage/outbuilding will require in order to keep the office warm.

 

The building is single story, detached and the office part of the garage is built to nearly the same standard as the brick/block cavity main house. According to my maths just 200 watts will keep it warm with the temperature at freezing point outside.

 

To simplify the ball-park maths let's assume:

 

1. The floor, walls and ceiling have a u-value of 0.2.
2. The room is 5m x 2.5m and 2.5m high = total surface area of 50 m2.
3. Temp difference is 20 degrees (c).

 

If a u-value of 0.2 = 0.2 of a watt of heat loss per m2 per degree of temperature difference then:

 

50 m 2 x 0.2 x 20 degrees = 200 watts or much less than I thought.

 

I will probably hook the heater up to an Amazom Alexa power switch which will allow me to say "Alexa heat the office" 30 minutes before heading out to do some work, which would be better than a timer switch scheduled for good intentions to work that never actually happen.

 

 

Edited September 17, 2019 by epsilonGreedy

[SteamyTea]

check your areas.

then multiply by temperature difference and U-value.

Edited September 17, 2019 by SteamyTea

[Jeremy Harris]

1 hour ago, epsilonGreedy said:

I am thinking ahead to what electricity capacity my garage/outbuilding will require in order to keep the office warm.

 

The building is single story, detached and the office part of the garage is built to nearly the same standard as the brick/block cavity main house. According to my maths just 200 watts will keep it warm with the temperature at freezing point outside.

 

To simplify the ball-park maths let's assume:

 

1. The floor, walls and ceiling have a u-value of 0.2.
2. The room is 5m x 2.5m and 2.5m high = total surface area of 50 m2.
3. Temp difference is 20 degrees (c).

 

If a u-value of 0.2 = 0.2 of a watt of heat loss per m2 per degree of temperature difference then:

 

50 m 2 x 0.2 x 20 degrees = 200 watts or much less than I thought.

 

I will probably hook the heater up to an Amazom Alexa power switch which will allow me to say "Alexa heat the office" 30 minutes before heading out to do some work, which would be better than a timer switch scheduled for good intentions to work that never actually happen.

 

 

 

 

What's the floor U value? 

 

What are the door and window U values and areas?

 

What allowance has been made for thermal bridging?

 

What's the ventilation heat loss rate?

 

The floor area can be considered to have a lower ∆_T than the walls and roof, as the ground temperature under a floor, pretty much all year around in the UK, tends to be about 8°C. 

 

Assuming that the floor U value is also 0.2W/m².K, that there is a 1m² area window with a U value of 1.6 W/m².K and a normal pedestrian door with a U value of 1.6 W/m².K, and with a low ventilation rate, for a ∆_T of 20°C between inside and out (ground temperature assumed to be 8°C, and internal to ground ∆_T of 12°C) then I make the heat loss about 345 W.  I've not included an allowance for thermal bridging.  Also, if the doors, window and floor U values/areas are not as assumed then the heat loss will be different.

 

 

[ToughButterCup]

"Naasah, yer wunt 10kW mate ... 's wot vey all 'ave round yur...... 10kW - at least."

[A_L]

I would use the full inside air/outside air temp difference for the floor as the full heatloss path is floor-soil-outside air.

 

Allow about 30% of fabric heatloss for thermal bridging.

 

Ventilation heat loss is Volume of structure (m³ ) x air changes per hour x temperature difference (°C) x 0.3 Watts

 

Remember you are an 80W heat source and the office equipment will contribute

 

The size of the heater will more likely be determined by providing a reasonable heat from cold performance than steady state loss. So a 2kW fan heater will probably do.

[Jeremy Harris]

1 hour ago, AnonymousBosch said:

"Naasah, yer wunt 10kW mate ... 's wot vey all 'ave round yur...... 10kW - at least."

 

 

Which raises a valid point, in that the steady state heat loss rate doesn't automatically define the heating power requirement.  There needs to be sufficient excess heating power available to be able to bring the space up to a comfortable temperature within a reasonable period of time.  This depends to a significant degree on the heat capacity, and thermal conductivity, of the materials that make up the first 50 to 100mm of the internal structure (ceiling, floor, walls etc).  Assuming a plasterboard ceiling, concrete floor and plastered or dot'n'dab boarded walls, then the internal fabric heat capacity could be anything from about 4 to 10 kWh for a 10°C temperature change.  If the room started off 10°C cooler than desired, then significantly more heat input will be required initially in order to get the internal fabric up to temperature.

[Jeremy Harris]

18 minutes ago, A_L said:

I would use the full inside air/outside air temp difference for the floor as the full heatloss path is floor-soil-outside air.

 

 

I have temperature sensors in the soil. beneath our house, put there out of curiosity more than anything else.  They have turned out to be the most pointless temperature sensors I've installed anywhere, as both of them stay within 0.2°C of 8.7°C all year around (I suspect that about +/- 0.1°C of this may just be sensor resolution fluctuation).  That suggests to me that, for a floor in contact with the ground using a typical UK soil temperature of about 8°C would make sense for floor heat loss calculations.

 

Clearly this doesn't really apply to a suspended floor, where depending on the effectiveness of the underfloor ventilation the ∆_T may well be about the same as for the walls and roof.

[MikeSharp01]

4 hours ago, AnonymousBosch said:

"Naasah, yer wunt 10kW mate ... 's wot vey all 'ave round yur...... 10kW - at least."

Yep this will allow you yo keep warm in the upcoming ice age of course there might not be any power generation by then. Naturally ensuring you have a 10Kw supply will allow you to run on hell of a workshop / model railway / tanning salon / steel works / sound system / radio station in there until the time comes.

Edited September 17, 2019 by MikeSharp01

[epsilonGreedy]

23 hours ago, JSHarris said:

What's the floor U value? 

 

 

Not sure because I have wandered off piste with the garage design as it has morphed from a double garage into an outbuilding. Probably 100mm of a solid PIR insulation sheet. It is a suspended block and beam floor and the Kingspan u-value calculator gives 0.15.

 

23 hours ago, JSHarris said:

What are the door and window U values and areas?

 

 

One external door = 1.6 m2

 

The area will be a bit under illuminated with about 1.5 m2 of non fancy double glazing, the main window will be a wooden sash so best assume minimum building regs performance.

 

23 hours ago, JSHarris said:

What's the ventilation heat loss rate?

 

 

As there will not be MVHR in this outbuilding this is something not yet considered, probably two trickle vents in the windows at either end.

 

23 hours ago, JSHarris said:

Assuming that the floor U value is also 0.2W/m².K, that there is a 1m² area window with a U value of 1.6 W/m².K and a normal pedestrian door with a U value of 1.6 W/m².K, and with a low ventilation rate, for a ∆_T of 20°C between inside and out (ground temperature assumed to be 8°C, and internal to ground ∆_T of 12°C) then I make the heat loss about 345 W.  I've not included an allowance for thermal bridging.

 

 

Thank you for this. The floor is suspended hence a greater heat gradient. I am hoping this will be mitigated by one 5m long wall being internal to the garage with an open cold loft space which will I hope be a few degrees above outside temperature. The "office space" is also divided 60/40 to create a small utility space with a chest deep freezer which will emit a few watts of heat into the area.

[epsilonGreedy]

23 hours ago, A_L said:

Allow about 30% of fabric heatloss for thermal bridging.

 

 

Wow that high!

 

23 hours ago, A_L said:

Ventilation heat loss is Volume of structure (m³ ) x air changes per hour x temperature difference (°C) x 0.3 Watts

 

 

I previously knew that air change heat loss was a significant factor in a building with half decent insulation, now with these figures I can see it is a major consideration.

 

Given 31.5 m3 of volume x a conservative 2 acph x 20 degrees x 0.3 = 375 watts just to heat up the fresh air. Ouch.

 

Edit: @JSHarrisestimates later in the thread that trickle ventilation would be more like 0.2 to 0.3 ACH hence more like 45 watts of heat loss.

Edited September 18, 2019 by epsilonGreedy

[Jeremy Harris]

Best estimate is that the heating requirement for a  ∆_T of 20°C will be a  bit higher, at 428 W, allowing for typical thermal bridging and ventilation heat losses.  The slightly better floor insulation is offset by the fact it's a suspended floor, plus the window area is slightly greater than I'd assumed (door area's the same as I'd used).,  That's for doors and windows with a U value of 1.2 W/m²∙K, about as good as any DG window is likely to be.

[epsilonGreedy]

22 hours ago, JSHarris said:

... Assuming a plasterboard ceiling, concrete floor and plastered or dot'n'dab boarded walls, then the internal fabric heat capacity could be anything from about 4 to 10 kWh for a 10°C temperature change.  If the room started off 10°C cooler than desired, then significantly more heat input will be required initially in order to get the internal fabric up to temperature.

 

 

This is a depressing insight. The heat loss is modest, heating up fresh air could be an equivalent cost but this issue of warming up the building from a cold start is a major consideration.

 

So heading out to a thoroughly chilled down outbuilding office for three hours of work during a frosty evening would cost about £0.25 to counter heat loss and possibly £1.00 to warm up the structure. Hmm time to rethink the building fabric which is just a block & beam floor at present. PIR backed plasterboard would help thermal boot up lag I hope.

[Jeremy Harris]

Just now, epsilonGreedy said:

 

This is a depressing insight. The heat loss is modest, heating up fresh air could be an equivalent cost but this issue of warming up the building from a cold start is a major consideration.

 

So heading out to a thoroughly chilled down outbuilding office for three hours of work during a frosty evening would cost about £0.25 to counter heat loss and possibly £1.00 to warm up the structure. Hmm time to rethink the building fabric which is just a block & beam floor at present. PIR backed plasterboard would help thermal boot up lag I hope.

 

 

This is where a fan heater works well, as they heat up the air quickly, and even though the structure will carry on absorbing heat for a long time, the warmer air makes the room feel more comfortable.  This is just the solution that @SteamyTea prefers, as it's an effective way to heat up a relatively small space to a comfortable temperature pretty quickly.  In my old workshop (a single skinned, brick built, uninsulated, garage) a 2 kW fan heater was enough to get that up to a comfortable temperature within about 20 minutes or so.

[SteamyTea]

3 minutes ago, JSHarris said:

This is just the solution that @SteamyTea prefers

Too right, fan heater costs a tenner to buy, even on day rate only about 25p/hour to run.

Heats my kitchen up in less than 20 minutes from as cold as it gets.

My experience is that, even when I leave the windows open at night, the house is 3°C warmer than outside.

So the temperature difference is never quite as bad as it seems.  Probably why I don't have to start heating my house until the mean temperature is below 10°C, and that is for when I am sitting and not when I am active.

The only downside of fan heaters are the noise, can trip over them (an amazing amount of times in my experience) and if you get one that turns the fan off when it reaches temperature, you can forget to switch it off properly when going out.

[epsilonGreedy]

6 minutes ago, JSHarris said:

This is where a fan heater works well, as they heat up the air quickly, and even though the structure will carry on absorbing heat for a long time, the warmer air makes the room feel more comfortable.

 

 

The problem is they can create a slight burnt smell in a room due to the exposed element. An oil radiator would only exacerbate the thermal lag problem so I will look into a panel radiator though I have not used one before.

[SteamyTea]

Just now, epsilonGreedy said:

The problem is they can create a slight burnt smell in a room due to the exposed element

Not my experience.  Sometime they smell if I drop sawdust down one in my shed.  

[epsilonGreedy]

2 minutes ago, SteamyTea said:

Probably why I don't have to start heating my house until the mean temperature is below 10°C, and that is for when I am sitting and not when I am active.

 

 

10 degrees! That sounds like balmy luxury ?

 

The internal static caravan temp was 7 degrees this morning and Swmbo is still stating we should get through to the end of September without turning on the bottled LPG central heating.

[Jeremy Harris]

8 minutes ago, epsilonGreedy said:

 

The problem is they can create a slight burnt smell in a room due to the exposed element.

 

Not my experience, unless the thing hasn't been turned on for ages and has got a bit dusty.  I still have one here that I used to try and speed up the drying of the plaster, as we had water running down the windows for a couple of weeks afterwards.  I'll dig it out and try it to see if there's any smell.

[SteamyTea]

4 minutes ago, epsilonGreedy said:

10 degrees! That sounds like balmy luxury

That is the external temperature.

I like my house to be around 21°C.

 

Monday was a pretty miserable day, grey and drizzle.

Mean temp was 16.5°C, min of 2°C and max of 18°C.

House was 18.5°C on average (was painting windows, so they were open most of the day).  Min was 17°C and max was 20°C.

Apart from the rain, Cornwall has a great climate.

And the wind, mist, fog, cloud....

[SteamyTea]

Here is my fan. Been kicking around since March. Got a good bit of dust in it, but hardly a whiff when I switched it on.

Had a few embers pop out, wish I had videoed it.

[ToughButterCup]

Got a few cats?

It's about 15 Watts a cat innit? A dog maybe 30 - 50 Watts. Put their beds close to any potential airflow round the doors....

Well on the way to smelly warmth.

[Jeremy Harris]

57 minutes ago, epsilonGreedy said:

 

I previously knew that air change heat loss was a significant factor in a building with half decent insulation, now with these figures I can see it is a major consideration.

 

Given 31.5 m3 of volume x a conservative 2 acph x 20 degrees x 0.3 = 375 watts just to heat up the fresh air. Ouch.

 

Missed this earlier.  Trickle ventilation shouldn't be anything near 2 ACH, probably more like about 0.2 to 0.3 ACH at a rough guess.  We ventilate our house at about 0.45 ACH and find that keeps the air nice and fresh, but the heat loss penalty for that ventilation rate is small, as about 85% to 90% of the heat is recovered by the MVHR system.