How much heating and cooling do I need « Alternative Technology Assn Forums

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How much heating and cooling do I need

(22 posts) (9 voices)

Started 10 years ago by tom
Latest reply from dlvb19

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tom
Member

I was hoping someone could please help me work out what sized heating/cooling unit I will need for the house we will be building. All I've seen is a recommendation of 125W of cooling for the average house, but no guidelines for heating.

The house will have R6.5 Walls (R4 panels and R2.5 ecofoam), R12 ceilings, and double glazing with a U or 1.8. There is 49 sqm of windows and glass doors. House is 253 sqm, but ignoring walls and garage we have 192 sqm of space. Exclude laundry and robes which will be shut up and it's around 175 sqm. Ceilings are 3m. Concrete slab. Solar access isn't great unfortunately, although we will get a bit in the northern living room.

Hoping to have ducted reverse cycle, and the unit that looks the best to me is rated at 14KW heating (18KW max) and 13KW (14KW max) of cooling. If I use the 125W recommendation I'd need 22KW, which would be 3 phase and costly.

Does anyone know of any rules of thumb for well insulated houses? I don't have the energy rating, so maybe that will tell me or help?

Posted Friday 15 Jun 2012 @ 2:26:14 am from IP #
ghostgum
Member

You haven't mentioned the location, which affects the answer. The following are very rough calculations.

If you assume that average inside to outside temperature difference is 7C (Melbourne) during the heating period, then average heat loss through the windows is 49 * 7 * 1.8 = 617W, roof is 192 * 7 / 12 = 112W. Heat loss through the walls will be similar to the roof, so assume the total heat loss is 1000W. Over 180 days of heating that would be 180 * 1 * 24 * 3.6 = 15500MJ. Divide by floor area 192m2 gives 81MJ per m2 per day. In Melbourne that would be 7 stars.

Using the same calculation on my 4.5 star house with 220m2 area gives 2400W. I have a hydronic heating system with 14kW output that is adequate.

I think a 14kW system is large enough for heating your house. A 7kW might also be large enough.

Some other thoughts:
Is the heating even, or will there be hot and cold spots?
How quick can it heat the house from cold?
Could you make it work with one or two smaller split systems on the wall?
How much heat loss through the ducting? With that much insulation, you also want better than average duct insulation.
With poor duct insulation, warm air from the room rises into the duct, cool, then fall back into the room. You ceiling insulation will be less than the R12 you expect.
Your heat loss is dominated by the windows. Window coverings reduce this heat loss.
How much cooling will you need in your climate?
Is ducted heating the best arrangement?

Posted Friday 15 Jun 2012 @ 3:41:56 am from IP #
Buzzman
Member

Or you could just wear more clothes and add an extra doona to the bed...;)

Jokes aside, heating the whole house is the LEAST sustainable way to survive winter.

It follows that if you focus on ONLY heating the actual area of the house you are using (ie: main living area/kitchen + a bedroom or two), then calc the area of those spaces and buy a heating system that is capable of heating THAT volume of space, not the entire house.

This may mean, if you go for ducted, that you will absolutely require multiple *closable* ceiling ducts, and possibly even more than 1 return duct, so you can *physically* 'zone' different areas of the house such that you aren't unecessarily heating areas that aren't being used.

This will be way cheaper than a 'whole of house' system both to purchase and most importantly for future power increases, cheaper to run both now and in the future.

And remember the 'energy minimiser rule' - heat the *body* FIRST, before you heat the *space* the body occupies....

That means utilising much warmer (and more layers) of clothing and ONLY using the heating system when that no longer keeps you comfortably warm.

Wandering around a centrally-heated house in shirt-sleeves in the middle of a Melbourne winter is not only environmentally unsound it is morally questionable and definitely not-cost-effective.

However, if you have a large solar PV array and are exporting to the grid sufficient power to run your heating AND other domestic usage, feel free to dance naked on the living room carpet, if that takes your fancy.

Posted Friday 15 Jun 2012 @ 4:45:24 am from IP #
tom
Member

Thanks Guys. I did leave out a few points.

I'm in Melbourne. I tried working out what I needed by adding up the U's and R's and areas, assuming a worst case of a 20 degree temperature difference and end up with heat loss way less than any house sized heater/cooler.

Perimeter of house is 76m, 3m high ceilings gives 228 sqm for the walls, take of 49 for the windows. Leaves 179 sqm of walls. Assume 180 and R of 6 for simplicity, give Uw x A of 30 for walls. Round Uw of the windows to 2, and area to 50. To get Uw x A of windows of 100. Round roof to 250, round insulation down to 10, to get Uw x A of 25, for a house total of 155. Assume 20 degree difference gives a max of 3100W.

Problem is I don't know how to model the heat loss/gain via by convection. Rendered panels have very low air infiltration. The windows are all casement, so well sealed. So left with the front door, which will be sealed also, so Pessimistically I can't see myself losing more than 50% of the 3100W lost by conduction.

As for radiant loss, I'm not too bothered about cooling in summer, more than cold winter nights, when all curtains/blinds are shut, so can't see a lot of radiant loss then. So really can't see why I couldn't get away with 5KW for the entire house. But that seems pretty low for a 4BR house. I'm in an old poorly sealed double brick 2BR house, and the 14KW gas heater can't keep up on really cold nights. Find it hard to believe plastic walls and extra glass can more than halve that in a house double the size.

Enough rambling!

Looking at the efficiency side, the best 2KW split systems have COPs of 5.5. The bigger whole house units hit around 4. So looking at the alone, I'd be best with an independant split system in every room. Being less convenient would also make us turn it on less often as well. Aesthetically they're butt ugly though, compared to some non-descript vents in the ceiling. I'm leaning towards 6 split system units, but the wife wants ducted.

Anyway, I do ramble. Appreciate the responses.

Posted Sunday 17 Jun 2012 @ 9:40:59 pm from IP #
Catopsilia
Member

Hardly anyone is doing the calculations to save on heating and cooling, tom.
The fact is, energy and appliances for heating and cooling are so dirt cheap that there is no incentive to economise (yet!).
Things were different a century ago.

From "Boarding House Geometry" by Stephen Leacock
"If from the opposite ends of a boarding house a line be drawn passing through all the rooms in turn, then the stovepipe which warms the boarders will lie within that line."

Posted Monday 18 Jun 2012 @ 12:32:39 am from IP #
ghostgum
Member

Tom, 6 split system units sounds excessive and ugly. Can you do it with two? For example, one for the main living room, and one for the passage outside the bedrooms? Leave the doors open when you want the air to move into the rooms. Can you use fewer split systems and a ducted heat shifting fan?

Do they make 5kW ducted units, or is that considered too small for a whole of house system?

Posted Monday 18 Jun 2012 @ 2:40:43 am from IP #
dubbo teacher
Member

Tom are you serious? With that amount of insulation in the walls and ceiling you shouldn't be worried about heating OR cooling...not for more than 2 or 3 weeks each peak summer and winter.
If you have a half decent amount of north glazing and not excessive thermal mass in areas without direct sunshine, you may not need ANY heating, even in Melbourne.
In Dubbo we've been getting down to 2-5 degrees overnight; 12-17 during the day and up til now haven't needed any heating, haven't even closed the blinds yet and sleep with the sliding door open in the bedroom.The flat is double brick with glazing mainly on east side except south side in bedroom mentioned.
Do wear a beanie if cold.
Are we getting a little soft in society these days?
I'm a babyboomer not a Gen Y so maybe that accounts for a little wider comfort zone...

Posted Monday 18 Jun 2012 @ 9:36:59 am from IP #
tom
Member

Solar access is pretty limited. The block is a 12m wide inner city block with the houses on each side quite close and quite high (1890's houses). The rear unfortunately has a LARGE tree over the back fence which will shade the north windows in winter. I mean 30m+ large heritage listed tree. Also a 10m tree 5m from the north windows on our block that the council refuses to let us remove. So I'm not expecting a lot of solar gain in winter. Hence the use of excessive insulation and the need for heating. Cooling? Meh, I can deal with being too hot, but with reverse cycle heating it comes for free so might as well make sure it can keep it cool for the other inhabitants. (the wife!)

I haven't seen 5KW ducted units, I think the smallest is around 8KW. 14KW seems to be the sweet spot in terms of efficiency on ducted units. 2KW is the sweet sport for wall hung units.

Posted Tuesday 19 Jun 2012 @ 2:23:49 am from IP #
ghostgum
Member

Tom, do you think a heat shifting fan would work?

Adding clerestory windows may allow you to get light and heat into the middle of the building, but would also increase heat loss at night. My need some insulating curtains (honeycomb/cellular blinds) on the high windows.

Could you install an active solar heating sytem? Now is the time to look at where the shadows are falling on the existing roof.

If you don't need an air-conditioner, have you figured out how to get cooling breezes into the house, and let hot air out the top?

Posted Tuesday 19 Jun 2012 @ 3:15:05 am from IP #
tom
Member

Hi Ghostgum,

I'm not familiar with heat shifting fans, so don't know if they would work. But based on their name, I'd assume they're just moving heat from one room to the other. It might help, but given we don't have good solar access anywhere I don't think so.

Clerestory would be a good idea, but unfortunately would require a design change that might not get through council. We're under a heritage overlay, so the design of the house is a little restricted - Victorian style hip roof and 250mm box eaves. The short eaves aren't too much of an issue given the shading from neighbouring houses and we do have a short veranda at the northern rear.

Haven't looked at active solar, but will check it out - thanks.

There will be a decent amount of cross ventilation to aid getting hot air out in summer. Could do with more, but given our constraints it's acceptable.

Thanks for the pointers.

Posted Tuesday 19 Jun 2012 @ 9:55:41 pm from IP #
ghostgum
Member

A heat shifter fan is just a fan that takes air from one part of the house and moves it via a duct to another part of the house. The duct needs to be well insulated, otherwise it acts like a hole in your insulation. It doesn't matter whether the air is hot or cold. You could have an air conditioner running in the main room, and a heat shifting fan moving cold air to the area near the bedrooms. I was thinking of using one or two wall mounted reverse cycle units plus a heat shifter with ducting, rather than a big ducted reverse cycle unit.

With a heritage overlay, you might have trouble putting things on your roof unless they can't be seen from the street. There was an article I saw (I think in ATA ReNew) about a resident in a heritage protection area who wasn't allowed to put up a solar hot water system on their roof, yet the neighbours were allowed to put up external TV antennas.

Posted Tuesday 19 Jun 2012 @ 11:44:47 pm from IP #
tom
Member

We're allowed things on the roof if they're not visible, but as it's a heritage area still need a permit. So up to the whim of the council person deciding. The neighbour on one side has vacuum tube hot water already, so at a minimum we should be able to do the same.

As for panels, that will be different. The house is long with a 4.4m x 16m "flat" bit down the centre (ie the two ridges you see on old hip roofs, rather than one ridge down the centre). I'm hoping I can 4-5 row of 4 panels on that. Will be interesting though. "Flat" means there's a 2 degree gradient in towards a central box gutter. Hoping to be able to attach frames to the metal ridge caps, but the frames I know of aren't designed to be fixed at only the ends. But that's a different topic ...

Posted Wednesday 20 Jun 2012 @ 2:31:37 am from IP #
ghostgum
Member

On that different topic, at 2 degree pitch that sounds like a tray deck roof. I'm planning on using Klip Klamp to mount a home made solar air heater on the flat section of my roof.

Posted Wednesday 20 Jun 2012 @ 2:40:24 am from IP #
tom
Member

Thanks Ghostgum, those clamps look exactly like what I need.

Posted Wednesday 20 Jun 2012 @ 9:23:21 pm from IP #
alanpaul
Member

have you considered a HRV?

i made one for my house (~200m2) for about $500, and it reduced my heating requirements by 75%. since then costs have come down, could prob do it now for about $300

paid for itself in half a winter

or you can buy one, but they are the smae price as a ducted system.

Posted Wednesday 20 Jun 2012 @ 11:38:57 pm from IP #
ghostgum
Member

An HRV/ERV (heat/energy recovery ventilator) is not a heater. It brings fresh air into the house, warming it with the air being pushed outside. They recover 70-90% of the heat from the air being exhausted to outside.

Why do you claim it reduces your heating requirements?
What design did you use?

Posted Thursday 21 Jun 2012 @ 1:08:03 am from IP #
Buzzman
Member

tom

You are correct in assuming that multiple stand-alone split systems will be more cost-effective and MUCH more cost-efficient to run.

One thing that is NOT widely know about splits is that it is possible to get a single base unit with multiple HEAD units; ie: one box outside, multiple indoor fan units. Normally two to three per compressor unit...

So for bedrooms and living areas, get a single base unit with multiple head units BUT (and this s critical for cost-efficient running) get a base unit that will heat/cool ONLY the volume of the 'largest' are within each 'zone'.

Generally most people find they aren't utilising the entire house ALL of the time, so only needing to heat/cool the area/volume ACTUALLY being used at any given time means that the base units can be minimised and thus be cheaper to buy and to run.

So if you had a multiple head unit for say three bedrooms, having the power unit 'sized' only for the Master bedroommakes sense. Here's why. Kids go to be earlier than parents. Kids used bedrooms when parents are elsewhere. So only the head units in both smaller b/rms will be roughly equivalent to the Master in voumetric airflow requirment. Make sense?

Ditto the living areas. base unit that is big enough to cool main kitchen/dining/TV room, but not necessarily the dining room and formal lounge, as these tend to be only used together on one or two days per year.

You just have to accept that on those two days, the house might not be warm/cool enough, but 99% of the time it will be fine.

Rationale for this approach is solely to minimise the size and therefore the consumption rate of the compressor in the AC unit. Larger the compressor more it tends to use per hour/per day.

Also, for modern houses with open plan living areas, this method will require temproary-to-permanent (your choice) physical zoning bewtween each head unit's 'zone' to minimise loses between zones. Hanign curtain in winter for example, or folding dors between living areas for summer - that type of thing.

Posted Saturday 23 Jun 2012 @ 6:38:14 am from IP #
tom
Member

Thanks Buzzman,

I have checked out the multi-splits but according to specs, they aren't as efficient as multiple single-splits. Both approaches have the same ugliness factor. I think what we will do is buy a 14KW ducted split system with a separate zone controller for every room. Installers all recommend all bedrooms be in the same zone, but we will have each bedroom be it's own zone. Will cost more upfront, but I don't see the point in heating/cooling 4 bedrooms at once, if most of the family is in the lounge watching a movie, and little Johnny is alone in his room studying (or on facebook more like it).

Posted Monday 25 Jun 2012 @ 9:50:52 pm from IP #
john de bruyn
Member

Hi All,

I've just responded to some of the above in a similar thread 3612.

http://www.ata.org.au/forums/topic/3612?replies=16#post-29030

I'd love the input.

Thanks,

John

Posted Tuesday 3 Jul 2012 @ 3:28:24 am from IP #
StephenF
Member

Hi Tom, What way did you go with the heating? Did you end up using something smaller than all the professionals were claiming you needed?

We have a similar problem but with the cooling, no one wants to look at it from a heat load perspective, they just say 100 watts per square meter regardless of the quality of your build or insulation. They don't seem to care that houses have changed a lot in recent years, new house should simply need less.

The other claim is about fan strength. Apparently undersizing the cooling, even when the energy calcs are right, causes problems with being unable to move enough air around? Just don't really know what to believe sometimes.

Just can't bring myself to put in 32ķw of cooling for 240 Square metres with r4 all the way around, double glazing, etc, etc. Like I am being pushed on. I think I will hardly need half that.

Posted Sunday 16 Mar 2014 @ 9:35:14 am from IP #
tom
Member

Hi Stephen,

We were pressured in to a bigger unit, and ended up with the biggest single phase ducted reverse cycle unit for sale which is 18KW heater/16KW cooling unit. We were told it wouldn't be big enough to do the whole house. We have 7 zones. 1 always on, the other 6 controllable. We didn't use it very much, but when we did it was able to drop the temperature from 25-23 degrees in about 2 hours while the temp was 43 outside. (More an experiment, than a real need to cool the house). The fan is a bit weak for the whole house at one time. But also too strong if we close all the closeable zones. No idea how it works as a heater yet. Given the mess of ducts in the ceiling and holes in the insulation, I think a couple of single splits would have been better. It blows warm air for 5 minutes when you turn the air-con on. Duct work is R1.5, but the air in the ducts still warms up too much.

Problem is air-con guys use a sizing program that does not have any way to enter the level of insulation or glazing. It's like insulation? yes/no. Window shading? yes/no.

Actually looking back we should have got a Zehnder air-exchanger/ERV and a single 8KW single split for the open plan area, and relied upon the air-exchanger to distribute to the rest of the house. I think the ERV would have meant we wouldn't have needed anything in summer. Most of the discomfort was humid/stationary air, not heat.

Posted Sunday 16 Mar 2014 @ 9:09:08 pm from IP #
dlvb19
Member

Standard recommendations for heating load are over-blown.

Our main living room is 54m^2, which according to conventional sizing would require a 5kw RC-AC however we have gone with a 2.5kW. In reality though this system does the entire house at 130m^2, which would warrant (according to convention) a 10+kW ducted system.

The limits of our 'undersized' system are seen at 40+ degree and 5- degree days when the living room can only be conditioned to 24 and 21 degrees respectively. Fine with me, although I would actually prefer a toasty 24 degrees during winter, but that seems extravagant.

The other limit is the speed at which the temperature is "conditioned". Takes about 2 hours to warm house from 13 degree overnight low to 22 degrees for the morning routine. If we had less thermal mass it might warm slightly faster, but the room might cool down even further at night without it. It might have been more economical to get a more powerful system and have the place warm up faster (saving lost heat over the longer period of time required with smaller RC-AC). If I could pick again I think I would get a ~4kW system to increase the response time of the heating/cooling, albeit at a slightly lower COP/efficiency.

Also should mention that room of main room with RC-AC maintain a temp about 1-2 degrees cooler/warmer (depending on season). Also walls are R8, ceiling R5, elevated timber floor R2.5 and 18m^2 of R0.5 windows, with 12m^2 of them in room with heat source.

Hope my self-indulgent ramblings are contributing somehow

Posted Monday 17 Mar 2014 @ 12:07:17 am from IP #

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