Heat loss calculations

[RogerS]

Been having a stab at calculating the heat loss and hence radiator requirements in the cottage but am not confident I've got it right.

The thing that is puzzling me is that, after applying the calcs for the main bedroom (with all its cosy insulation factored in) that the heat loss due to ventilation (500 watts) is more than twice that due to the fabric loss (200 watts). Given that the emphasis in Building Regs is moving towards 'air-tightness' and I can't find any reference in the Regs Part L regarding allowance for ventilation, should one ignore or at least factor downwards the ventilation heatloss ?

To further undermine my confidence, entering in my room parameters into an online heat loss calculator, the results are 1300 watts - which is double what I calculated.

Has anyone done their own calculations recently?

 

[Woody Alan]

Roger

A little reluctant to reply as it is a minefield, but I have very recently done the calcs for my downstairs rooms as I have just upgrade from a one pipe system to a two pipe. I used the online calcs too, but the radiators I ended up with I think are oversized. They work but I am in a cavity house with insulation and a pre condensate boiler (runs at a slightly higher temp) worth knowing if you are used to calcs on older boilers. I think you've got to rely on a bit of "feel" I wish I had, modern rads are so efficient now..
I did the manual calcs and the online calcs are pretty accurate for ball park, but if you have a room which always feels warm spec it down a bit and one that's always cold (north in the shade with two external walls) spec up. If it doesn't feel right to you and you think it's OTT it probably is IMHO. If you haven't got any draughts and you don't intend to create any then don't allow for it. If I had the cash I would use a control system that goes on the TRV's and has a stat in each room. I assume you are doing current regs stat in main areas with no TRV and TRV's in other rooms and seperate zone on upstairs?
Secondary loop on towel rads? or are you putting one of those combi's in.
EDIT just reflecting a bit more and would conclude that it's a bit of an opposite to diminishing returns. In a small room the online calcs seem spot on but in a larger room e.g twice the respective small room don't doublr the BTU spec it down a little if that makes sense

Alan

 

[bjm]

Roger

I haven't done these calculations for many years now but I think you are talking about the air exchange factor which, given suitable ventilation, will be about two per hour(?) - normal ventilation plus air currents within the building (opening/closing doors etc). The air you warm up will actually need to be done a few times an hour (albeit a continuous process) so that will explain the discrepancy between what you calculate as a minimum and what you require in reality. It's always best to have to much rather than too little capacity in the system anyway. Besides it's not healthy to live in a hermetically sealed box!

Brian

 

[Jenx]

Hi There Roger ....

I'm a heating engineer -- (old school :wink: ) -- to check your calculations, as a "rough rule of thumb" to establish your heat requirements and thus allow you to size your boiler ( regardless of the type of fuel used to fire it ) and your Hot water requirements :

Rooms
Forget tying yourself up in Air changes per hour and design-temperature recommendations for the moment..

Take the cubic volume of your room in cubic feet, then multiply that by
5 - this will give you an approximate BTU/hr requirement for that given room

EG : Room is 15' x 11' x 7'6"
(15x11x7.5) x 5 = 6187.5 BTU/hr
( to convert to Kw - multiply your BTU figure by 0.000293 )
So this room will require an approximation of 6187.5 BTU's/ hr
or effectively 1.81 Kw

-You'll 'know' you're about right, when you see it in terms of thinking of a 3Kw electric fire, for really 'simple' thinking.

Its acceptable to reduce that x5 calculation to x4.5 for bedrooms and still be within a hair's breadth of being as accurate as wont make a hillock of difference to the end result.


Add your total rooms BTU requirement up...
then, depending on how you want to consider how you deliver Hot water.. i.e. you may want to go for a 'Combi', which only heats the water which you require, at the point in time you wish to use it... or if you want to have a storage ( cylinder ) of hot water.

A word of caution - a 'combi' may sound like an economical 'ideal' way to handle your HW requirements.. but bear in mind the number of occupants in the house, frequency of Baths / showers required and the general 'demand'.
The downfall of the combi against a stored HW system is easy to visualise.. the average 'mean' temperature of UK mains water is 4 deg C. Domestic Hot water need really never be anything hotter than 65 deg C.. hot enough to melt the Bacon grease off your dishes after a 'full english' :wink: . Any hotter and you physically have to 'dilute' it again with cold water... so --- the combi, if you choose to go that way, has to instantaeously heat water from 4 deg C to 65. Thats a tall order... and the 'payoff' is that you receive a much reduced flowrate at the HW outlets.. or in other words, it takes forever to run a bath.

There are some combis available now with a small level of storage, but this is never going to be as 'convenient' as having a full 36+ gallons of stored Hot water. ( personally i would go for nothing less than a 42 gallong cylinder if you go the 'stored' route.)

Next HW issue to consider is that - depending on the number of occupants, and assuming you go for 'storage' as opposed to combi, is the 'recovery rate' of that HW Cylinder.. or in other words.. how long will it take it to recover from full depletion from down to just above 4 deg C again, back to the desired 65 deg C. - obviously then, the 'more available heat' you have from your boiler, thats NOT going to the central heating circuits, the better - with regards to the HW... well, within an acceptable reason .... too much available heat when the CH is 'satisfied' or off, as in summer, will only result in the Boiler 'short cycling, or coming on and off repeatedly, which highly fuel in-efficient.
Many of todays 'heating thinkers' will tell you to allow 10,000 btu for Hot water... I would advise and advocate 20 / 25,000 and this will allow a more suitable recovery rate of HW, during periods of High demand on the boiler, without overly thermally stressing the boiler through short cycling during the summer months.

Heating is all about 'trade-offs' ....
Its all well and good to consider all the air changes per hour and other supposed necessary design criteria, but these take little or no consideration of 'real world' scenarios of grandkids who leave windows and doors open, a perticularly active cat that wont leave the catflap alone for 2 minutes, a busy housewife who is out to the washing line more often today than usual... you get my drift, its all 'theoretical' and 'ideal world scenarios'.
Most important to consider is the electrical / electronic / temperature controls.. and these are what will save money... there's only one real way to save money after all, and thats 'turn the thing off completely'...
but again, to emply the 'trade-off' mentality.. the more accurate and good quality controls you can employ in your systems, the better the performance of it will be ... but again, bear in mind, the more 'automation' there is to anything, the more problematic it will ultimately become, and the costlier to repair.

Again - to think of the very basic terms ... the object is to put some kind of 'fire' beneath a box of water, and circulate it to wherever that water's heat can be exchanged either to water, or to air.
You can make this as simple or as complicated as you wish...

so to summarise.. to check your calcs ...

CUBIC VOLUME OF ROOM ( in CuFt ) x 5 = BTU requirement for that room ( you can drop to x4.5 for a bedroom, quite happily )

Allow 25000 more BTU's for your HW

Add them all up.. that'll give you your heat requirement for your house.. you can, if you so wish, factor in a small amount of additional safety ( an extra 10,000 or so... but really, you won't need it.. its done by people who cant add up as a 'cover my backside' exercise...
select the boiler to suit, ... job done :wink:

All online calculators / Mears wheels etc etc will oversize your system by a minimum of 15%, cause they dont want any 'comeback' from a dis-satisfied houseowner who is 'shy' of enough heat to be comfortable.. as I say, heating a domestic dwelling is not an exact science.. it cant ever be due to the amount of variables involved.

Beware the nasty 'Gas reg's that now prevail of course, if you go the Gas route.. The powers that be are hell bent on the usual 'blame culture, but of course its still OK for B&Q to flog a gas boiler to anyone willing to pay for it, and merrily go off and connect it up with jubilee clips and some garden hose.. but that is another story !

If you need any help - don't be shy in PM'ing me... i'll help you wherever i can.

Alun

 

[RogerS]

Thanks, Jenx, for the very detailed reply.

Does your approach factor in the latest Regs re huge slabs of Kingspan, for example?

 

[White House Workshop]

I like your way of looking at the problem, Jenx, as t's a nice simple way of doing it, but what about taking into account temperature differentials?

It takes a lot more heat to keep a room warm when the outside temperature is consistently below zero. I did this for our house in Massachusetts when I threw out the stupid baseboard and put in radiators. The heat requirements for an average outdoor temp of 20 degrees F were much higher than at 32, for example.

There are also heat gains to be considered from adjoining rooms, downstairs versus upstairs, attic insulation factors etc. Then there's the question of what temperature do you want your room at? Some people like 21 deg C, others (like me) are very happy with 18 in the living areas and 15 in the bedroom, although I do like the bathroom warm!

Here's a link to a heat requirement calculator that uses Jenx's approach - but note the long list of cautions at the bottom!
http://www.herman-nelson.com/btu_calculator.cfm
Note also that a critical factor is the outside temperature. Play around with it and you'll see what I mean.

 

[Jenx]

Hi Roger & WHW ...

The approach was given to let Roger see if his calculations were 'wildly adrift' of the mark, or reasonably accurate.
Heating is indeed a very 'non-exact' science, as there are so many variables that can change from minute to minute, never mind day by day. That 'rough' formula is applicable to 'mean average UK' situations, and I would be highly inclined to 'up' the multiplication factor for regions such as WHW describes, where the outside ambient temperature is as low as 20 Degrees F.

On the matter of Kingspan ( Insulation panels ) --
Insulation is another thing often mis-represented in the heating of buildings - .... Insulation in whatever form, will Not prevent heat loss, nothing can.. what it will do, is Slow Down the rate of heat exchange between areas of differing ambient temperatures.
To expand - take for example - two adjacent rooms, one heated to lets say 28 degrees C and the other having no form of heating within it at all... 'Heat' wants to equalise that difference, buy losing temperature from the hot room, transferring to the cold one.
The greater the difference in temperature between the two areas, the faster the rate of heat exchange will be, and as the difference begins to equalise, the rate or speed of this exchange begins to tail off.
Insulation, regardless of its construction, density, etc will seek to slowthis rate of exchange down.
This is true for the losses between attics and the heated area, underfloor voids and the heated area, adjoining ( abutment ) garages or storage areas on the side of the building etc etc.

When designing systems - Ideally, one takes into consideration all the materials of construction of a given building, and each one has a 'U-Value', which is a figure given to represent that particular material's insulative properties. next, the system is configured to take 'complete volume' air-changes into the equation, i.e. - to allow for completely removing and replacing the volume of air in each living space of the building 'x' times per hour, with fresh air at the average outside temperature, and to subsequently have enough heat input to re-heat that air to the design temperature.
As WHW says " Design Temperatures" are again a 'balancing act', as what is comfortable for a young lad of 21 years of age, is generally accepted as not warm enough for an older lady in her 80's...
( you can see then, when considering all these variables, of how much of a 'black art' heating is )
To go even further into it.. the design engineer should be seeking to achieve an optimum temperature of the 'flow' leaving the heat source, and an optimum temperature difference between flow and return pipework, as they enter and exit the heat source. ( from memory. thats 22deg C, but i won't swear to it :wink: ) .
You then have to consider the speed of water circulation ( generally regulated by the variable speeds on the pump, and the need to 'balance' the system in order to avoid whats called 'index circuits', or circuits of the greatest frictional resistance in the pipework against the developed head ( circulating force ) of the pump. - Blancing was traditionally achieved by varying the open orifice of the individual lockshied radiator valves, but more recently has been easier to achieve by the use of Double regulating valves & metering stations installed into the pipework.
As you can see, with all these variable controls on flow rate via the pump(s) and variable orifices in valves etc... the facility to 'compensate' in many areas of the system is almost 'overly provided-for'.

Then of course, what happens is, we fit TRV's to all of our radiators.. and the lady of the house covers the heads with her curtains, and the thing 'hunts' all over the place or worse, shuts off far too early.
( remember - these controls are sensing the surrounding AIR temperature, NOT the water temperature within the system... obstruct the free movement of air round about it, and it'll never work in a month of sundays.

Assuming you gat all these calculations right or slightly wrong.. your heat source or boiler is still able to come to the rescue.

Coal / Solid fuel : -- build a bigger fire in the firebox !

Oil - "Up" the pressure developed by the burner's oil-pump, and re-adjust all the combustion settings to suit ( i.e.- re-comission ), and you can achieve a huge variance in input/output

Gas : As per oil - 'up' the burner pressure within the permissable perameters of that given appliance to achieve the variances in input / output. ( equally applicable to atmospheric or 'blown gas' burners )

As we mentioned before - heating is all about trade-offs and achieving an 'optimum balance' to find what best suits you and your family members... the system will by default, be very 'forgiving', in allowing quite wide variances in what it can deliver.

To Summarise -
1.Insulate as best you can

2. Ensure your pipework / piperuns are sized to be able to deliver enough volume of heated water

3. Try to position your radiators where they are most effective ... remember, they are 'misnamed' .. they're convectors really - 30% of their heat output is by radiation, 70% is by convection. - try and fit them under the windows of possible - that is the optimum position for their siting, as they will 'warm' the colder spots of the room

4. Ensure to utilise the best 'controls' setup you can afford -- programmable roomstats, etc etc.. the greater and more effective your control system - the better your system will perform

5. Its worth having the system comissioned by a qualified engineer - essential in the case of gas in the Uk of course, and well worth it in the case of oil, as an incorrectly setup burner is potentially very messy.. Oil is however, very very safe - try as you might, you will never be able to make an oil burner produce carbon monoxide - it cant !

Again - Please PM if you need specific help ! :wink: 8)