Calculating Energy Demand from Heat Load

[Samo117]

Please help, I have a designed new build domestic home which I have calculated the Heat Load for according to Eurocode standard BS EN 12831 2003 which gives a value in Watts.

I'm now trying to work out the energy demand of the space heating in Kwh/year to establish the cost and carbon emissions of heating the house, but can't work out how.

I have the Heating Degree days for my location, NDD and the average 24 hour temp for each month, Tavg.

I have calculated the heat load for each month HL using the average 24 hour temp as the external design temperature. Then to calculate the energy demand, Q, I thought you did the following

Q=HL*NDD*24/1000

but the results i'm getting are far too high (564,000 kWh/a) When I know the average gas consumption for heating a domestic home is 16,500 kWh/a). What am I doing wrong?

Thank you for any help you offer

 

[bengasman]

You can not calculate this. Why on earth would you want to anyway?

 

[Samo117]

Im trying to work out how much gas fuel it would take to heat the home in a year?

 

[Agile]

Why not tell us what you have calculated the heat loss at and the basic size of the property ( e.g. 26 room castle ! )

Tony

 

[bengasman]

Im trying to work out how much gas fuel it would take to heat the home in a year?

It is impossible to calculate that with any accuracy. Apart from that, why would you want to know how much gas you may use?
A much quicker and accurate way is to use the estimation of 3 large gas suppliers for your type of house.

 

[Samo117]

Sorry guess I havn't explained very well. I'm trying to establish how using different materials it a buildings envelope effects the energy required to heat the home and therefore the carbon savings achieved by the different materials. I thought this could be done using the heat load for the different setups.

The building is a 3 bedroom domestic home about 170m2 internal floor space and when using 'typical' building materials I've calculated a heat load of 15.39 kW and using highly insulating materials I get 9.6 kW
both are calculated uing an external design temp of -1 deg C

I was trying to calculate the gas fuel a heating system would use so I could quantify the savings a higher insulated building would have.

I apologise if i'm way off the plot here, I'm a civil engineering student designing a home for my dissertation and have very little knowledge in this area

 

[bengasman]

As it is only theory, it doesn't matter if it matches the reality I suppose.
Calculating the gas use would be unnecessarily complicated as you could just as well stick to the difference in heat losses.

If you don't mind me asking: how can you do a dissertation on a subject you know nothing about?

 

[Samo117]

Haha yeah good question! Its more of a literate review and this design is only one part of it to help enforce some concepts im discussing.

So can I not use the heat load to estimate carbon or bill savings in any way then?

What is a heat load typically used for?

Thank you for your help by the way

 

[bengasman]

...So can I not use the heat load to estimate carbon or bill savings in any way then?

Not with any degree of accuracy; main reasons:
Price per kW for one house can be 40% higher than next door.
Gas use depends on many factors, most of them due to fluctuations of 10% or more each e.g. temperature differential between design temp and real, easily 10% and temperature differential between design temp and outside temp. Average for today is 13 outside and 21 inside -> 8 degrees. The mean deviation over the past year has been 5 degrees below average which would bring the differential to 13 i.e. and therefore a 60% increase.
Those 3 alone add up to 100% variation of the calcs.

What is a heat load typically used for

Define " heat load for general use"

And a tip for you about "carbon savings" , the general public is rapidly realising that the whole anthropogenic global warming malarkey is little more than a ploy to hit us with taxes.

There is no evidence that will stand up to scrutiny to prove that global warming is anthropogenic.

There is no evidence that will stand up to scrutiny to prove that CO2 is the main reason for global warming.

And even if somebody managed to "prove" the above, oil comes from prehistoric flora that absorbed that CO2 from the atmosphere. Burning it will only put the carbon back to where it was originally and thereby restoring the planet as "nature designed it to be" if there is such a thing.

 

[Samo117]

Well the heat load is the energy required to achieve required temperatures in a building at any one time, is that right?

So would you use the heat load to size a boiler?

or what other uses does knowing the Heat Load have?

 

[bengasman]

Well the heat load is the energy required to achieve required temperatures in a building at any one time, is that right?

Not a specific definition; different people use different names for different uses

So would you use the heat load to size a boiler?

No, not usually, for reasons stated above; it is a purely theoretical exercise with lots of allowances, tolerances and guesses.
Then there is the matter of available space, specific wants, needs, uses and habits of the owner e.g. born in Scotland or Jamaica, doors mostly closed or open, home all day or rarely.

I ask the owner how he uses the house, look at the room for 3 seconds, and guesstimate the size of the rad needed; not had a complaint yet.

New build projects do get "carefully" calculated, and are notorious for heating problems. It is a futile exercise where the problem is matched to meet the solution.

 

[Agile]

What is a heat load typically used for?

The heat load we use is the instantaneous heat requirted to heat to 21° when its -1° outside.

Thats all you need to compare insulation!

A typical house with a 15 kW heat loss ( inc 2 kW for water heting ) costs about £1000 a year in gas.

I also wonder if you might not be better doing a dissertation in something you know about.

Maybe we should do your dissertation and get awarded the degree?

Tony

 

[Samo117]

Well thats my questions answered
Thank you both your time and help

 

[ianniann]

So it looks like you came to the wrong place, Samo. These guys can get within about 30% by sticking their finger in the air, but seem to lack the intelligence to realise that heat in = heat out and it can be calculated. Of course whether the added precision is meaningful or not only you can tell.

You say you have calculated the heat load, but it seems to me that you have only calculated one heat load and that is the one required to maintain (or achieve?) the set temperature when it is -1C outside. Clearly you don't run your heating at that level every hour of every day, so your total is massively overdone. That heat load even looks more like the required boiler power rather than the average heat load (excluding hot water?) over a 24 hour period which would be more like 2kW.

You need to calculate the heat load daily or monthly based on the average temperature for that day or month. Or better yet on a statistically valid distribution of temperatures since heat loads are not quite symmetrical about an average temperature. I don't know if you have calculators for that but it can be done from scratch using a model of the house and the various U values, plus the temperature differentials between inside and out, plus ventilation. Then you can add up your 12 or 365 heat loads to get the total heat requirement for the year. At your desired level of precision you must also take into account the efficiency of the system at converting fuel into heat inside the house, probably around 90%.

 

[Samo117]

Hallo ianniann

That does sound more like what I assumed the procedure would be.

My calculations are for the space heating only yes

I've calculated the heat load using the monthly average temperature and summed them as you said and I get 85.8kW.
But this value doesn't seem to take into account the period of time at all?

The calculation doesn't use any input for the value of time and the resultant Heat load is given in Watts. That's why I thought you had to multiply the heat load for the month by the number of heating degree days for that month, which would add the time period into the equation?? Times this value by 24 to convert from days to hours and I get a heat supply requirement in kWh for the year. No??