Heat Gain in Buildings

Squall Leonhart

Hi,

I am really hoping that somebody will be able to assist me in working out the heat gain of a building, as the transmitted heat gain through a structure.

I.e., the sun will be shining on a building all day (well, ideally.. not so much in Ireland ), heating the walls, and the roof of the building. The heat of which will slowly be radiated into the house in the evening/night. Perhaps not to any perceptible level (winter) but can be quite notable (Summer).

Can somebody assist with formula for this?

It's a standard 300mm block wall, with a U-Value of 0.21 w/m2K.
The roof has a U-Value of 0.16 W/m2K, and is just tiles on timber battens/rafters.

Any further information needed I'll provide it as needed, I've quite a lot of information about the building.

Thanks,
Paul.

roy rodgers

yeah i remember there was a formula for that when i was in college doing my plumbing course . all i remember was each building had a differen U-value and there was a table to work off from. i'llhave to go and look for that and let you know.

Squall Leonhart

Ah that'd be great if you could, cheers man.

FISMA

Paul,
Probably best to ask a civ-e this question. I could help you with heat flows/time or inside outside temperatures.

Also, what is the "u-value?" Is that a thermal conductivity of the material? The units of thermal conductivity are W/mK - you look to have m² where I have meters.

In Physics I have heard of the "R" value (thermal resistance). R = l/k where l is the thickness of the material and k is the thermal conductivity.

Finally, what exactly are you looking for: (a) heat flow/time, (b) total heat flow, (c) temperatures inside or outside, or something else?

Give this link a read - this might be what you need.
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatcond.html

Sample Calculation
What is the rate of heat flow through a window 2.00m x 2.50m, 3.50mm thick, if the temperatures at the inner and outer surfaces are 15.0°C and 14.0°C?

Basically, if this is your setup, you can plug and chug into the above equation. You want to find the flow rate of heat.

Kappa (that funky k above) is just a constant - thermal conductivity. I pulled a generic value of 0.84J/smC°.

So,

ΔQ/Δt = kA(T1-T2)/l
ΔQ/Δt = (0.84J/smC)(5.00m²)(15.0°C -14.0°C) / (3.50 x 10-³m)

ΔQ/Δt = 1200J/s = 1.2x10³J/s (only two sig figs here)

Hope that helps...

I could spend a bit more time on this over the next few days - more on the weekend. Let me know if this is in the ball-park of what you are looking for or if I am barking up the wrong tree.

Where are we going to start? Are you going to be able to give me the temperatures as shown above as a function of the time of day? Wouldn't that be nice!:D

Or, are we going to try and figure out the energy that is coming from the sun and heating up the wall?

A problem figuring out temperatures wouldn't be too bad as well.

Slan

dathi

paul post your request on the construction forum one of the lads with the passive house software will help you

Sev

It seems to me that if you simply multiply the U-value by the area of the walls and ceiling of your building and then by the difference in the temperature between inside and outside you will get the rate at which heat energy is being transferred to or from the inside of the building.

This value is of no practical use unless you know the quantity of air in the building and it's heat capacity. This will then give you an idea for how quickly the temperature inside the building will change.

As for your mention of the walls storing heat from the sun during the day and reradiating itat night. It doesn't seem to me as if you have provided enough information to speak about this quantitatively. You would need to know about the heat capacity and thermal emissivity of the walls.

Squall Leonhart

Hi Sev,

Thanks for the response.

The wall will be a 100mm concrete block outer leaf, 30mm cavity, 70mm insulation, 100mm block, with a 15mm coat of plaster/render on each leaf.

Specific heat capacity of:

concrete block 1000 joules/kg K
Plasterboard 840 joules/kg K
external render 1000 joules/kg K
insulation 1400 joules/kg K

Volume of air within the house, 413.34

....where do we go from here?

Sev

If we ignore radiation and convection and the heat capacity of the walls etc. the problem is simple. Given outside air temperature, inside air temperature and the areas of the walls, we have a handle on the rate of energy going into or out of the house. If you simply divide this by the volume of air in the house and it's heat capacity (per volume). You will have an idea of how fast the temperature will change.

But to be honest, I don't know anything about civil engineering and that was just guess work on my part based on an education in physics. So I don't really know where to go next!

I would expect a full treatment of the problem to be very complicated. Trying to decide how fast the house heats up during the day depends on so many factors: whether it's sunny or cloudy, the outside air temperature, the emissivity of the roof, the heat capacity of the walls, whether its windy or raining etc.

I suspect civil engineers use a number of approximations to make these estimates, but I have no experience with it.

BryanF

hi paul,
as suggested above the easiest way of calculating the heat gain is to input the building data into PHPP (passivhaus software)
May I ask what this calc is for? as imo you seem to have a standard enough construction going on there, so it'll be heat loss you should be worried about.

have you some particular building materials you are measuring to retain the 'the heat gain of a building', generally most buildings (even in Ireland)

Squall Leonhart

The specific purpose of trying to calculate this is to flesh out a dissertation I'm working on for an architectural technology course. The title of which is "Is mechanical ventilation with heat recovery a viable alternative to natural ventilation in new build houses in the southeast of Ireland?"

Working with a set of drawings I have, I have been trying to calculate the heating requirements of the building by determining the total heat loss through the fabric of the structure, taking into account the solar gains, as directly made through glazing and any heat that may be re-radiated into the house due to the effect of sun shining on walls for a few hours. Also taken into account is thermal gain from occupants and occupancy behaviour. Run these calculations with gas as the fuel source @ €0.04 per unit, taking into account the efficiency of the gas boiler, to obtain a running cost for a year.

Compare this with an MVHR system with 90% heat recovery efficiency, taking into account also the initial cost for the unit and installation, compare the figures for both scenarios... and boom, an answer

That's the plan anyway!!

FISMA

Paul,
I will get back to you in a day or two, just a bit busy...

BryanF

Great question, conclusion: depends if by 'new house' you mean to current building regs?
check out PHPP, the main factor in whether MVHR is appropriate is not the heat gains, its the air-tightness. + occupant behaviour, maintenance, and product specified.

Irish Building regs & DEAP are crap and do not consider heat gains as being a major factor. they do not require a low enough air-tightness factor either.

first things first transfer your questions to the arch tech section (one of the mods will oblige), and explain your studies. I think you need to do alot more reading before you attempt this question though.

I'm not particularly biased towards the use of MVHR, i just find that the natural/passive alternative systems are not quiet there yet.

hope this helps and the best of luck

see some links and papers for your consideration below:
www.aecb.net/PDFs/9Jan2009_MVHR_Final-2.pdf
www.veetech.org.uk/Building%20Regulations%20Veetech%20Response%20Web.pdf
http://webarchive.nationalarchives.gov.uk/20110118095356/http:/www.cabe.org.uk/sustainable-places/advice/overheating
http://constructireland.ie/articles/0213mater3.php
http://www.gaiagroup.org/Research/RI/DI/index.html
http://constructireland.ie/Articles/Part-L-Building-Regulations/Examining-the-key-changes-in-Irelands-new-energy-standard/Page-3.html
www.greenbuildingpress.co.uk/article.php?article_id=286
http://www.cmhc-schl.gc.ca/publications/en/rh-pr/tech/96215.htm
http://www.building.co.uk/passivhaus-diary-part-14-ventilation/3153359.article
http://www.engineeringtoolbox.com/heat-recovery-efficiency-d_201.html
http://www.sei.ie/Renewables/Renewable_Energy_for_the_Homeowner/SEI_Passive_House_A4.pdf
http://www.passivehouse.us/passiveHouse/Articles_files/EDU%20Jan%2008.PDF
J. Korsgaard, (1991) Mechanical ventilation and house dust mites: a controlled investigation. In: D Van Moerbeke, Editor, Dust mite allergens and asthma., UCB Institute of Allergy, Brussles 1991, pp. 87–89

TAE Platts-Mills, ML Hayden, MD Chapman and SR. Wilkins, (1987) Seasonal variation in dust mite and grass pollen allergens in dust from houses of patients with asthma, J Allergy Clin Immunol 79 1987, pp. 781–791.

A. Leaman, B. Bordass, (1999) Productivity in buildings: the ‘killer’ variables, Building Research & Information 27 (1) 1999 4–20.

M. Paciuk, (1989) The role of personal control of the environment in thermal comfort and satisfaction at the workplace, Ph.D. thesis, University of Wisconsin-Milwaukee, 1989.

G.S. Brager, G. Paliaga, R. de Dear, (2004)Operable windows, personal control and occupant comfort, Ashrae Transactions 110 (2) 2004 17–35.


Bornehag, C.G. et al., 2005. Association between ventilation rates in 390 Swedish homes and allergic symptoms in children. Indoor Air, 15(4), 275-280.


Niven, R.M. et al., (1999). Attempting to control mite allergens with mechanical ventilation and dehumidification in British houses, , Journal of Allergy and Clinical Immunology, 103(5), 756-762

Erhorn, H., 1988. Influence of meteorological conditions on inhabitants' behaviour in dwellings with mechanical ventilation. Energy and Buildings, 11(1-3), 267-275.