IES-VE User Forum

I may be wrong in this because the mechanics are not well documented, but I think the problem here is that the temperature profile you applied in Apache Systems will still be counted as a heat load. It's designed for systems with an air handling unit with local heating (either a fan coil unit, radiators, etc). So if you have an Earth tube, the way you've done it will not show any energy savings because the computer still thinks that it needs to heat that supply air.

If you want to stay out of Apache HVAC (which I think is the way to go here, but it may take too much time), the easiest way to go about it is to substract the heating energy by post-processing the Vista results in Excel, using the equation q= m' × Cp × (To - Ti)/η-boiler, where m' is the air mass flowrate, Cp is the specific heat of air, To is 15°C, Ti is the continuously fluctuating external air temperature as per your weather file, η-boiler is your boiler efficiency, and q is the energy delivered to this air.

This won't be perfectly exact as there are internal coefficients within IES that I haven't been able to figure out, but you'll be within 2% of your requested figure which is close enough for me.

Hi Silvestre,

I think using natural ventilation would work as well, I didn't think of that. The only thing is that you will then need to account separately for the energy required to run your system. Unless your system truly is natural ventilation (which I doubt) it will use auxiliary energy, and if you are trying to calculate the energy savings there is an energy cost to mechanical ventilation. You could calculate it by running a separate simulation with mechanical ventilation and extracting the auxiliary energy from there.

A side note, I wonder where you got 15°C as output from the earth tube (aka puits canadien). Assuming you're doing this project in southern Québec, I very much doubt (a) that the ground will even reach 15°C in February (it varies seasonally), and (b) that the air coming out of the heat exchanger would even reach 15°C (if the ground is 15°C you need to account for heat exchange effectiveness and it's not 100%). I'm assuming here that you're asking my advice, and if you are I'd have the air supply vary seasonally using a profile.

Here's a very rough draft off the top of my head

(a) Calculate the estimated ground temperature 2m below ground (or more) using published weather data appropriate for your site. Calculate an average temperature for each month.

(b) estimate the output from the puits canadien using the relationship Te = η × (Tg - To) + To, where η is the heat exchanger effectiveness, Tg is the average monthly ground temperature (you'll need to repeat this profile equation for all 12 months to cater for variable ground temperature), and To is the outside air temperature taken from the weather file.

Apologies if you've already thought of this or already have a way better solution. I just thought 15°C was an aggressive assumption, and I'm just trying to help.