Planning stage for Geo Thermal

[kimana]

Hi,,Just joined the forum.I have lurked in the past and always enjoyed the members contributions and the great advice.My 25+yr old oil fired hotwater furnace took a poop and now I am looking at the trend in geo thermal.At the present I am doing my heat -loss calculations.I have downloaded a program to determine the size of the system that I need,so my question is..I am using70F as my indoor design temp,but what do I input here in South central Pennsylvania (Hershey)for my outdoor design temp?Is 13F good or too high?

[Bill Arden]

The outdoor design temp is the lowest temp that you have.
http://www.everything2.org/e2node/outside%2520design%2520temperature

Here's a list
http://www.engineeringtoolbox.com/us-outdoor-design-temperature-humidity-d_296.html

Pennsylvania = 0F

Edit: another way is to use past fuel costs to estimate your heat loss.

[sixlashes]

Where I am, the power company will give you the design temperature(s). I would start with them. I suspect you are using the same (HVAC) Manual J calculations software I used. The design temps it listed for Pensacola, Fla were 2 degrees different than the what power company used to complete their energy audit. I went with the power company's numbers.

Good luck on your journey. I am installing a open loop system and am at the point of obtaining a permit for reinjecting the (cooling) water back into the ground. The guys in this forum are invaluable in pointing you in the right direction concerning design strategies.

[jadnashua]

If, for example, you use 13 degrees, (and I think that may be low), and the system ends up being exactly the right size for that load, and it ends up being zero out one day, the house temperature may drop that 13 degrees if it is that cold long enough and your insulation isn't great. Most of the boilers end up being at least slightly oversized.

One thing that can greatly increase the efficiency is if you have a modulating boiler - it can adjust the water temperature to match the load which increases comfort and efficiency.

[Bill Arden]

Just remember that the efficiency decreases as the temperature of the water or air that heats the room increases.

This effect is a lot worse with heat pumps than with oil boilers.

A oil burner's efficiency decreases only a small amount.
For example Increasing the heat temperature from 120F to 160F will reduce efficiency by the amount of air being exhausted times the temperature difference. This is probably about 10% to 20%

A heat pump, on the other hand, looses efficiency a lot faster.
For example with a ground temperature of 60F and Increasing the heat temperature from 120F to 160F will reduce efficiency using the ratios.
(160F - 60F) / (120F - 60F) = or ~60% of the 120F version
This is ~40% reduction in efficiency.

Just remember that heat pumps are the most efficient way to heat a house as long as everything else is also done well.