|
You are not authorized to post a reply.
|
 Prev Next
|
| Author |
Messages |
|
gregj
Registered Users
Basic Member
Posts:200
 |
| 11/19/2008 2:30 PM |
|
I'm in the somewhat upper midwest where our heating requirements are approximately 2 1/2 times the cooling requirements. I haven't gotten a solid load calc done but it will be very close to 2 1/2 ton cooling and 75,000 BTU heating.
I know I'm not suppose to oversize my A/C so if I go for a geo system how is the best way to handle the large difference in heating/cooling requirements?
I'll also throw in another twist. If I wanted to supply at least some of my heat through a floor radiant system what would you recommend a water to air unit sized for the A/C load supplemented by a water to water unit sized for the difference between the heat and A/C load so that on the coldest days both the radiant and forced air units run? My concern with this approach is that two units totalling say 6 tons might cost substantially more to install than one 6 ton unit.
What approach do you pros advocate? |
|
|
|
|
|
|
tuffluckdriller
Registered Users
Advanced Member
Posts:593
|
| 11/19/2008 5:43 PM |
|
My first question is regarding the load. What was the outdoor design temp for the heating load? Also, how accurate is it?
Assuming your loads are correct/accurate, you'll find that heating load takes more than a 6 ton. More likely, it'd be 7 to 8 tons, depending on where you need the balance point to fall and still be efficient.
If the loads are correct, I'd suggest 2 heat pumps. If you wanted both units to heat water for radiant, and one of those two to cool with forced air, then I'd suggest the EarthLinked product. I'd use a 4 or 5 ton HC unit to heat radiant water, and a 3 ton SW model for heating/cooling air AND heating radiant water. This is one of the great features EarthLinked offers. Plus, you'll likely save money on the ground loop. In fact, if you wanted the 4 or 5 ton to heat the domestic water, too, NOT DESUPERHEATER, I'd use the HW model. It will heat hydronic water, AND it will heat domestic water. |
|
Clark Timothy (clark@pinksdx.com)
VP sales, Tuff Luck Geothermal Drilling
Geothermal Heat Pumps: Heating and Cooling that's Dirt Cheap! |
|
|
engineer
Registered Users
Advanced Member
Posts:911
|
| 11/19/2008 8:43 PM |
|
You may find size of your present ducts (if any) limits amount of tonnage you can supply via forced warm air. Heating air from a geo is warm (~100 F) vs that from fossil forced air, which is often hot (~150 F) That means much more airflow is needed to supply a given Btuh load.
Definitely be certain of your load calc with correct indoor and outdoor, summer and winter design temps.
The hybrid system you described (some heat via water to water) could be a winner. Do definitely also consider NOT meeting your entire heat load with geo tonnage - perfectly OK and generally cheaper in the long run to let the last ton or so be met using heat strips - they'll only operate for a few hours on the coldest days, and the upfront cost reduction of knocking a ton or so off a geo system will generally more than cover the slightly increased operating cost on the coldest days.
Somewhat oversizing for cooling can be mitigated via a two stage system, but meeting a 2.5 ton cooling load with a six ton machine is far from ideal. |
|
Without data, you only have an opinion. |
|
|
joe.ami
Registered Users
Veteran Member
Posts:1105
|
| 11/19/2008 10:03 PM |
|
Not sure where you are located, but in MI, a 70ishKBtu load would generally be met with a 4 ton to 5 ton system. AUX heater makes up the difference for $75ish/year. Duct work also key in determining system size (if retrofit). Water to water has lower COP's with geo, but mix and match can be done.
Good luck,
Joe |
|
Just a Mechanic;
Geothermal; Savings Underfoot |
|
|
geodean
Registered Users
Veteran Member
Posts:1030
|
| 11/20/2008 12:37 AM |
|
Some heat pumps are combo units capable of doing both forced air and hot water from the same unit.
As others have said, a two stage unit helps to handle the disparity between heating and cooling modes.
|
|
Dewayne Dean
PalaceGeothermal.com
Why settle for 90% when you can have 400%
We heat and cool with dirt!
visit- http://welserver.com/WEL0114/- to see my system |
|
|
gregj
Registered Users
Basic Member
Posts:200
|
| 11/20/2008 1:12 PM |
|
Thank you all. This gives me a lot to think about.
I still need to get a real Manual J done so the loads I gave are ballpark based upon how well my current 2 1/2 Ton A/C works (barely does the job on the hottest days - but does it) and a "typical" figure a local HVAC guy gave me for a propane furnace. I'm near Lincoln Nebraska. Don't know the design temps for the area. How would I go about finding them? I also need to find out more about this "balance point" and how it impacts sizing - I'm not familiar with the term at all.
If I understand ductwork sizing right when you have propane furnace with A/C the ductwork is designed for the A/C cooling requirements of around 400 cfm per ton so around 1000 cfm for my 2 1/2 ton. And that flow also works well for the hot propane heat but not for the lower heatpump heat temperatures. So do the lower heat pump temps also require 400 cfm per ton of heat? Then I'm sure I need improvement in the ductwork. Basement isn't finished yet so access is pretty good.
Electric rates here are only around 5 cents per kwh so I guess I shouldn't be too afraid to rely on help from the electric resistance strips for the coldest days.
I clearly need to get that manual J done because it looks like the best direction to go will probably depend on the ratio of cooling to heating I need. It sounds like if I get real lucky and my actual heat load was a bit smaller (say 50000 to 60000) then I could possibly use a 4 ton 2 stage with electric strips to make up the difference. But I may be a bit too far North for that.
So when doing the manual J calc to determine heating and cooling requirements does the ratio between heating and cooling requirements come down to the difference between the design temps and the desired house temp? For example if the A/C design temp is 95 and designed to achieve 75 degree temps (20 degree difference) and the heating design temp is 10 and designed to achieve 70 degree temps (60 degree difference) then will the manual J heat load be three times the cooling load (60/20)?
Last question, most geo pumps seem to use freon circulating A coils for cooling just like a regular air source heat pump or A/C but there are water to water geo pumps that use a an A coil through which cold water rather than freon circulates. Do these water circulating A coils (don't know the right name for these) remove humidity better, worse or the same as the freon A coils?
I guess where I'm going with this is if I go with a large water to water unit for heating is it practical to also use it for cooling with a water A coil even though the geo heat pump will be way oversized for cooling? I assume the water coiled units have the same humidity control issues as the freon coiled units when oversized? If so could this be mitigated by somehow using a buffering tank and pump to a smaller A coil so that the air handler/ACoil/buffer tank pump unit runs longer but the actual compressor runs less frequently to maintain the buffer tank temp? |
|
|
|
|
engineer
Registered Users
Advanced Member
Posts:911
|
| 11/20/2008 7:26 PM |
|
"So when doing the manual J calc to determine heating and cooling requirements does the ratio between heating and cooling requirements come down to the difference between the design temps and the desired house temp? For example if the A/C design temp is 95 and designed to achieve 75 degree temps (20 degree difference) and the heating design temp is 10 and designed to achieve 70 degree temps (60 degree difference) then will the manual J heat load be three times the cooling load (60/20)?"
It is a bit more complicated than that - a good part of AC load is dehumidifying, and the geo unit moves a bit more Btuh per CFM in heating mode than cooling.
I'll hazard a guess and figure that your ratio will be on the order of 2:1
"Last question, most geo pumps seem to use freon circulating A coils for cooling just like a regular air source heat pump or A/C but there are water to water geo pumps that use a an A coil through which cold water rather than freon circulates. Do these water circulating A coils (don't know the right name for these) remove humidity better, worse or the same as the freon A coils? "
That can be done, but it takes quite cold water (~42 F) for good dehumidification - most groundwater isn't that cold. Big commercial systems often chill and circulate water as you describe.
"I guess where I'm going with this is if I go with a large water to water unit for heating is it practical to also use it for cooling with a water A coil even though the geo heat pump will be way oversized for cooling? I assume the water coiled units have the same humidity control issues as the freon coiled units when oversized? If so could this be mitigated by somehow using a buffering tank and pump to a smaller A coil so that the air handler/ACoil/buffer tank pump unit runs longer but the actual compressor runs less frequently to maintain the buffer tank temp?"
Oversizing causes short run times resulting in poor dehumidification in some climates. What you describe could work but would be non-standard, complicated, and expensive - typical HVAC contractor would not know how to make it work.
My advice:
1) Get a proper Manual J
2) Consider 2 stage
3) Compromise a bit by having resistance for last ton or so of heating.
If for example it turns out your heating load is 5 tons, get a 4 ton 2 stage with 5-10 KW of strips. Could even go as low as 3 ton for better cooling but somewhat more expensive heating.
There may also be so low hanging weatherization fruit to pluck - caulking air leaks, adding attic insulation - those can be cheap ways to reduce heat load.
At some point Manual S needs to be done as well - matching the unit to the actual waterside conditions to establish actual capacity rather than just going by nominal tonnage. |
|
Without data, you only have an opinion. |
|
|
|
| You are not authorized to post a reply. |
|
|
|
ActiveForums 3.6
|
Professionals Serving
Your Location:
|