The geothermal discussion thread

[jkish]

If your unit is sized properly it should run close to 100% of the time during your hardest season.

While a properly designed system with the right size unit is critical to energy savings, a properly designed system should not run 100% of the time at the extreme. At least not if your house is properly insulated and as air tight as possible. The closer it runs to 100% of the time, the less likely your system is designed properly, or the less air-tight your house is.

A properly designed system runs at 100% of capacity when the outdoor temperature reaches the design temperature.

Typically, a Manual J calc is done that determines BTU loss at the design temperatures. Manual J suggests the 97.5 percentile outdoor design temperature for cooling, 2.5 percentile outdoor design temperatrure for heating, and 75 degrees for indoor design temperature. The equipment is picked to match the BTU gain/loss at design temperture.

 

[beelzerob]

So then, *should* the spec'd BTU's of the unit match the BTU loss projected for me in the heat loss calc? because I was pretty sure it didn't. I'd have to go look at it again.

 

[jkish]

So then, *should* the spec'd BTU's of the unit match the BTU loss projected for me in the heat loss calc? because I was pretty sure it didn't. I'd have to go look at it again.

Yes, the heating capacity of the system should at least be equal to the BTU loss at the design temps. It would be interesting to know what design temps they were using.

From what I've seen, on the heating side they sometimes even oversize by a small amount as a safety factor.

On the cooling side, they don't because it affects humidity removal capability - longer run times are better there.

 

[apostolakisl]

If your unit is sized properly it should run close to 100% of the time during your hardest season.

While a properly designed system with the right size unit is critical to energy savings, a properly designed system should not run 100% of the time at the extreme. At least not if your house is properly insulated and as air tight as possible. The closer it runs to 100% of the time, the less likely your system is designed properly, or the less air-tight your house is.

No offense, but what you say is not true. By insulating your house you get the privalege of putting in a smaller system. Even if your house is poorly insulated, you should not put in a system that is bigger than the smallest necessary to do the job. The most efficient unit is the smallest unit possible to keep the temp right. Large units cycle often which is not efficient and is more wear and tear and results in a damp house in the summer because of failure to dehumidify. Plus a larger system costs more to install and uses more resources. A good HVAC guy will run a very complex formula (manual J as mentioned above) looking at the square feet of wall, the square feet of window, the square feet of roof and calculate heat loss using the r-value appropriate to each of those items. They will also use house orientation and how sun affects things as well as predominant winds. In addition they will look at things like using spray foam becuase air infiltration is lower than with fiberglass at the same r-value. It is very complicated. I paid a smart guy to do that because my builder insisted we use the not so smart guy to install the system.

What jkish said above about maybe going a bit over on heating makes sense. If your house is a little warm in the summer, not a bid deal, but if it can't heat in the winter, that would be a problem. Despite the best effort, a manual J is going to have an error margin and you might just have that rare -20 degree day that the manual j didn't really take into account. My system even on the coldest days probably runs 2/3 of the time becuase the main stressor here is the load on a 110 degree day. Plus we have a fireplace and this house is so buttoned up that on a 20 degree night a fire heats the entire 6000sf of house so that none of the units run or at most only the one on the opposite side of the house.

So looking at Rob's situation where heating is really the driving force and his temps go down to zero in the winter, an air to air heat pump will be pathetic on those days regardless of the efficiency. Air to air units just don't work very well when your approach freezing and definitely not below it. If nat gas is not available, propane and heating oil too expensive, geotherm might just be the best thing even with the 2000 watts on the water pump.

It is too bad that you can't do a hybrid system where you use a radiator of sorts on the mild winter days to pull heat out of the air and run that water into the geotherms heat pump instead of ground water. When the temp goes lower you start pulling water out of the ground. There would be some perfect temp where your 2000 watts of well pump will make the system more efficient than trying to use outdoor air temp to grab heat.

 

[jkish]

So looking at Rob's situation where heating is really the driving force and his temps go down to zero in the winter, an air to air heat pump will be pathetic on those days regardless of the efficiency. Air to air units just don't work very well when your approach freezing and definitely not below it. If nat gas is not available, propane and heating oil too expensive, geotherm might just be the best thing even with the 2000 watts on the water pump.

Some of the non-US heat pump manufacturers have some fairly impressive products. Outside of the U.S., they have been building modulating systems with high performance for a number of years (they are way ahead of the U.S. in this area). They can run effectively into some pretty low temperatures. For example, Daikin's VRV-S system has a COP of 2 and a heating capacity of 35K BTUs at 17F. These are very interesting systems that connect multiple indoor units to a single outdoor unit. Each indoor unit can act as a zone and can be ducted or un-ducted. Maybe that performance still isn't good enough for your environment (I don't know the specifics), but it might be worth checking out as an alternative.

 

[apostolakisl]

So looking at Rob's situation where heating is really the driving force and his temps go down to zero in the winter, an air to air heat pump will be pathetic on those days regardless of the efficiency. Air to air units just don't work very well when your approach freezing and definitely not below it. If nat gas is not available, propane and heating oil too expensive, geotherm might just be the best thing even with the 2000 watts on the water pump.

Some of the non-US heat pump manufacturers have some fairly impressive products. Outside of the U.S., they have been building modulating systems with high performance for a number of years (they are way ahead of the U.S. in this area). They can run effectively into some pretty low temperatures. For example, Daikin's VRV-S system has a COP of 2 and a heating capacity of 35K BTUs at 17F. These are very interesting systems that connect multiple indoor units to a single outdoor unit. Each indoor unit can act as a zone and can be ducted or un-ducted. Maybe that performance still isn't good enough for your environment (I don't know the specifics), but it might be worth checking out as an alternative.

That's curious. I guess I could have one unit outside instead of three. There is advantage and disadvantage to that. With one unit, if it breaks, you are not happy. With one unit you get less ugly stuff outside. I still would have to believe it isn't very efficient near and below freezing becuase no matter what you would have to run heat strips and defrost cycles or the outdoor unit would turn into a giant iceblock.

 

[beelzerob]

Ok, so from the most reliable of the quotes I've gotten:

Using the information from the survey of your home the residential air conditioning guide calculates your home will gain 34,194 BTU/H at 97 F outdoors and 75 F indoors and will lose 62,165 BTU/H with -7 F outdoors and 70 F indoors.

We propose to install a four-ton (49,000 BTU/H) Geo-Thermal heating and cooling system for the first floor. This WATER FURNACE unit model NDV049A includes 15KW of electric heat. This unit has a variable speed motor and two-stage compressor for increased comfort and humidity control.

So, see what I mean? Im losing 62k BTU/H and they're only proposing a 49k BTU device. Granted, I think it got below zero only once this last winter...if even that. But still.

One of the other contractors I trust at this point recommended the Climate Master TT049, which according to the spec is 37.5k BTU/H.

 

[apostolakisl]

Probably the 15kw of electric heat aren't part of the 49,000 btu's? What is your btu loss at say 10 degrees? They may have sized the expensive geo therm system for the 99.9% coverage and added the electric heat for that .1% of the time it gest rediculously cold. Sizing the geotherm system for the -7 degree would mean a way oversized system for the vast majority of the time.

 

[beelzerob]

that makes sense. I guess this would be a good question to ask them!

 

[apostolakisl]

I was playing with the settings on my HVAC system and found that it maintains a log of everything. It keeps track of number of hours the units have been attached to electricity and the number of hours running in all of the various modes (high heat, low heat, high cool, low cool, defrost). I did the math and it turns out that my units have been running about 20% of the time since power was turned on. The first couple of months the units had power we hadn't moved in but the heat was on periodically. Most of the time was spent in the low modes (they are two stage units). My guess is that over the course of a year of actually living in the house they probably spend something like 23% of the time operating.

It also tells you rpm's on the fan, voltage levels, cubic feet of airflow, any faults that occur, and other stuff. It is kind of interesting to see all of that stuff.

 

[beelzerob]

Interesting! I can accomplish all of that data logging through CQC, just about, but what system is yours that it automatically logs that stuff? That seems really useful. Is it just the thermostat or is it your entire hvac system? I've yet to decide on a thermostat.

 

[apostolakisl]

Interesting! I can accomplish all of that data logging through CQC, just about, but what system is yours that it automatically logs that stuff? That seems really useful. Is it just the thermostat or is it your entire hvac system? I've yet to decide on a thermostat.

It's the carrier infinity system. You access the info from the thermostats. The thermostats are not compatible with other systems. Your standard thermostat just has open or closed circuit signalling to turn the units on and off. The infinity system is a data cable. I don't know if the data is tracked on a cpu in the unit or if it is stored in the thermostat. My guess is that the thermostat just displays the info. Anyhow, you can't use those thermostats unless you have the full carrier setup.

There is quite a lot of stuff you can set in there. You can have it run on humidity, you can tell it how to operate at different outdoor temps. All kinds of stuff.

They also have an ethernet module so you can get to all of this stuff via IP. It is stupid expensive though. I can't remember the exact price but it was like $1200. That's just nuts, $1200 so you can adjust your HVAC from anywhere in the world. Seriously? Just pay some dude to come to your house and change the temp! It probably has $100 worth of parts in it. If they sold it for $200 probably everyone would just buy it. At over $1200 they probably sell 2 a year. And because I have three units I would need 2 of them becuase each one is only capable of running 2 HVAC units.

Carrier has a geotherm heat pump. I don't know for sure, but assume that the same type of stuff is on that unit.

 

[beelzerob]

Hahaha...oh, that figures it's Carrier Infinity. I'm writing a CQC driver for that system right now for someone. I've been meaning to ask them if it is a standalone thermostat or part of the entire system. I had hoped maybe I could put it on my list of hopefuls. *sigh* I know there's a carrier geothermal system, but no one around here has offered it, so that would mean buying it myself, which I still lack the smarts to do.

 

[apostolakisl]

I just checked the upstairs unit and it also has been running a little over 20% of total power on time. As you might expect, it is skewed heavily to the AC side. Of the 2500 hours the unit has operated, only 500 of them were in heating mode. And that is with nearly 2 winters and only one summer.

 

[WayneW]

I was playing with the settings on my HVAC system and found that it maintains a log of everything. It keeps track of number of hours the units have been attached to electricity and the number of hours running in all of the various modes (high heat, low heat, high cool, low cool, defrost). I did the math and it turns out that my units have been running about 20% of the time since power was turned on. The first couple of months the units had power we hadn't moved in but the heat was on periodically. Most of the time was spent in the low modes (they are two stage units). My guess is that over the course of a year of actually living in the house they probably spend something like 23% of the time operating.

It also tells you rpm's on the fan, voltage levels, cubic feet of airflow, any faults that occur, and other stuff. It is kind of interesting to see all of that stuff.

It's the carrier infinity system. You access the info from the thermostats.

Lou, where or how were you getting all those juicy tidbits of runtime, RPMS, etc? From the thermostat LCD itself? From a serial port?