Question on heating capacity
Hi there, homeowner here. I am trying to nail down some basics to make sure that I read different specs correctly and evaluating options/costs accordingly.
1. A GSHP system with a stated heating capacity of 50,000 Btuh at COP 4 can produce a maximum of 1,200,000 Btu if run continuously for 24 hours?
2. 1,200,000 Btu = 351,600 Watts, thus at COP 4 the system has used 351,600 / 5 = 70,320 watts or 70.3 KWatts?
3. A house with a Manual J estimate of 55,000 Btuh heat-loss will need to use the emergency heat-strip for about 10%-11% of time as at the extreme will require 55,000 X 24 = 1,320,000 Btus?
Questions 1 and 2, short answer yes, the reality is no. Keep in mind most systems rated efficiency is at a 50 degree entering water temperature, so if your loop temperatures drop to 35 degrees(fairly common) not only will your heating capacity diminish but so will your efficiency. You also have to account for costs to pump the water which if i remember correctly are not part of the COP number you see at rated but I could be wrong on this. Also keep in mind that rated efficiency depends on what stage the heat pump is in, sometimes it can be a 4.8COP in first stage but only a 3.8 COP in second stage so there is a lot more to take in than rated numbers for a system, but most two stage systems are fairly close in efficiency except for the WaterFurnace 7 Series that is up to 30% more efficient than other top of the line systems(5.3COP 41 EER at part load)
Question 3 If your heat loss is 55,000 BTU's this is only the heat loss on the design day(usually coldest average day of the year or a 1% design load) so if on the coldest day of the year your house needs 55,000 BTU's then 50,000 BTU's is heated by geothermal and the additional 5,000 BTU's is heated by strip heat. Of course most days it is warmer than the coldest day of the year so lets say your design temp is 10 degrees outdoor temperature and the house needs 55,000 BTU's when its 17 degrees out your house may only need 50,000 BTU's, this is called your balance point, the temperature at which your house does not need backup heat so in theory you only use backup heat about 1% of the time not 10% of the time.
If you are looking for costs a professional in your area should be able to model your costs based on your heat load based on your specific efficiency.
What area of the country are you in. Are you doing an open loop or closed loop geothermal system?
1. Depends on EW and RA temps on the GSHP when it is runninng for the 24hrs.
2.By a book I have a unit with 50.51k cap, has a 3.28 Kw rating and cop of 4.5 - mfr rating
3. A house with a 55k load at design conditions will need 10-11% more capacity than what the unit is rated for assuming that EWT and RA temps are correct for the unit to produce the 50K, so that would mean the adding 5Kw additional strip heat would be all that is needed.
Thank you both for "dumbing" this down for me. I think it is clear(er).
So I live in central NJ, heating dominated climate, on a dense diabase rock hill (it is quarried about a mile away) that's how I know)
My single story house based on manual J needs about 51 Kbuth total cooling and 81 Kbtuh heating (3,500 sqft, lots of windows, open floor plan, average construction, before any insulation improvements, but some are planned) Currently, the HVAC system is zoned into two separate zones, and is about 25 years old, thus sooner or later will need replacement. I have natural gas available. It is not my intention to eliminate the gas meter.
I am taking the cost of drilling and installing a vertical loop out of the calculation ($21/foot, 2X300feet), to be able to compare the cost of GSHP to a conventional highest-efficiency system.
Considering my gas bills, this past winter in January averaged at about 900,000 Btus daily (adjusted for the AFUE efficiency) which translates to 37,500 Btuh.
I got proposed a 6-ton 2 stage unit (with 2 X 300 vertical closed loop) which at best produces 52 Kbtuh at COP 3.8, in stage 1 the COP is 4
The current conventional natural gas hot water heater will need a chimney upgrade to function, thus considering for a bit more cost to upgrade it to a tankless unit and also running a hot water coil to function as back-up heat, instead of electric strip.
I have 2 quotes on the table. There is a bit over 10K difference between the two. It is also still over 5K difference between a conventional system and the lower of the two estimates for above with the hot-water upgrade (or 17K difference if I do not take the loop cost out) and the operating expense between the two is minimal (something like $300 a year)
Trying to justify it, but it is not easy...
It may be becasue of the different soil type but 2 300' wells is not near enough tube. We figure 400'/ton so you would need at least 4-300' wells to transfer enough heat to keep the system working properly.
Where are you going to get the water from for the water coil, most codes do not let you use potable water recirculation for heating, ie taking water out of the water heater, running it through a coil and putting it back in.
What is the efficiency of the conventional system and how does that operating cost compare to the geo. We can cut operation cost in half just by going from a furnace a/c to heat pump and save at least 25% from heat pump to geo.
I am no expert, but from what I (re)searched this topic, rock, especially the hard, dense diabase, a form of granite, has very good thermal conductivity. The bedrock is very close to surface, in fact sticking out at most places. This type of rock has also high water table level, which in my case is about 20'-30' below ground. My private water well is slow producing, but is deep.
The 2 wells would have 1,200' of 1.25" pipe, which translates to about 200'/ton. I believe I have seen this size recommended here.
I believe there are some tankless water heaters that have a dedicated loop or some other heat exchanger on a closed loop to function as boilers and DHW heaters. I will have to research this with my town, to see what they say on this.
The current HVAC system has two 100K Btuh units with 2X3-ton A/C with a 64% AFUE and 9-10 SEER. The proposed conventional would combine these into a 120K btuh 98% AFUE modulating furnace with a 5-ton 17 SEER ASHP as dual fuel, and separate conventional DHW. This is expected to significantly reduce the operating cost, actually to less than half.
The geo system described above can only further reduce that with about ~$300 or so per year which in fact is close to 20% of the new conventional system yearly operating cost.
Here again, it may be different in your area but here and at the IGSHPA training I took they recommend 400'/ton vertical an 500'/ton horizontal, minimum. Also 1-1/2" line is piping to and from the well field, 3/4" in the wells. The larger pipe will not provide the turbulance needed for good heat transfer.
I ran into a system a couple of years ago, 3 ton 900' horizontal loop. The unit could not keep up because the water temps were too high after running most of the summer, over 110*.
Just want you to make sure you have all your ducks in a row.
According to my software if you have a 6 ton system with that heat load seeing a minimum EWT of 28 an a max EWT of 95 assuming dense rock I show you would need at least 650 feet of bore holes with 1.25" pipe so 600 feet or boring seems fairly short and again that is assuming dense rock. Switch to average rock which is much more common and now you need 845' of bore holes or 3X300.
When you say you see the 200' number on here a lot it is referring to 200' of bore hole(400 feet of pipe as KLS-CC has mentioned) and that is in average rock with .75" pipe. No matter what I would be very skeptical of that short of a loop even with great rock and the loop they designed is less than the bare minimum. I also agree that 1.25" pipe might be a bit large for the turbulence needed, also 1.25" pipe has a thicker sidewall and does not transfer as fast as .75" pipe. In my area we design on a horizontal loop for 720 ft per ton minimum and shoot for 800 and with that much loop rarely get below 38 degree EWT but its less expensive for us to go horizontal at a savings of almost 5 to 1 over vertical.
What brand equipment are you looking at?
With a WaterFurnace 2 stage 6 ton system I show at full capacity 54K BTU's of heat at a 30 degree entering water temperature but a COP of 3.5 at 40 degree EWT I show 63K BTU of capacity with a 3.85 COP
At low speed(partial capacity) I show at 30 degree EWT 37K BTU's of heat with a3.2COP and at 40 degree EWT I show 43.5K BTU's of heat at a 3.65 COP
I would be very skeptical of that loop design without some real world data/ customers who were happy to back it up. Drilling in rock is costly as you know but shorting the loops to that extreme will cost you even more. As was said the average is 400 feet of loop in the ground per ton for vertical.
Thanks for the explanation. The proposal still in contest states that final loop length will be decided after field thermal conductivity test, which did not happen yet.
The Waterfurnace dealers came in consistently at much higher cost (I mean from crazy high to 10K(20%) difference), so I am now considering the two stage system from Geosystems.
Sorry, but I have quoted the efficiency numbers from top of my head, I have now look it up and they are:
Model # RGT-V072
Heating at 41°F EWT in stage 1 with 1,400 CFM, 18 GPM 43,500 BTUH COP 3.9
Heating at 32°F EWT in stage 2 with 2,110 CFM, 18 GPM 53,500 BTUH COP 3.6
The issue is, that with the current utility rates of $1.2/therm and $0.18/KW, the balance point is tipped toward the gas heating.
Even if I take the cost of installing the loop out, sort of defining as a lifetime investment, that will not need repair or replacement, I have hard time to justify the remainder of 5K difference to break even in 10 years. And when I include additional cost of operating the loop, it is even worse.
The only case this may change, is if I install solar PV electricity generation, thus reducing the cost of electricity to about half of what is today. But this will require additional investment.
Wow $.18 per KWh for electricity? I don't know if geo is the way to go for you from a purely economic standpoint. The Dual Fuel heat pump may be the best option for cost effectiveness. Keep in mind with a geo you have no outdoor noise from AC or heat pump and at the present time you can not generate your own natural gas but you can generate electricy as you alluded too. Geothermal can also heat your water if setup properly and this factors into savings as well as future sustainability. I personally think and have seen less issues with geothermal(assuming loop is correct) than gas or air source heat pump and most geo equipment has a 10 year parts and labor warranty vs a 5 year parts 1 year labor(10 year parts with registration) on a furnace or heat pump.
I certainly think looking into things like insulation and air sealing and duct sealing would be the best payback in your case and would help bring your load down. Conservation of heat should sometimes happen before generation of heat, things like Windows are usually your slowest payback with air sealing being your fastest.
I must have missed the part about the thermal conductivity test? This is crazy expensive for a resi project. We typically charge 8-10k for that testing. What are the costs associated with your Testing? Takeing that testing out of the proposal should lower your estimate big.