GSHP ? for ME

Rich L.

Hi Mark, You posted this statement awhile back on a discussion regarding GSHP's. I've never heard of this before. Would you please explain?

"Also, a ton of GSHP is 12,000 btu's when you are doing AC, except when you're trying to do heat, in which case it is roughly 80% of a ton, or 9,600 btu's."

I don't understand why a ton of energy isn't created regardless of which way the refrigerant flow is directed. Your (or anyone else's) clarification would be greatly appreciated!

Thanks, Rich L.

[Deleted User]

It's the mechanics of the beast...

Heat flows from hot to cold. So when in AC mode, you are sending high grade heat DOWN hill to an excellent heat dump, and the C.O.P. is relatively high.

When you are extracting BTU's UP hill, you are taking extremely low grade heat (40 to 45 degrees F) and bringing it to the surface and storing it in a storage tank. The C.O.P drops off significantly under this scenario. depending upon your final temperature needs. It is a function of final temperature needs. The higher final temperature need, the lower the COP. You can increase the COP by putting an ODR in the mix, but then you would circumvent the preheating of the DHW.

A ton of refrigeration is a ton of refrigeration, which is really all that a GSHP is, but if the unit is rated (in my experience) as a 5 ton unit, it will easily move 5 tons from the house to the hole, but when taking heat from the hole and putting it into the house, it is only worth roughly 80% of the listed tonnage. Check the manufacturers performance/specification literature. If you find a unit that can maintain a high COP regardless of approach temperatures, let me know and I will start specifying it...

If you assume full capacity during the heating mode, you will come up about 20% shy of your target, which is why we don't recommend doing 100% geo projects. Although technically feasible, they are tough to justify economically. So, we sized the GSHP to handle the AC load, and use the heating capacity to base load the homes heating neeeds and DHW preheat needs, and blend a high efficiency boiler (soft interface) into the mix. THis way, you end up with an auxillary back up, which will be needed at one point or another. It also serves to "top off" the DHW temperature neeeds to avoid legionella and other water born diseases.

"People want to be "GREEN" until they find out how much of their own green it's going to take to be green, then they start asking if they can be, maybe a lighter shade of green, like pale green..."

HTH

ME

Rich L.

Thanks Mark,

I understand what you're saying now and have seen it first hand. Thank you for the clarification.

A side question, on a W to W-GSHP, what do you recommend for a temperature setting on the buffer tank? I know I need to keep from freezing my unit but do I want to get the water as cold as possible for best performance, both the GSHP as well as the "AC" operation? I have sensors set up to monitor discharge water temp of the unit as well as the tank temp itself so I can set for either.

Thanks again Mark, I appreciate your response, Rich L

Unknown

Yep,

taking heat from 70* and moving it toward 55* is a lot easier than taking heat from 55* (or lower) and moving it to 70*.

[Deleted User]

As usual....

It depends. If you design your air distribution system to work with an average of 40 degree F water, and your split is 20 degrees, you could be in trouble because the split would require 30 degree supply water with a 50 degree return. Possibility of freezing the coaxial heat exchanger... You could go with a tighter split, but that will require a higher flow rate and more parasitic cost of operation. Most units have an internal freeze protection capacity to avoid freeze ups. It shuts the compressor off and allows the refrigerant temps to come back up. Some units will do a hard lock out on third freeze default in one hour.

I wouldn't go with a supply temperature much lower than 40 degrees F for cooling, and 115 to 120 for heating.

You can size the fan coils for what ever water temperature you are comfortable with. Remember this, the closer the loop temps are to the load temps, the higher the COP. The farther apart they are, the lower the COP. So, design your cooling systems with larger FCU surface areas, hence requiring less cooler approach temperatures, resulting in higher performance, and on the heating side, use the most efficient, lowest temperature of operation possible for the highest performance. This is why WarmBoard is mandatory on our GSHP applications with framed radiant floor heating systems. Even with a slab, you may need to increase the tube density to 9" OC in order to guarantee you can deliver comfort with low water temperatures.

You can't afford to skimp on anything when you are using GSHP as your energy source. There is little to no room for error. It's not like a boiler where you can "turn it up..."

ME

Unknown

Good read,

thanks!

How much differential are you using? Have you done any multiple staging with them? If so same question, how are you staging them?

[Deleted User]

You're welcome, 10 and 5

I've used a 10 degree diffierential and a 5 degree interstage differential on a tekmar 2 stage controller (152) with no time delay.

ME

Unknown

Thanks again,

I have to check, but I think that's exactly where I am at with mine. I happen to be going there today. What I've found is when either of my concrete loads come in they can lower water temp enough temporarily to cause stage 2 to come in unnecessarily if the diff is too narrow. Especially the garage, which we're only heating to 55*.

Rich L.

Thank you,

Thanks Mark, I appreciate the info!