correct gpm for open loop geothermal

zepfan

I was working on an open loop geothermal today (5 ton) on an open well system. Systems like this are no longer permitted in our area, but this was grandfathered in. The water comes from the home's well goes through the unit, and then gets discharged into a remote drain field. My question is what would be the correct GPM for a system like this? According to what I can find on the unit the correct GPM is 2.2 gpm/per ton. So the unit should have a little over 10 GPM. The unit is running at 5 GPM and the well company is stating that it should have 1 gpm/per ton for an open loop. The company had also put a flow regulator set at 5 gpm. While I would have thought that too little of flow would with the unit operating in the cooling mode, it would have gone off on high head pressure, the opposite is happening. The unit is going off on low pressure and the condenser coil is sweating. The reversing valve is working as the discharge line going into the condenser is very hot, the liquid line that comes out of the condenser before it reaches the TXV is ice cold. Could it be that the unit is just low on refrigerant and there is so much heat being rejected in the condenser that it is causing this sweating, or is this condition with the 5 gpm going to be a real issue when the unit switches over to heat soon? Also with this system being an open loop would it be necessary to flush the loop if a water side repair needs to be made?

any assistance would be much appreciated, and thanks to all

leonz

Is this system a water furnace? How deep is the well, how far away is the dump point?

Check the gas pressure first before you do anything.

The well water is going to be cold 52 degrees or less so please keep that in mind when looking at the whole picture.

I guess you could flush the exchanger with strong vinegar and see how much build up is in it, you may very well have no restriction of flow.

DCContrarian

2.2 gpm per ton is a temperature change of 11F.

1 gpm per ton is a temperature change of 24F.

For heating, in many places that 24F drop would freeze up. Even at 2.2 gpm your source water can't be below 43F or you risk freezing.

ethicalpaul

yeah what is your temperature delta ?

not sure what your system “likes” but I adjusted the flow rate on my until the delta was 10F, for better or worse I don’t know

HVACNUT

What's the make and model of the system? Is there a manual on site or on line? It will tell you exactly what all the pressures and temperatures should be at X flow rate.

You haven't checked refrigerant pressures yet?

What is the condenser, coax coil, plate HX?

What you're describing seems like the reversing valve is in the heat position. What air temperatures are you getting?

Jamie Hall

As @DCContrarian and @ethicalpaul have stated, the gpm from the well for geothermal is related to the heat added or rejected. Exactly the same equation as standard hydronics. Any refrigeration problems with the condenser or pressures or all that magic will have nothing to do with it.

It would, though, be worth checking what your water intake and discharge temperatures are. You really don't want to freeze that drain field or the piping to it…. and, oddly, you don't want to cook it either. Discharge water temperatures much over 80 F will sterilize the soil in and over the drain field, which is not good.

EBEBRATT-Ed

Standard condenser water flow when you recycle the water is 3 gpm /ton nominal. Older ac systems that used city water and dumped it were based on 1.5 gpm/ton because the city water was colder.

So heat pump or not it doesn't change much your between the 1.5 and 3 gpm/ton. Don't forget 1 ton of cooling is 12,000 btus but you have the added heat of rejection from the compressor which is usually figured at 1.3 x 12000 btu or =15,600 btu/ton which used to be referred to as "tower tons" as a cooling tower was used.

Well water may be as cold as city water or it may not be. I seriously doubt the 1 gpm/ton the well guy said is correct

The manufacturer would be the best place to go to get specific information based on water temp and gpm flow. There will be some leeway on this

zepfan

thanks to all that responded. The unit looks like a water furnace, but the nameplate states that it is a Heat Controller, made in Florida. There is a manual with the unit with temperature deltas that range from 8 degrees needing 19.4 GPM and 20 degrees needing 7.8 Last time I checked the unit it was running a 17 degree delta and had 7 GPM. The unit is 5 ton with a coax condenser Would a an open system like this need to be flushed with a flush cart if the system is replaced? Or could it just be piped in and valves opened up? Thanks again

EBEBRATT-Ed

@zepfan Should be able to hook the new one up and just open the valves. But with an open system you will have to see what's lurking in the piping. Any filters on the system?

Jamie Hall

On the water side of things — I don't know much about the refrigerant side! — as noted it depends a lot on the water quality whether anything in the heat exchanger might get gunked up. Likely not, but a reverse flow flush would do no harm.

However, on the source well side, things can get stickier. From time to time you should check the dynamic drawdown level in the well and keep track of that. If the well is screened, screens can and do clog, which will cause the dynamic drawdown to increase. A well completed as an open hole in rock likely will not do that, but it does no harm to check. A clogged screen can be redveloped, but that is a task very much best left to a really good well driller.

zepfan

Thanks for the responses. No I do not see any filters or strainers on the system