geo and outdoor reset

Mike T., Swampeast MO

<I>a mixing valve doesn't make the geo provide lower temperature water</I>

Of course not. Wouldn't that require that the geo unit actually modulate?

My understanding of geo it wants as constant a temperature as possible to work as either the source (during heating) or the sink (during cooling). In other words you can either extract or dump the maximimum possible number of BTUs regardless of the weather or the load. "Reset" would seem to thus require a continually variable resizing of the system that's moving the energy.

Unknown

Geothermal heat and Outdoor Reset

Ok, I understand a couple of things, I think.

1. Geo gets much more efficient if it produces lower water temps.
2. Outdoor reset could tell the geo to produce lower waters when it was warmer outside.

Except, no one I've talked to knows of any outdoor reset controls for Geo.

WHY???? Does anyone know?

Paul Rohrs_4

TN4

Tekmar mentioned a stat they were coming out with that will complement geo equipment and the TN4 system. IIRC, Paul Williamson told us that last month at a training in Des Moines, IA.

Maybe page HydronicsMike to verify.

Regards,

PR

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Mike T., Swampeast MO

Perhaps because the geo units don't or can't modulate?

Perhaps because there's not too much room for reset with geo systems connected to forced air?

Surely with a hydronic system you could use a motorized mixing valve on the emission side. The imagineer in me tells me that such could be made really sweet with the addition of a buffer on the geo side...

Unknown

We can certainly do radiant with outdoor reset, but adding a mixing valve doesn't make the geo provide lower temperature water. And if you can set it to provide 90 degree water or 120 degree water, all I want to do is vary that based on outdoor temp. Seems simple to me.. but I don't really understand how geo works internally I suppose.

Unknown

I wonder if that just means the heat/cool tstats? I'd imagine they would need their algorithms in the "boiler control" to do what I'm asking about here..?

this would be easier if I really knew a darn thing about geo itself! Normally we just pick it up at an injection circuit or mixing valve.

Craig R Bergman

For hydronics...

we use a buffer tank. The Geo unit and hydronics are seperate systems. The Geo will keep the buffer tank at whatever temp the reset controls call for and the hydronics simply draw what is in the buffer tank.

Unknown

Hi Craig,

I get what you're saying, but then the geo is producing the same outlet temperature under all conditions, right? So all you get is reset to the hydronics.

So the 64k question that is driving me nuts is.. why can't we reset the geo outlet temperature directly? It is settable.. so why not variable?

Or, am I missing something really really basic?

Mike T., Swampeast MO

Thanks for the great diagram of nearly the sort of "sweet" system I was imagineering...

I know this would add to complexity, but could efficiency be maximized by heating the buffer to an optimized and fixed temperature then mixing down for the hydronics? Am I off the deep end again?

Rob: Constant temperature geo source or dump + fixed ability (due to size of the "heat pump") to either draw or dump to that source.

Craig Bergman

I don't know..

of any variable heat pump. The heat pump will heat the buffer tank to what the reset curve is. The radiant floor gets what the buffer has...
We design our systems for a max temp of 118 and adjust the reset curve from there.

With the geo piping as shown we send hot water to the buffer tank first. From there it can go to the radiant system or the air handler. In the cooling mode we send the chilled water to the air handler first. This allows cooling to start right away. Otherwise we would have to cool the buffer down before getting temps cold enough for cooling.

Bergy

Unknown

But you can set the fixed temp the geo is to provide, correct? For instance, if your radiant system only needed 100 degrees, you could get the Geo to spit out water that is cooler.. right?

or is that where I'm missing something?

Craig Bergman

No

The Geo only provides a single water temp.(I can't remember off the top of my head what that is)An aquastat simply tells the Geo when to turn on or off, depending on what temp the reset wants the buffer tank at.

Bergy

Craig Bergman

P.S.

The reason we adjust the buffer tank temp rather than mix the radiant is energy. If I need 118 deg @ -5 and only 90 deg @ +25, why hold 80 gallons at the higher temps? The Geo simply runs less if it has to hold the buffer at 90 deg.

Bergy

Unknown

Really. Hmm. So if your *design* temp were 100 degrees, you couldn't set up the geo to provide only one temp, but a lower temp?

Hrmph! That's dissapointing. I wonder why that is. Seems wasteful with the number of 100 degree radiant systems I'm doing these days if you can't then turn the geo down to gain extra efficiency.

I understand why you are resetting the storage tank, and that might have some benefit (probably depending on tank size, outdoor daily temp swings, building loads, and reset curve).. we have just injected using reset on the radiant, out of a fixed temp storage tank, typically.. but that's a band aid. Seems like we've moved beyond fixed-temp conventional boilers (and heck, we can even reset those).. there *must* be a way to do the same thing with the Geo directly.... someday, anyway.

I guess I have to do some serious studying on how Geothermal systems operate. any suggestions on good tech lit?

thanks, by the way, for sharing your knowledge Craig.

jp_2

not there yet?

maybe they're just not there yet NRT?

with heat exchangers and pumps you can surely vary the output.

maybe you are the first to ask?

Simply Rad_2

Controls

NRT Rob
The Geo does not modualate. But when you add a buffer it more thatn keep the ssytem form short cycling. YEs the Geo puts out one temp, BUT if it is heating a 120 gallon buffer tank it does not get high temps. I have found that the tank temp is about 5-10 degrees cooler than the heat pump. Heat pumps are slower to come up in temp than boilers. SO you are not going directly to 120 degrees and shutting off if you are heating you system to 95 degrees. The mass slows the heat pumps reaction time and lengthens the run times. I have used tekmar controls a simple 260 controls-it has outdoor reset and many options for adjustment. Good luck Jeffrey

BC

reset is critical to good efficiency on hydronic geo systems

Rob, I’m just a geo enthusiast, not a pro, but I’ve done a lot of research designing my own system. The short answer is that by using a buffer tank with a reset curve, you will indeed get all the efficiency advantages of running the heat pump at a lower temperature. While all the residential heat pumps that I know of are strictly on/off devices, the COP of the unit is based on the amount of work the compressor has to do, which is related to the difference in temperature between the ground loop water coming in and the hydronic water leaving. Even though the heat pump is always at 100% output when it is running, it will only heat the hydronic water 10 degrees or so, depending on flow rate and heat pump capacity. So, if your buffer tank water is coming in at 80 and leaving at 90, you will get a much higher COP than if the tank was at 110 and leaving at 120. I am also informed by one manufacturer that lower hydronic temps are also very beneficial for compressor life – it has to work much harder to provide hotter water. Are there further efficiency gains to be had by using a variable speed compressor and modulating the output of the heat pump itself? Probably, but perhaps not enough to justify the added cost and complexity.

Many of the manufacturers publish detailed specs showing heating capacity and COP vs loop and hydronic temperature. Econar’s are at: http://www.econar.com/InstallationManuals/90-0112, Engineering Specifications 10-05.pdf – you can see how much the COP improves with lower design temps.

I wrote a simple program to model my system that takes into account outdoor temp and loop temperature over the heating season. At a 120F radiant design temp with no reset it predicts a COP of 3.0 averaged over the season. Simply adding a buffer tank with outdoor reset raises the COP to 3.9, a huge improvement. In my case it is about a $500/yr savings on electricity. At a 100F radiant design temp the COP goes up to 4.7, or another $250/yr savings.

So, if you’re not using a buffer tank with reset, you’re not even coming close to the best efficiency the heat pump is capable of providing.

Unknown

Jeff and BC.. just the info I needed (at least, the info presented in the terms I needed).

Thanks everyone!