Washington Hp Home DIY Geothermal

If all 12 circuits flow at the same rate at peak load then 9 gpm/ 12= 0.75 gpm.

So at that flow rate on the longest loop-

1) 250' of 1/2" pex

H= 0.055 x 0.71213 x 250 x (0.75 ^ 1.75) = 5.914

2) 100' of 1" pex

H= 0.055 x 0.04318 x 100 x (2.25 ^ 1.75) = 0.982

3) 20' of 1.25" copper

H= 0.055 x 0.0068082 x 20 x (9 ^ 1.75) = 0.3502

5.914 + 0.982 + 0.3502 = 7.25 feet of head

Or on the slab with 6 circuits x 0.75 gpm = 4.5 gpm

H= 0.055 x 0.71213 x 300 x (0.75 ^ 1.75) = 7.1

Plus the 60' of 1" pex

H= 0.055 x 0.04318 x 60 x (4.5 ^ 1.75) = 1.98

7.1 + 1.98 = 9.08 feet of head

 

That is a bad assumption. Below is a partial table for head loss in half inch pex. I took a regression to find relationship between flow rate and pressure loss in 100' of pipe. The equation is here and calculated values appended to the table, so a good fit.
Head Loss/100' = 0.8790019153· x^2 + 1.142854637 x - 0.3386142087
Assuming equal flow would result in 3 times the pressure drop in the 250' vs the 90' pipe section which is impossible. You would have lots of trial and error or some serious math challenges with pressure and flow balancing against the pump curve to determine flow for each of the 3 main branches and then further down to each leg within those branches. It is either trial and error or some hefty pipe flow modeling software.

GPM..................Ft/100'....................Calculated
0.5.....................0.51..........................0.452575
1 ........................1.7........................... 1.68325
2........................5.3........................... 5.4631
3.........................11............................11.00095
4........................18.4.........................18.2968
5........................27.4.........................27.35065
6........................38.1.........................38.1625

Similar regression for 1" pipe give a good fit at
Head Loss/100' = 0.04456668135· x^2 + 0.1084413596· x - 0.06093317515

Discount the 1.25" pipe and plug some assumed numbers in and you'll get a quick idea which direction to start adjusting your assumptions.

Attached is a starting point for the simultaneous equations you'd need to solve to get balance.

 

The math is a bit more than fifth grade, but not much go look over a fifth grade mathematics text.

 

Mtrentw- thank you It's going to take me a minute to digest it all. There must be software out there that does this as well?

Mark Custis- Yes probably true but I am still trying to figure out what numbers to give that 5th grader. Exponents and basic algebra might be more like 6 th grade?

 

Not sure if your slab will put out 32,000 BTUs/hour at 90F ingoing water temperature. But I am not sure if it has to do this either.

I did not say the same flow rate per circuit, I did say the same pressure drop. The flow rate per circuit will be all over the place, creating different delta Ts, but the BTU delivered will be half way similar (within certain ranges), since the lesser the flow rate the higher the delta T.

Pushing water through a 250' pipe and a 90' pipe, which means that the 250' has almost 3 time the pressure drop, or flow resistance, if it would have the same flow. But in reality that means more water will flow through the 90' pipe, and less through the 250' pipe, increasing the pressure drop of the through the 90' significantly, and lowering the pressure drop in the 250'. The question is how much.

Since mtrentw is starting to let the cat out of the bag, lets look at this a bit closer.

In the example (Table) from mtrentw above lets say you try to push 1.5 gpm through both the 90' pipe and the 250' pipe. Lets say if you assume 0.5 gpm goes through the 250' the pressure drop is 1.275, and the remaining 1 gpm goes through the 90' pipe, resulting in a PD of 1.53 (1 gpm x 90'/100' x 1.7). So it becomes clear quickly that a bit more than 0.5' goes through the 250' (probably 0.6 gpm), and less flow will go through the 90' pipe (probably around 0.9 gpm).

For flow, the difference between the 2 extreme circuits would only be 50% (0.3 gpm) more flow through one pipe versus the other (and much less for the other circuits).

From a heat transfer perspective, as long as we have turbulent flow, the predominant resistance to heat transfer here is the delta T between the water and the space (which is assumed the be the same for all the circuits) and the slight difference in forced convection heat transfer coefficients due to different flow rates is literally trivial.
This fact combined with the fact that the same temperature water will flow into each group and thus the different flow rates may result in a slightly different temperature change and thus the average temperature in each loop may be different by a degree or so.

In other words it all does not matter very much for the heat transfer!

But we follow the illusion of precision! We now put balance valves on each circuit, with flow meters, increase the cost of the install, and increase the total system pressure drop, only to now require a bigger pump and increase the operating costs for the life of the system.

I will certainly be receptive to any arguments that could justify the use of balance valves and flow meters even on a misbalanced system like this one. Like I said, get the flow to a certain point, and everything will balance itself out.

Now the simple question: What pump do I need for that? We are back to 5th grader math....

 

Back to where we started. Here's what I know-

The Eco is too small, the Stratus costs $800, the Alpha may or may not be big enough. Eenie meenie miney moe

 

That is the way pumps have been sized for years. Or the bigger is better school of thought. I like the idea of increasing the flow enough to create the thrust to leave earth orbit.

I think if I wanted to know the answer to the puzzle I would ask a plumber.

 

We don't want to put a big pump in there which costs a lot of money to buy an/or a lot to operate. Do we.
We don't need much thrust, just enough so the loops start to balance themselves.
We want to do 5th grader math first to save our customers some money.

 

This part of the we cares not what is planed for installation. Not my electric bill.

If you are not able to measure the flow your design results are guesses. See bigger is better. How do you guess when to stop going bigger? Flow does not become thrust until the system springs a leak.

 

You need 9 gpm (at least that is a good number) for the whole system, the question is at how many ft/hd. You also know that the circuit with the highest pressure drop, which has the highest flow resistance, sees the least amount of flow. You have a total of 12 circuits, so just assume for now that you have 1 gpm of flow in that circuit. You know it will be less, since all other 11 circuits have a lesser pressure drop and will see more flow, so just stick with the conservative 1 gpm for now and simple pick that number. Mtrentw provided a nice table which shows 1.7 gpm/100ft, so you have 4.25 ft/hd for the worst flow circuit upstairs, or 5.1 ft/hd for the worst one in the slab.

Again, it is gonna be less, since the longest run will see much lesser flow, just think about worst case scenario. Now you look and you take 1 gpm per circuit for the 1" header pipe, so 6 gpm, which has less 2.5 ft/hd per 100ft. It will be much less, since flow will go to the other circuits on the 1st floor too, again just picking a worst case scenario here.

So 65 ft each way, 130ft total, for 3.25 ft/hd total. Add that to the 5.1 ft/hd. The 1.25" pipe is negligible, so just ignore it. So you are at 8.35ft/hd the pump has to make to get 1 gpm through the worst circuit. In reality it is much less, since the worst circuit in terms of pressure drop sees less flow, but this gets you in the ballpark. So which is the cheapest pump to operate which can make 8.35 ft/hd at 9 gpm of the 3 pumps you mention above?

 

This is obviously the fundamental difference between us, since I care what the electric bill of the geo system is which I design and put in.

Obviously the fluid follows the physics. You should know ahead of time what the flow is in each circuit.

No bigger is not better. And again, if you do the math you know that the circuits' pressure drop will equalize. The penalty you get for the balance valves usually is much higher than their benefit. I would argue they have none.

But please, anyone prove me wrong.

 

I am not designing this system, so I will allow you to have my share of caring.

I agree about knowing the flow ahead of time. The system was presented here as built. I make all loops in a radiant manifold the same length. This was a DIY new by error. The question was can we make this work. I said yes, so you said NO. Know you say yes because I will not lower myself to your level, so you can gloat.

Math does not matter to you as you wanted to increase the flow to overcome the length differential. Tell me how you know when to stop increasing the flow without being able to test the flow?

So you said bigger was better but could or did not tell us when to stop. I hope you measure some stuff. What is the penalty for being able to do arithmetic in your head? Low flow? What is the penalty for not having a compass in with your fishing gear? Go over the falls?

I need not prove you wrong. My ego does not require that I do so, sorry.

 

Thank you for helping me figure this out. It's simple math but it only works if you use the right numbers.

The 65' of 1" header is round trip so it might be more like just under 7 feet at 9 gpm.

So the Eco might be big enough after all, but the Alpha might actually be cheaper to operate.

Now that I have learned a little bit about the relationship between delta T, flow and pressure it makes me wonder more about the pros and cons of delta P versus delta T pumps?

The amount of energy required to do work should be the same but Grundfos says the delta T is more efficient. But would this be a problem with my unbalanced circuits to have a pump setting pressure based on delta T?

 

I don't think it matters, I prefer to set a pump to constant P which is kept content no matter if 1 or 3 circuits are open. I envision that delta T would have issues at different supply temps when you use an outdoor reset. On the other hand you seem to have a very efficient radiant system, you might not need an outdoor reset. But I do like to turn off the pump(s) when there is no call from any zone.

 

No so quick here. I took the numbers from the above table for granted.
Trent (mtrentw), are you sure those are the numbers in ft/hd, and not psi?
Hp_Home, before you buy new pumps here, lets figure this out.

 

OK, I finally sat down and run the real numbers.

this is for about 120F water temp, but your system pressure drop is about 5.60 ft/hd at 9 gpm. This includes the 1" headers but not the 1.25" header/manifolds.

You can see that the 90' and 130' are the main trouble makers. You will see flow rates between 0.6 (300ft circuit) and 1.2 gpm (90ft circuit).

Hope this helps.

 

Why would it not need an outdoor reset? I thought that was by far the best way to go with a slab and geo system.

My radiant with all the messed up balancing issues is actually efficient?

 

Wow yes that helps thank you. That is a cool spreadsheet I figured there was software out there that could save pages of scrap paper doing math.

Looks like it falls within the curve of the Wilo Eco as well so that is now a viable option.

 

The efficiency will be determined by the capability to transfer (loose) heat to the space. If 90F leaving load will heat our space, how much lower do you want to regulate down the temperature in the system on the warmer days? Make sure it is insulated below the slab, and the pipes are about 2-2.5" below the surface. You are good to go.

 

While you suggested to put balance valves on all the circuits, I suggested to make the physics your friend and allow the flow to go up in the shorter circuits so they see more flow which causes a significant increase in pressure drop. Thus all the circuits would balance themselves, although at a different flow rates. We know now that at 9 gpm, the heat pump makes about 28,000 kbtu under rating conditions, and he has 2,990 ft of pipe in the radiant, which gives you 9.36 BTUs/ft, maybe some more or less in the slab, maybe some more or less in the wall. We also know that the temp going into each circuit is the same.

SO the key question is: What is the impact of the higher or lower flow on the average temperature in the pipe?
We
So the most a pipe sees is 1.2 gpm, the least is 0.6 gpm. If all the circuits would be the same length and have the same flow it would be 0.75 (0.75 x 12 = 9 gpm) and loose 6.22F while flowing through the pipe (9 gpm x 6.22 delta T x 500 = 28,000 BTUs), and each circuit would be 249 ft long.
Some circuits have lesser flow, namely 0.6 gpm, and a much longer length (up to 300ft), and therefore will loose more temperature (delta T goes since each gallon of water stays in the pipe a bit longer ), namely around 9.36 F delta T. So lets say you put in 90F water and you have a 9.36 delta T, the average water temp in that pipe will be 85.32 degrees F.
Others will have twice as much water flowing through, (the 90' circuit will have 1.2 gpm flowing through) thus it has not much chance to loose the heat, and its delta T will be around 1.40 F. And the average water temp will be 89.30 in the pipe. All other pipes will be somewhere in between. So the average water temp will vary between 85.32F in the longest pipe (300ft) and 89.30 F (shortest pipe, 90ft).

Now lets put balance valves on it, because we live under an elusion of precision, and reverse return piping for the manifolds, and make sure each circuit sees the same amount of flow (0.75 gpm). Now you delta T in the longest pipe is 7.49F for an average pipe temp of 86.26F, and the delta T in the shortest 90" run is 2.25F for an average water temp in the pipe of 88.88.F

So after increasing the pumping power (I think you quoted your Wilo rep "there is no such thing as an oversized pump") and after significantly increasing the pressure drop for the whole system (balance valves everywhere), even in this very unbalanced system, and do not forget the higher costs for larger pumps, the balance valves, the reverse return header piping, and don't forget to pay for the higher electricity consumption of the larger pump for the life of the system, what did you gain?

All you did is increasing the average temperature in the pipe by not even 1 degree on the longest pipe and you increased it by less than 0.5 degree in the shortest pipe, with the others somewhere in between. That would change not very much the heating of the house, especially when the shorter and some of the longer pipes serve the same zone.

So why do I need to test the flow again, in each circuit, when 5th grader math will tell you pretty quickly that a lack of balance does not matter very much for heat transfer?
So it might daunt in you that balancing with zone valves and installing larger pumps (and many pumps) for 35 years you might not do much but increasing the upfront and operating costs of your customers systems.