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Old 01-08-21, 06:41 AM   #1
solarhotairpanels
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Question Delta T Winter settings Questions?

I installed a drain back solar hot water system consisting of:

2 - 20 tube evac units. Total 40 tubes
6 gallon drain back tank
40 gallon storage tank
Grundfos variable speed pump

My question is has anyone here had better luck creating hot water in the storage tank by changing the Delta T settings to what I am calling a winter setting?

I'm talking during 20 degree weather and colder.

During the warmer months I leave mine set at: 12/8
When the collectors are 12 degrees warmer then the storage tank my pump kicks in and pumps until the collector is within 8 degrees of storage tank temp then pump shuts down and waits for heat to build up again.

Has anyone experienced better results heating their storage tanks by using different setting during winter and if so what settings are you utilizing to accomplish your goals?

What's happening right now with my system is if the sun is beaming my pump fires up on cue and hardly ever shuts down. Water just keeps circulating from my storage tank up thru the collectors and back but the collector temp never gets cooled down enough to shut the pump down.

Initially, when the controller turns on the pump I am pumping 45 degree water from my storage tank up thru the collectors which are staying right around 74 degrees.

My storage tank temp increases to 65 degrees and sometimes a little warmer but pump just keeps on cranking..

Your thoughts, experimentations, test results during winter months would be appreciated.

Thanks!

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Last edited by solarhotairpanels; 01-08-21 at 06:50 AM.. Reason: Update to initial post
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Old 01-09-21, 05:41 PM   #2
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My best guess is, your system is suffering from transportation losses. The collector temp is X, but you lose so much on the way back to the tank that you are pushing a rope uphill, so to speak.
I don't recall what you did to insulate the piping, but that is my guess nonetheless.
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Old 01-10-21, 03:15 AM   #3
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Does your controller always run the pump at full speed? Or does it keep pumping but modify the pump speed based on the delta? Is it capable of changing the pump speed?
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Old 01-10-21, 12:31 PM   #4
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Quote:
Originally Posted by SDMCF View Post
Does your controller always run the pump at full speed? Or does it keep pumping but modify the pump speed based on the delta? Is it capable of changing the pump speed?
Thanks to both of you for responding.
Transportation Loss:
My water returns very quickly to the drain back tank.. no problem there I don't think. Pipes are rapped tight with foam insulation and taped real good at all of the seams.

Pump Speed:
Next, my pump pumps 100% power at initial start up then I have it dropping down to 50% pump speed so the water running thru my evac tube header moves slowly in an attempt to pick up more heat.

I'm wondering if I change the 12 degree differential to like 18 degrees the collector header would have more time to heat up thus higher temps before pump turns on. The higher temps might help to heat the storage tank quicker... don't know for sure but I'm trying different settings and monitoring storage tank temps.
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Last edited by solarhotairpanels; 01-10-21 at 12:37 PM.. Reason: update
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Old 01-10-21, 12:32 PM   #5
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Quote:
Originally Posted by skyking View Post
My best guess is, your system is suffering from transportation losses. The collector temp is X, but you lose so much on the way back to the tank that you are pushing a rope uphill, so to speak.
I don't recall what you did to insulate the piping, but that is my guess nonetheless.
Skyking I responded below / thanks for jumping in here with your input. I appreciate it.
If your transportation theory is right, what's the solution? Faster pump speed? Pump the heated water back to the tank quicker?
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Old 01-10-21, 06:28 PM   #6
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you clarified your setup, and I agree that raising the diff temp is the first thing to try.
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Old 01-10-21, 08:26 PM   #7
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Main idea is: if it's working, play with it until you make it short cycle. Then dial it back until it doesn't. Pick a sweet spot and drive it like you stole it.
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Old 01-11-21, 11:28 PM   #8
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Quote:
Originally Posted by solarhotairpanels View Post
When the collectors are 12 degrees warmer then the storage tank my pump kicks in and pumps until the collector is within 8 degrees of storage tank temp then pump shuts down and waits for heat to build up again.
If your pump isn't shutting down then it sounds to me like there is a fault with the controller. From above, with an 8 degrees difference the pump should stop, you say it continues to run (but without having or with a minimal difference in temperature). For the pump to continue running there should always be 8 degrees delta so your heat must be going somewhere (you say it is all well insulated so this shouldn't be the case) or it's faulty.

Check the temperatures and see what they are.. is it a commercial unit or homebrew?

I run my system with 10/3 and when the delta is < 10C then I modulate the pump (it's a basic on/off pump, not variable speed) between 10C and 3C and it works very well 365.
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Old 01-12-21, 08:37 AM   #9
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Quote:
Originally Posted by Acuario View Post

Check the temperatures and see what they are.. is it a commercial unit or homebrew?

.
Thanks for your input, I appreciate it.

To answer your question (homebrue vs factory)
My drain back tank is nothing but a used/reconfigured 6 gal electric hot water heater. Electric element disconnected / sight glass added to monitor water level. This is mounted just above my hot water storage tank which is a factory 33 to 40 gallon insulated tank.

My controller is a Resol Plus / works perfectly. It will drop down the pump speed after start up to a percentage of my choice. Right now I have it set so that the second water starts returning from my collectors to my drain back tank the pump speed drops down to 50%. (I've thought about increasing pump speed to 80 or even 100% to see if that helps. )

Next,
The temp in my storage tank DOES increase very slowly while the pump is running but it just seems to take forever to get close to the 8 degree shut down temp difference. The pump DOES shut off eventually (8 degree difference is met) just takes forever.

I was thinking that if I changed my settings from 12/8 to 14/8 or 16/8 to allow the collector to heat up more before (pump on) maybe the collector would pick up hotter water running thru the collector manifold on route back to the storage tank..... but I'm guessing and testing different settings to see what happens.

Your thoughts?
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Old 01-13-21, 12:25 AM   #10
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Have you measured the entry/exit temperatures of the water passing through the tank? And the temperature at the lowest point of the heat exchanger?

What is the delta across the heat exchanger? The water from the panels should enter at the top of the tank heat exchanger (I assume this is what you have).

If the exit temperature is the same as the temperature at the lowest point of the heat exchanger then you have removed all the heat somewhere in the heat exchanger. Try increasing the pump speed until the exit temperature is slightly higher, then reduce it a little, then you know that you have the maximum possible flow rate to get the maximum transfer from the heat exchanger.

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