HWS pumped split system pump speed, low, med, high?

[rgormley]

i build a DIY evacuated tube solar hot water heater system back in 2010
piped fully lagged, non return check valves, basiclly all the stuff myself...works great!

15 X evacuated tubes
200L ground reticulating tank
connected to a instantaneous gas heater
all still working great!
(recently the gronfos circulationg pump failed, and also took out the differential control board.)

made steam for a day or so!! tripping the PTR valve like a kettle due to no water flow in the collector (incredible how well those evacuated tubes work) (hit 126deg on the collector) but the emergncy re-cool function could not cool it down as the pump failed

That has all been replaced and is working again, but it raised a question in my mind...

There is no speed control on the differential controller as it compares the water temp coming out of the collector and the temp at the bottom of the reticulating tank and just turns on a output relay that applies power to the pump. simple on/off based on temperate

Q: what is the optimum pump speed? Low, med, high ? (has been set on high for the entire install)

what’s the general consensus?

Cheers,
Richard

(next project a solar powered pond fish feeder!)

 

[OzSolar]

Glad you got the system going again! Nice work.

Delta T x GPM (you'll have to convert it LPM, sorry) x 500 is the formula for BTU's when you're circulating water. You don't happen to have a flow meter on your system?

You could try some tests to see how many BTU's you put in your tank in an hour on various pump speeds and the same solar conditions but the difference might be so small you wouldn't see it easily. BTU = 1 lb of water 1 degree F (again sorry for the non metric)

Low speed will probably be fine as long as it overcomes the head losses and keeps pumpin. Plus it will use a lot less pumping energy thanks to the cubed power law for pumping.

 

[rgormley]

ok thanks for the info,,, for the moment i will go more simplistic "look and see and feel" evaluation

we had a 25deg partly cloudy day yesterday pump on high, the tank went to 43deg c
today is a similar day temperature and i have the pump on low....stay tuned for the end of day tank temp
(i know this is a small sample amount, 2 days) but they are basically the same day time temperature and "cloudiness" (is that a word?)

update: 7:50pm sun is sinking (well of the collector anyway) and the water tempin the tank is 46 deg C
i will we have a few warmer days coming up, so i will what tempo it gets up to..

So basicly it was a few Deg warmer 43 yestersday to 46 today...
to early to tell if it`s was due to slower circulation, or more uv hitting the collector?


25/01 hit 48deg in the tank today! all looking great... 4 showers again today no gas used!!

 

[WindWizard]

I run mine on low and it works fine. I lost my circulator pump due to the controller circulator pump relay going bad, and I got steam shooting out of the relief valve outside. I also have a UPS tied to it in case of a power failure.

I took a tarp and covered the tubes to prevent the steam from continuing. I also replaced my propylene glycol solution due to overheating.

When the sun shines it can heat up to 150 to 160 F and that supplies us with hot water. I have a oil boiler or wood boiler backup.

 

[smr.ruby]

Q: what is the optimum pump speed? Low, med, high ? (has been set on high for the entire install)

faster flow will get you more heat from the system (log mean temperature difference and all that). But it could be overkill if you don't need it and slower flow could allow you to discharge hotter water from the collectors.