Discuss Solar thermal flow meter blocked in the Renewables area at Plumbers Forums

Hi, Solar thermal not so big these days but a couple of Q for anyone who still works in it.

Background. I'm an Electrician not a plumber.. I've been maintaining my otherwise excellent solar thermal (Original install MCS by Others), for the last ~ 18 years, over which time I've needed to clean and flush it, re-fill it, replace expansion vessel and pump, and more recently re-flush it due to "blockages" (more on this below). My drill pump lash-up does a perfectly adequate job of getting 2 bar into the system without air, and I'm familiar enough that I can set-up, and re-pressurise in half an hour.

I last flushed properly with solar cleaner in 2014 and am running Fenox Solar S1

The "blockage" today occurred also last summer after the extremely hot spell following heavy periods of stagnation. Last time it went away after pumping it through but not this time.

The pump is fine, the glass window flow meter appears to pass ~ zero fluid where I expect to see ~ 5L/min. The disk will not lift. I was hoping the flow was there regardless of the disk, but with pump settings all as before, the temperatures in the system indicate not much is getting through (Resol brand VM1020 which has integral fill/drain cock). The stuck disk then does seem to be impeding flow.
Flows during a fill-pumping (while bypassing the flow meter) through the panels and through the solar heat exchanger are fine. I just did that this morning, it's flushing through fast and no air.

My Questions:
Resol VM1020 flow rate meter https://www.resol.de/Produktdokumente/11200176_VM1020.daten.pdf I have not removed it yet. The lifting disk is purely mechanical, what's the reasonable chance of getting it cleaned out properly and working? Any advice about clearing crud or other when I remove it? (solvents, mechanical cleaning?)

Re-sealing: Any advice about re-sealing the above (PTFE, or best alternatives, O rings etc) if/when I clean it out and/or replace it. I am thinking that it must be fibre washer top and O ring bottom, are further sealants on these items advisable?

Replacements: I can't find a Resol VM1020 anywhere. Data says it's 1" flat upper union and 3/4" with O ring bottom. What fits? I need one with a fill valve.

Best value flushing and fill? My stock of S1 is low and that is it's rather re-used/pre circulated and probably not in great nick any longer. Fernox solar cleaner and a new barrel of S1 maybe £££.


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I have a self installed 12 year old system, flat plate array, nd only replaced the solar fluid once because of a leak on the deaerator, do you have evacuated tubes?, which can reach stagnation temperatures of +200C and degrade the solar fluid very rapidly and make it very viscious, you should be able to clean out the flow meter, I didn't use anythinhg other than the fibre washers (new) on mine for sealing and one O ring on the deaerator which I reused. You say no/poor flow as shown by the temperature gauges, what was/is the collector temperature, if no flow then would expect +200C with E>tubes and~ 120C with F.plates.
You also say you pumped it up to 2bar, did you then repressurize the air end?, I pumped mine to ~ 1.5bar, no pressure at E.vessel air end and then pumped the air end to give a cold pressure of 2.5bar, this results in a hot pressure of ~ 2.8/2.9bar. When you start the circ pump do you still have a system pressure of ~ 2bar?.
Hi John

Yes I have evacuated tubes. They are extraordinarily good, but in these mid summer blue days can produce many times the hot water we'd ever need (220L tank) and the system often stagnates by 1PM. The upside is that it works very well into the cool "shoulder" months and give us all our water from mid March till late October. I switch on the PV immersion diverter to fill any winter gaps, so very rarely burn any gas at all..

When the tubes stagnate they can reach 160C very easily, more up to 200+ when it's steaming and blue. I have Ritter tubes and I spoke to Ritter tech about 17 years ago (when new) and the engineer there was totally "cool" about this, and he declared no problem whatsoever (for the health of the tubes themselves..).

I play a trick with it in mid summer to try and make it as "inefficient" as I possibly can, to reduce stresses - I set the temp differential up to it's maximum (20C), I set the tank temp limit to minimum (about 20C) and I set the "re-cooling" option on to trigger at about 120C. The system thinks it's up to temperature at 20C so spends it's whole time trying to "re-cool". This means rather then fully stagnating, the pump flows occasionally to keep the tubes below 120 but, meaning a massive differential between tank and flow, and more heat losses (which is good). The tank still heats but rather less fast. It's only limited then by the ultimate safety temperature (90C in my case). Unfortunately it can get too hot even when doing this, so I get scared about having only one "layer" of safety left (the final 90C switch everything off limit,), and I go back to "conventional" stagnation settings which ultimately may be better for the system (not sure). (We did go away last summer in the hottest part and I unwisely left it tuned this way. Well, the 220Litres at ~ 95C did alarm me considerably on return!. I might not do that in future :)

The fluid yes, I think it can be an issue. I noted when I last did a proper clean and brand new fluid, it became more efficient. When I bought it the Fernox S2 was purportedly better then standard old-school glycol.

My pump is an old drill with an Ebay drill pump. It is capable of 2 bar but no more. I set the vessel to somewhat less than the 2 bar (Can't quite remember exactly) and then pressurize the system to 2 bar cold by closing valves in the right order. It normally holds close to 2 bar whatever when hot, occasionally may reach 2.3 or so. Flushing and re-filling is not my problem, I've done it a few times now. The spare fluid I have is what has remained in my "tank" after previous re-fills so it has been rather "used" even though ostensibly it's my stock. - It probably should be changed. (Disposal is a problem) .

I think you have answered my question about the flow meter though. Thanks. I'd better disassemble it and have a really good clean out somehow, then start with some fresh fluids. It's a shame the solar fluid (and cleaners) are quite so pricey.



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