Has anyone any comment on the best pump flowrate setup? I hear near
1L/min is good. My installer left the system pumping 3.5L/min (with pump at 30%, it modulates).By turning the pump from power III down to power II I get flow rates about 0.9L/min to 4L/min. The SDHW system still seems to do its thing but I can't tell if it is 'better' or 'worse'.
Checking against a standard is hard as weather is so variable.
Thanks Phil
Isn't the flow rate supposed to be set so there is a certain temp difference across the boiler? The boiler install manual should tell you this.
On Thu, 06 Sep 2007 12:13:09 -0000 someone who may be muymalestado wrote this:-
Depends on the setup (and as you say the weather).
If you go to the forums on
Thanks David Hansen, I will consult navitron. Have done so and conclude they are heavily evac tube oriented whereas I have a serpentine flat panel collector. The opinion seems to be varied rather than a simple confirmation of one strategy (fast flow) over another (slow flow).
OldBill - it is the solar side which I am addressing here, the boiler side has been going well for years.
Phil
I'd have thought with a flat plate it's best to keep panel temp down to minimise re-radiation losses from the panel, so not too slow pump rate.
Having said that, on a dull day if the pump is tooo powerful, it will use a fair bit of power even though solar gain is quite low.
May be worth having a cheap IR thermometer to measure temp rise across the panel.
cheers, Pete.
If
On Thu, 06 Sep 2007 19:57:41 -0000 someone who may be muymalestado wrote this:-
With a flat panel a slow flow is generally considered best. There isn't much to be gained by pumping lukewarm water to the cylinder. It is better to give the water enough time to get hot in the panel and then pump the hot water to the cylinder. This strategy does involve having good insulation of the pipes. It may be desirable to have a higher flow rate in the summer than in the rest of the year.
The consensus on the Navitron site for evacuated tubes is that a fast flow strategy is best, largely achieved by using microbore pipes.
One other aspect I neglected to state is that the cylinder is indirect
- the Navitron discussion centring on evac tubes seems also to assume direct cylinders as the talk so often mentions stratification which slow flow would aid.
Pete, as the controller modulates the pump hopefully a slower flow always results when temp diff is low. That is what I observe anyway.
The whole point is that the pump has powers (rpm's) I, and II, and III and so which range is best. From what you are saying here possibly III in summer and II in winter.
Phil
On Fri, 07 Sep 2007 00:24:53 -0700 someone who may be muymalestado wrote this:-
Just about the only direct system is the Solartwin. In the others the domestic hot water is kept separate from the working fluid in boiler and solar circuits.
Discussion about stratification on the Navitron site is largely to do with thermal stores. These are still indirect systems. Although the shell of the store has boiler water in it, the domestic hot water is generated via a coil in the store. The boiler water does not mix with the domestic hot water. The solar water doesn't mix with either, as it is fed through the solar coil in the store. Stratification is important for many reasons, not the least of which is to have the hottest water in the store at the top where the domestic hot water coil is.
You will only find out by experiment. What is best for your system may not be best for another system.
Sounds good, I'd have thought the lower the temp of the panel, the more heat is being moved to the rest of the system.
Though there must come a point where the cost of electricity used to run the pump outweighs the saving from solar heated water that's being used.
cheers, Pete.
May be best to do it by trial and error as David says.
In summer I'd try to see if a higher setting gives a better chance of covering daily hot water needs, if not a lower setting would do.
The rest of the year I'd try to see whether a higher setting just gives more solar heated water, if not a lower setting would do.
cheers, Pete.
I have now purchased a thermistor and installed onto the solar return pipe just away from the cyl. I hope to see what pump settings give biggest diff (Tcol - Tret).
Also from the outset I installed a datalogger and now have 6 weeks of spreadsheet rows. It will be interesting to watch the Tcollector - Treturn against various pump RPM's. Phil
Weather variations will create so much statistical noise that I doubt trial will tell you anything reliable.
Collector losses are proportional to T diff between the HW generated and ambuient temp. Flat panels have stagnation temps typically not much above HW output temp, so increasing flow rate makes a significant improvement with these type of panels. Vac tubes have much higher stag temps so flow makes much less difference.
As said there are more factors involved. Nonetheless you'll generally get more yield from flat plates with higher pump speed. OTOH more yield may in some cases mean more warm semi-heated water and less hot....
NT
Weather variations will create so much statistical noise that I doubt trial will tell you anything reliable.
Collector losses are proportional to T diff between the HW generated and ambuient temp. Flat panels have stagnation temps typically not much above HW output temp, so increasing flow rate makes a significant improvement with these type of panels. Vac tubes have much higher stag temps so flow makes much less difference.
As said there are more factors involved. Nonetheless you'll generally get more yield from flat plates with higher pump speed. OTOH more yield may in some cases mean more warm semi-heated water and less hot....
Best option really is to monitor temps into and out of the panel, plus know your HW stat temp plus panel stagnation temp. Then you can make rough efficiency calcs for different pump settings.
NT
Thanks all. If after a period some conclusoin emerges and has some framework of logic behind it I will try to relate that here.
Meantime I don't know what a 'panel stagnation temp' is.
Phil
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