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john of wessex

Solar Hot water Query

My system was fitted last Autumn.

15 Thermomax tubes & a 250L (ish) Heatstore, south facing

In the recent sun it seems to manage to get to about 56c in the store & 60 in the tubes but no more -assuming no water is drawn in the day. Obviously it's slightly better if we have no wind

Would I be right in thinking that there is an 'equilibrium temprature' in the tubes - ie however hot the sun is, in effect they will not exceed a certain temprature.
resistance is fertile

Has your store got a thermostat (normally around 60 degrees), and have you got a pumped system?

Ours has been running at around 85 in the collectors, 61 in the store and the rest blaring out of the heat leak towel rails in the laundry room.
Clara

Not sure if this helps....I have about the same size set up, it has a mixer tap that is set between 40 and 50, which is important because left untouched in the summer the water gets deadly hot here.

I think you're doing pretty good to get those temperatures in the uk.
Shane

Black body temperature (i.e. the temperature at which heat radiated from an object is equal to the heat gain from solar radiation) for carbon steel is typically around 85°C, so RIF's system (assuming that's what the pipes are made of) is doing pretty well.
resistance is fertile

Shane wrote:
Black body temperature (i.e. the temperature at which heat radiated from an object is equal to the heat gain from solar radiation) for carbon steel is typically around 85°C, so RIF's system (assuming that's what the pipes are made of) is doing pretty well.


We have 24 Glycol filled vacuum tubes heating a stat controlled header on each 6 tube panel, which is in a pumped loop (also glycol filled) which only pumps when the panels header tank temp is high enough and runs through a very large coil in a 250 liter Oso store.

Gives us hot water and laundry room drying most of the time Very Happy
Dee J

Thirty 58mm x 1.8m Navitron evacuated tubes (due south facing), 270l Gledhill thermal store, pressurised glycol water mix. Store easily hits 60 all through on any reasonably clear/sunny day - not necessarily recent hot and sunny. Excess heat leaks via Rayburn loop (un-intended) - but keeps the homebreww bubbling Wink

Sounds like you've got quite a small number of tubes for your size of tank... possibly not intended as sole source of water heating. Afraid I specified more on the basis of think of a number and double it! (plus ensure adequate, efficent heat dump - switches on underfloor heating - same as the boil protection for the Rayburn)
Dee
john of wessex

Funnily enough they originally specified 10 tubes.

The controller isnt set to stop heating the tank until about 90C

I done particularly want/need a heat dump but it seems then with more tubes it could get hotter
resistance is fertile

john of wessex wrote:

The controller isnt set to stop heating the tank until about 90C


Shocked
john of wessex

Why? There is a mixer to keep the hot water temprature constant & I dont see it ever getting that hot unless we had the woodburner - which also heats the store on at full blast for 24 hours and didnt wash
adrian

Well in the last few days of extremely hot sunny weather the sensor on our tubes has reached 133C, this is on a Riomay system (it has quite large diameter tubes). We have a 220 litre tank which we allow to get to 65C. We don't have a mixer having reckoned that it wouldn't be useful for many days of the year. Even at 133C we still use that heat until the whole tank gets up for 65C, it was still just a little under yesterday afternoon.

Interesting in the previous hot days of air temp 25C but cloudy the tube sensors were only at 45C and we had to top up the temp in the tank with the gas boiler.
john of wessex

Interestingly enough I had a chat with another installer yesterday.

While mine suggested 10 tubes, later upped to 15, he would have started with 20 or 25.

My installers were, to put it mildly 'laid back' and appear to have wildly undersized it.
RichardW

Re: Solar Hot water Query

john of wessex wrote:


15 Thermomax tubes & a 250L (ish) Heatstore, south facing



I have bad news.

Who ever spec'd your system wants shooting.

The average going rate for a two season set up is 8L per 47mm tube. For a 3+ season setup 5 or 6L per 47mm tube.

Yours works out at 16.6L per tube (if 47mm tubes) or adjusted for 58mm tubes is 11.1L per tube (58mm tubes are worth 1.5 47mm tubes, I doubt you have 70mm tubes so I wont work them out).

You need more like 40 X 47mm tubes or 26 X 58mm tubes.

The longer season you want the more tubes you need but you then also need to be able to either cover the extra tubes in summer or dump the excess heat.

Richard
tahir

Riomay has huge diameter tubes
sean

Lucky him.
resistance is fertile

If you are not getting a collector temp over 60oC then it sound like the thermostat that dumps the collected hot water into your store is not set correctly or is not working properly (or maybe that the pump is working when it shouldnt?)

We have been getting collector temps of up to 140oC lately and with the store thermostat set at 65oC this has meant that the heat leak rails in the laundry room have been drying everything full blast!
tahir

resistance is fertile wrote:
this has meant that the heat leak rails in the laundry room have been drying everything full blast!


Tell me more....
resistance is fertile

Our system has a simple excess heat sink in the form of a set of towel radiators mounted in the barn's downstairs laundry room.

When our solar water system has spare heat and the tank is already at temp then these dry all of the laundry.

For the extra cost of fitting some extra tubes, when you are installing anyway, this is a handy extra and well worth the extra fifty quid or so!
tahir

I'll get onto it right away Wink
vegplot

Always need towels drying.
john of wessex

As the controller kicks in when the collector exceed the temprature in the heatstore by 5C the max collector temprature is limited by the tank temp.

I imagine that if I were to turn the controller off in hot weather for a while the collector temprature could get quite interesting.
RichardW

john of wessex wrote:
As the controller kicks in when the collector exceed the temprature in the heatstore by 5C the max collector temprature is limited by the tank temp.

I imagine that if I were to turn the controller off in hot weather for a while the collector temprature could get quite interesting.


You got it. So the only solutions are

more tubes
or
smaller storage tank.

You have a small heat source feeding a large load. That load is pulling the temps down in the array. Turn the pump of & the array will get much hotter.

Its like one of those shower where the flow speed controls the tempreture of the water. Fast flow = low temps.

Richard
resistance is fertile

Is your system constantly circulating?

Ours only releases the heat from the collectors in bursts when the temp has got up to a decent heat, the pump only then cuts in to bring the heat into the store.

If it is constantly pumped then, as Richard says, you need to slow your pump down or activate it with its own thermostat (like I did) set at a higher temp at the collector.

We fitted cheap probe type stats at collector, at pre flow (before heat store), at main storage, and at hot water delivery this way we can adjust everything independently.
RichardW

resistance is fertile wrote:


If it is constantly pumped then, as Richard says, you need to slow your pump down or activate it with its own thermostat (like I did) set at a higher temp at the collector.



Not quite what I meant.
I was using the shower flow as an example of too much water trying to be heated by to little heat.

So that wont work. Yes the collector will get hotter but it will just get quenched as soon as the pump starts to circulate the cooler water. The total amount of available heat will still be to low to heat the storage tank.

You cant boil a kettle on a match.
resistance is fertile

Maybe we have different set ups Smile

I dont circulate the water itself through my collectors, its a glycol mix that stays in small, well insulated, header tanks on the collectors until the tubes have heated it to a certain temperature at which point the heat is transfered to the store by the pump.

This way we dont lose heat from the store to the system or constantly circulate luke warm temps to no effect.

If you have a system that constantly circulates (or circulates to fast) and is failing to build up sufficient heat then to either slow the pump (which should normally be on its lowest setting anyway) or thermostatically control it to allow effective heat to be built up and then transferred would be a way round things.
RichardW

Thats how most systems work (the one I have ordered works in reverse to that as the tap water will be at mains pressure & more importantly it does not need any power). But its still heating the TOTAL volume of water in the store. So if the store is to big for the power of the array it will only ever get to a low temp. The pump (prob cycling on & off lots as the collector gets cooled) will be on as the store will be cooler & the array slightly hotter.

Also they should be set to only run the pump when the collector is X deg above the store.

Yes pumps should be set to a low speed in most cases but it wont improve this system as its design is at fault.

If your array can only capture say 3kwh per day due to its size then it will never heat a 200L tank to a usable temp.
resistance is fertile

Sorry if Ive got confused/confusing.

JoW definitely needs more tubes. Very Happy

But, I'm also confused why his system output seems to be regulated by tank temp.

Maybe I haven't read it right Rolling Eyes
RichardW

No your right it is.


With no pump the collector will get hot.
But as soon as it does the heat is dumped into the store. As the store can take all the heat & more than the collector can provide the return flow to the collector is much colder than normal. This cools the collector.

Its a bit like a good system first thing in the morning with a fully cold store after all the hot water has been used up.

a, collector starts to get warm
b, collector is x deg hotter than store so pump comes on
c, collector is cooled by return flow pump goes off
d, collector heats up again & pump comes on
e, repeat b,c,d for a few cycles till heat input from array has risen the stores temp enough that the return water is still warm ish & the sun will by then be providing more power so the pump can stay on longer for each cycle.
f, eventually the pump can stay on as the sun & the return water are keeping the collector x deg above the store
g, eventually the store is at max design temp & the heat dump comes on.

A system with to few panels will cycle b c d as it can never raise the stores temp enough to get a warm return flow & the total heat from the sun is to small for the total system volume of water (IE power load).

Its like a kettle that should have a 2kw heater but has been fitted with a 100w one. It gets warm but never boils as the heat losses are bigger than the heat inputs.
Rob R

I've just been reading this, after BA's post in the energy supplier thread & I was wondering how a 100% renewables company could possibly supply gas...

...and apparently they do it by the Renewable Heat Incentive (RHI), which may be relevant to this thread.

Excuse me if this post is the equivalent of educating elderly relatives to consume poultry products in an unusual manner.
resistance is fertile

Interesting Rob, but I couldn't actually see what they were on about. Is it paying people for heat produced by solar panels to encourage them to use less gas?

Space heating is a massive Co2 issue, but solar panels are not really suitable for such applications. Specifically for the reasons Richard sets out in the water heating posts, effectively you are trying to heat a much larger area than can be reasonably done with a relatively small area of panels.
Rob R

Not quite, they are selling bog standard gas but for every unit of gas heat they sell they pay someone producing/using their own with solar panels so everyone switching to their gas is subsidising solar panel use.
RichardW

If I wanted to jump through hoops I can be paid for all the solar power I make & use myself off grid. The rate is lower than the on grid rate but its still money. Personally I dont want all the extra equipment you have to buy (and has a short in service life time) & dont want to sign up to the terms of the contract (large service charge) & you have to do all the work before you can apply & you are still not guaranteed acceptance even if you meet all the conditions.

The GOV are supposed to be bringing in (well forcing companies to bring in) much better rates some time around April next year.

Richard
resistance is fertile

so in effect its 'offsetting' rather than minimising gas use?

in fact its encouraging people to use gas by suggesting their is some fringe environmental benefit.

Confused
Rob R

resistance is fertile wrote:
in fact its encouraging people to use gas by suggesting their is some fringe environmental benefit.

Confused


A lot of people use gas anyway, I don't see the incentive to use any more, anyway, I've read further & they are only trialling the gas supply route at the moment...

It says they'll encourage people to use less gas & more solar. I haven't really looked into it any further (as we don't have mains gas or solar heat) so you'd probably have to contact the e-mail address on that page.
resistance is fertile

Rob R wrote:
A lot of people use gas anyway, I don't see the incentive to use any more


I was meaning it in the sense that they are a Gas suppying company who have found a way to make using their gas more attractive to potential customers by giving it a environmental 'offsetting' spin.
Rob R

Oh, I see, no, they're an electricity supplying company, gas is a new thing, in response to requests from existing customers as far as I can see.
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