Slow Recovery Time

[NormanPrice]

Thanks for all your replies, they are appreciated.

I'll try to answer your questions,

D_Hailsham
The problem just seems to be there all the time. If I switch the heating on then the flow temperature to the coil drops initially but does come back up once the heating circuit warms up. I guess this is just the water taking the path of least resistance and heading off down the 22mm pipe to the radiators rather than the 15mm one to the coil?

Yes its a SOL300i Stainless Lite Indirect Solar.

The boiler is just the existing one, it was under 4 years old and is sufficient to heat the house and seemed a bit of a waste to chuck it. Was thinking that having a coil with a larger rating than the boiler was a bit silly, perhaps you agree?! Would I be right in thinking the amount of heat that the coil can deliver/absorb will decrease with increasing water temperature in the cylinder?

ALEC1

With hindsight I should have just asked that question! It was the conclusion I was coming to, based on info I'd gleaned from previous posts on here.

Thanks for all your replies. I was banging my head against a brick wall with the installer. One of his excuses was that my solar thermal system hadnt been connected and that once it was things would be okay!

 

[NormanPrice]

Mine is 15mm plastic pipe most of the way. From what I can see 22mm copper to 15mm plastic is almost a 50% reduction in area. Would this be significant?

Norm

Sounds like another "numpty" installer , the plastic along with the 15mm says it all , the good guys loose work due to these types , did you pay him in nuts?

Why some of the experienced guys on here give free advice when clearly the 'installer' should be called back is beyong me , the bed was made so unfortunatly you'll just have to lie in it , how much did you pay for the new boiler install etc?

The 'installer' has been called back and will be again. I've told him what I thought the problem was, he disagreed. I came to seek a bit of advice before telling him to correct it. If he refuses then he won't get his nuts. Simples.

Its not a huge job to correct it, maybe 15m of pipework has to changed. One little piece of plasterboard will have to removed, that'll all. Just want it done before everything is put back together and he has his nuts.

 

[gaswizzard]

If the 'installer' can't get it right first time then what chance of getting it right the 2nd time?

90 minutes to recover 300 litres from 20c to 55c is around 8.1kw.

Recovery rate for a 300 litre cylinder with a 15kw input rating should take around 70 minutes (15c to 65c) , find someone who prefers to be paid in money and not nuts.

 

[Dan Robinson]

There you go again with the maths I feel my desk rising .

 

[Agile]

Its not quite a simple calculation.

The rated heat absorbtion is based on the water around the coil being at the inlet temperature.

When that starts warming then the power absorbed reduces. So it takes a little longer.

Regardless, in this case the heating flow rate is too small because of the 15 mm tube.

Although it sounds bad, in actual use its unlikely to be much of a problem unless you are using at least 300 li of hot water each day. Even then it will just take a little longer, but whats 30 min in a 24 hour day.

Its just wrong and should have been done properly. As stated why do these nupties get all the jobs when we quote to do it properly?

Tony Glazier

 

[sandybeach]

There are two valves on the flow and return pipes but they are open. Not sure what you call them but I think they are there so you can isolate the cylinder, similar to what I've seem on pipework running to a tap.

If those two valves are ball o fix type then the flow rate will be reduced even more so.

 

[D_Hailsham]

The spec for the cylinder says that the heat-up time is base on a coil rating of 20.5kW and flow temperature of 82C. You are currently running at 70-75C.

What happens if you turn the boiler temperature up?

 

[NormanPrice]

The spec for the cylinder says that the heat-up time is base on a coil rating of 20.5kW and flow temperature of 82C. You are currently running at 70-75C.

What happens if you turn the boiler temperature up?

Yes you are right that the quoted heat up and recovery times are based on a flow temperature of 82 deg. The times are also based on a flow rate of 0.25lt/s which would equate to a velocity of 2.6m/s in my super duper pipe. If I remember rightly I read a post from yourself on another thread that said you dont really want to go above 1.5m/s? Increasing the pipe to a 22mm would drop it to about 0.8m/s.

ANyway, I turned the temperature on the boiler right up (which is 82 deg I believe) and ran the pump on the fastest setting. Here are the results,

15 deg at start
20 deg after 5 mins (12.2kW)
25 deg after 12 mins (10.2kW)
30 deg after 19.5 mins (9.4kW)
35 deg after 28 mins (8.7kW)
40 deg after 37.5 mins (8.1kw)
45 deg after 49 mins (7.5kW)
50 deg after 62 mins (6.9kW)

After 75 mins I stopped it, it hadnt reached 55 deg and the boiler shut down/cycled which I've never seen it do before.

Would I be right in thinking that ideally you would want to slow the flow rate down as the temperture in the tank rises? If so then because the pipe is under sized its not a bit of a catch 22 - you have to run the pump faster to compensate but this starts to work against you once the cylinder get hot.

We'll see what the larger pipe does. I can let you know if your interested?

Norm

 

[Agile]

Tre is no need to slow the flow rate down as the cylinder heats up. True a lower flow rate would still enable the same heat to be transferred.

But the differential will reduce and the boiler will modulate back and possibly reach the limit of the minimum output and need to on/off modulate.

You dont buy a cylinder with a particular heat absorbtion. The makers design it as a cost/benefit. If thats higher then you need then thats no problem.

As you have noted the makers still quote specs based on inappropriate operating conditions like a 82 C flow temp. Thats general when anyone can get away with it.

The reality is that your system has been badly specced with 15 mm tube however the effect on performance is negligible in use.

If limited bore iso valves have been used then they will give a significant restriction. They should be replaced with full bore. Apart from that, whilst not correct I dont see much point in changing anything else.

Tony