Slow Recovery Time

[NormanPrice]

Hi,
I've just had a new central heating system installed and the recovery/heat up time for the water is very very slow.

My old system was a vented one and when the water was boosted the boiler would throttle up and run at maximum for a period of time (maybe 10 mins depending on the initial temp of the water in the tank) before gradually throttling/modulating down to its minimum as the tank heated up. With the new system when the hot water is boosted the boiler starts up but never really gets beyond its minimum setting.

The tank is an unvented Gledhill 300lt solar, the boiler is Worcester Bosch Greenstar 15Ri and all the pipework is new.

The boiler is heating a larger volume of water (175lt) than before so perhaps it should take longer but I don't understand why the boiler never gets about its minimum setting. From 20deg it takes 90mins plus to reach 55deg vs the published heatup time of 40mins to 60 deg. With my boiler I would never achieve the published heat up time but something doesn't seem quite right.

Any ideas welcomed!

Norm

PS The heating side of things works fine so its not the boiler. I hope . . .

 

[Dan Robinson]

but I don't understand why the boiler never gets about its minimum setting

Ｗｈａｔ　ｄｏｅｓ　ｔｈｉｓ　ｍｅａｎ？
Ｗｈａｔ　ｉｓ　ｔｈｅ　temperature　ｋｎｏｂ　ｏｎ　the　ｂｏｉｌｅｒ　ｓｅｔ too?

My guess for your boiler would be around 55 minutes for a 35 degree rise, but I don't know what the coil rating for the cylinder is. But then I have a very low opinion of Gledhill.

 

[NormanPrice]

The temperature is set to 70-75 deg. This may be below the optimum but this was the setting used previosuly.

By minimum setting I mean its always running at its lowest output level, 5kW I think.

The coil is rated at 20kW.

Norm

 

[Agile]

70-75 C is correct!

But to investigate further you need to measure the flow and return temperatures at the boiler ( or cylinder heating coil ).

If they are more than about 15 C then the circulating rate is too slow and either a restriction or the pump is not working well. What speed setting? Should work fine on "2".

Tony Glazier

 

[mysteryman]

Tony means that the temperature difference between flow and return should not be much more than 15 degrees.

 

[NormanPrice]

The temps have only been measured by hand so not very scientific. Next to the cylinder the flow pipe is piping hot and the return pipe temp varies from very cool when the tank is cold to hot when it's heated up. Wouldn't say the return is getting as hot as the flow but whether it 10 deg or 20 deg difference at the end I'm not 100% sure. I was thinking of getting a thermometer but wasn't sure which kind was best.

It's an alpha2L that's been fitted and I've tried all the settings. Though you can hear the speed change it doesn't seem to make much difference! I guess if the boiler is knocking out only 5kW the coil would easily absorb it?

Would the pipe diameter make much of a difference? 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

 

[Agile]

That could be the problem! But that will be indicated by an accurate temperature measurement. The best temperature measurement is with a digital meter and a thermocouple wire probe. Available at Maplins although they have discontinued the best/cheapest options.

Can you force the pump to maximum speed?

With a 15 kW boiler, 15 mm is acceptable if the distance is fairly short.

Tony

 

[NormanPrice]

I've tried force the speed to maximum. It seems to have made a bit if a difference as the boiler throttles up to a higher output initially. It's nowhere near maximum though that's for sure - gauged purely on noise!

The pipe run is about 6m horizontally and 3m vertically from boiler to cylinder flow inlet. There are probably six 90 deg bends as well.

Thanks for your comments

Norm

 

[D_Hailsham]

You say the boiler never gets above it's minimum output. Is it going through a cycle of running for a few minutes and then cutting out? If so this is because the hot water cylinder is not extracting the heat from the boiler water.

Are there any gate valves on the flow and return pipes between boiler and cylinder? If so, how far are they open?

 

[NormanPrice]

It's not switching on and off. It's just running constantly at minimum output level, it's throttled/modulated back as far as it can.

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.

Does the flow being piping hot and return one being cool not indicate that coil is extracting the heat from the water?

Thanks for help/advice/comments.

Norm

 

[D_Hailsham]

It's not switching on and off. It's just running constantly at minimum output level, it's throttled/modulated back as far as it can.

So the boiler can keep up with the heat extracted by the cylinder the boiler when running at min output.

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.

They're the ones.

Does the flow being piping hot and return one being cool not indicate that coil is extracting the heat from the water?

Possibly.

Are you getting the problem when the heating is on or just when it's heating the hot water?

You say it's a Gledhill solar cylinder. Is it the SOL300i Stainless Lite Indirect Solar? If so the spec says it has a 20.5kW primary coil and the reheat time after 70% draw off is 20.3 minutes.

Who specified the WB 15Ri boiler?

 

[Agile]

Your boiler is fine!

There is insufficient flow to deliver enough power to the cylinder!

Tony

 

[ALEC1]

why on earth whould any one connect a 20kw coil with 15mm pipe... as a rule of thumb in domestic installs a 15mm pipe only carries 5kw of heat..

that too may be the issue...

 

[Dan Robinson]

Glad I'm not the only one pondering that one....

 

[gaswizzard]

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?