De-scaling a heat store DHW coil

[diymostthings]

My son has a heat store with primary pumped boiler circuit to heat the store with a second pump serving the CH, controlled by the room stat. DHW is via an internal coil in the store, fed from the mains with a blender valve at the outlet. The problem is that although the store water heats up fine, and the CH works fine, the DHW will only give an initial "slug" of hot water after a period of non use, followed by a flow of at best mildly lukewarm water. I am almost certain the problem is a scaled up coil and/or blender valve.

Question: Is it feasible to just descale the coil (e.g. using citric acid or Kilrock or the like? I could make up a simple bit of kit to just circulate this solution around the coil until free from scale.

Any comments or advice (particularly on what de-scaler chemical to use) gratefully received!

diymostthings

 

[stevetheplumber]

yes you can descale the coil easyily with a descaling machine or as you say you could probably lash something up, personally id use spirits of salts you could just rig up a header tank with a slow running valve and a catch tank in the way that ascots used to recomend for their heat exchagers but think hiring a machine might well be as cost effective

 

[diymostthings]

Thanks Steve. Yes I was going to rig up a "closed circuit" of pump and polypipe with a vertical "leg" from a tee at the top to add the water and chemical. that way the CO2 will escape too.
Much appreciated - thanks again.

diymostthings

 

[Billy Bob Bob]

what make thermal store?

 

[diymostthings]

Its a Gledhill direct with integral header. Obsolete I know but we can't afford a shiny new unvented!

Does this make a difference to my de-scaling plans?

diymostthings

 

[Billy Bob Bob]

yes its not scale its a knackerd thermister or more probably a knackerd dhw pump i have lots of experiance with gledhills dont put a standard grundfos pump on its a special for this i get them of gledhill for £110 cheaper than merchants but this is probably your fault de scaleing is not the issue

 

[stevetheplumber]

yes its not scale its a knackerd thermister or more probably a knackerd dhw pump i have lots of experiance with gledhills dont put a standard grundfos pump on its a special for this i get them of gledhill for £110 cheaper than merchants but this is probably your fault de scaleing is not the issue

ive got a gledhill boiler mate and theres no hot water pump just a main pump and heating pump but mine needs descaling every five of six years its the stationary water in the coil that causes the problem i belive

 

[Billy Bob Bob]

only basing this on the faults that i do,bearing in mind i am in a soft/medium water area most boiler mates i see are unvented [dhw] side with 3 pumps and its normally the domestic pump which causes this fault

 

[Blackcatgas]

yes its not scale its a knackerd thermister or more probably a knackerd dhw pump i have lots of experiance with gledhills dont put a standard grundfos pump on its a special for this i get them of gledhill for £110 cheaper than merchants but this is probably your fault de scaleing is not the issue

very nearly fell foul of this, special pump that modulates down at low voltage.

ive got a gledhill boiler mate and theres no hot water pump just a main pump and heating pump but mine needs descaling every five of six years its the stationary water in the coil that causes the problem i belive

Could be a pulsa coil 2000, no heating pump, just a pump that circulates the stored heat through a plate type exchanger like in a combi.

 

[diymostthings]

yes its not scale its a knackerd thermister or more probably a knackerd dhw pump i have lots of experiance with gledhills dont put a standard grundfos pump on its a special for this i get them of gledhill for £110 cheaper than merchants but this is probably your fault de scaleing is not the issue

Hi gas man and thanks for your input. No it can't be a thermistor or pump problem - it hasn't got either on the system. The DHW simply comes straight off the mains, through the coil and out to the taps (via a blending valve). There are two pumps but one is on the boiler primary to the store, the other pumps the CH. The boiler and CH are working fine - i.e. full of hot water, so i think it must be scaling in the DHW coil in the store.

thanks again
diymostthings

ive got a gledhill boiler mate and theres no hot water pump just a main pump and heating pump but mine needs descaling every five of six years its the stationary water in the coil that causes the problem i belive

Hi Steve - looks lik eyou have confirmed my diagnosis! could I ask what you use to descale? (chemical used and kit)?

diymostthings

 

[stevetheplumber]

Hi gas man and thanks for your input. No it can't be a thermistor or pump problem - it hasn't got either on the system. The DHW simply comes straight off the mains, through the coil and out to the taps (via a blending valve). There are two pumps but one is on the boiler primary to the store, the other pumps the CH. The boiler and CH are working fine - i.e. full of hot water, so i think it must be scaling in the DHW coil in the store.

thanks again
diymostthings



Hi Steve - looks lik eyou have confirmed my diagnosis! could I ask what you use to descale? (chemical used and kit)?

diymostthings

i use a flushing machine with any proprietry descaler