Search the forum,

Discuss hot water cylinder return valve setting in the Central Heating Forum area at PlumbersForums.net

Messages
14
h all ,
i have a faily old s plan fully pumped system with a worster bosch r 18 heat only boiler,

the boiler was replaced about 4 years ago had a potterton neta heat before. 16/18 i think,

what i want to know it has a gate valve coming from the cylinder back to the reurn to the boiler , what should be the bet setting for this fully open as per the lasted regs or about half way as it is at the moment,
which setting will heat the hot water fasted fully open or half way , one thing i noticed on the the old bullet proof neteheat the boiler would run at full steam but on the newer boiler on hot water it modulates down is this due to return thermister sensor .im talking when only hot water is called for thanks baz
 
Fully open if you want quickest HWC warm up, throttled in (balancing) if you have a problem with rads heat up, IMO, anything more than 1/2 open (say 2 full turns) has the same effect as fully open, 1 full turn open might give you adequate HWC heating time and adequate CH. I run mine fully open, doesn't seem to noticeably affect the CH but I time the hot water on for 45 minutes before the CH in the morning, then both on as required by cylinder stat and TRVs.
 
Fully open if you want quickest HWC warm up, throttled in (balancing) if you have a problem with rads heat up, IMO, anything more than 1/2 open (say 2 full turns) has the same effect as fully open, 1 full turn open might give you adequate HWC heating time and adequate CH. I run mine fully open, doesn't seem to noticeably affect the CH but I time the hot water on for 45 minutes before the CH in the morning, then both on as required by cylinder stat and TRVs.
Fully open if you want quickest HWC warm up, throttled in (balancing) if you have a problem with rads heat up, IMO, anything more than 1/2 open (say 2 full turns) has the same effect as fully open, 1 full turn open might give you adequate HWC heating time and adequate CH. I run mine fully open, doesn't seem to noticeably affect the CH but I time the hot water on for 45 minutes before the CH in the morning, then both on as required by cylinder stat and TRVs.

thanks for your reply ived notice there is not much difference between half open or fully open one thing the hot water cylinder is very close to the boiler would that be anything to do with this also the hot water cylinder is about 20 years old , i have noticed the boiler seems to modulate down when in hot water mode only
 
I have attached a (excel) spreadsheet which shows the performance of my very standard DHW coil and you can see that the coil input varies from ~ 7kw to 2.5kw depending on the hot water temperature. Rapid heat recovery coils can input > 20kw but that is often based on a boiler temp of 85C and flowrates of > 35 LPM with coil heating surface of 1.0 to 2.0 M2.
 

Attachments

  • Hot Water Cylinder Tests Nov 2015 Extract1.zip
    9.6 KB · Views: 12
Your old cylinder probably has a low KW rated coil, even if you have say a 15kw boiler and 3kw coil it will only transfer at 3kw, which will take a set time for a certain volume to reach, basic laws of thermodynamics. As John is saying most modern day cylinders have high rated coils, like he said down to surface area. A cylinder could have a coil rating >20kw but if your boiler is only producing 15kw of power in an hour then it will transfer at boiler rating, this ofcourse will be less and more time required if the CH was running at the same time. With the valve fully open that circuit resistance will be minimal and water will naturally take that path but could starve the CH if the hydronic system is not properly designed. If both CH and HW are fine with valve open fully then leave be, it's really only there to help balance.
 
i thanks, ive attached a photo of my cylinder , telford cant help me with the spec on this at the moment but yes this was bought 20 year's or more ago , what i would like to know is normal for condensing boilers to modulate down when the hot water only is called for , and as i live in the south east could the cylinder be a bit scaled up now baz
 

Attachments

  • Iteloford cylinder.jpg
    Iteloford cylinder.jpg
    84.5 KB · Views: 5
Hot water only small circuit so boiler will modulate it doesn't know the difference but the return temp gets back quicker so boiler modulates
 
Your old cylinder probably has a low KW rated coil, even if you have say a 15kw boiler and 3kw coil it will only transfer at 3kw, which will take a set time for a certain volume to reach, basic laws of thermodynamics. As John is saying most modern day cylinders have high rated coils, like he said down to surface area. A cylinder could have a coil rating >20kw but if your boiler is only producing 15kw of power in an hour then it will transfer at boiler rating, this ofcourse will be less and more time required if the CH was running at the same time. With the valve fully open that circuit resistance will be minimal and water will naturally take that path but could starve the CH if the hydronic system is not properly designed. If both CH and HW are fine with valve open fully then leave be, it's really only there to help balance.
i thanks, ive attached a photo of my cylinder , telford cant help me with the spec on this at the moment but yes this was bought 20 year's or more ago , what i would like to know is normal for condensing boilers to modulate down when the hot water only is called for , and as i live in the south east could the cylinder be a bit scaled up now baz

You have a 140 ltr cylinder with a 0.7M2 coil. If one assumes a recovery time of 30 minutes from say 35C to 60C then by calculation the average coil output is 8.1 kw, (140*(60-35)/860*60/30)). So, based on this your boiler would modulate to ~ 45% output (8.1/18*100). The only time, realistically that I would see your boiler running at 100% output is if you start with a cylinder full of cold water, like in my test above, then as the cylinder heats up the boiler will modulate down to match the coil output, also, after 20 years or so there is almost bound to be some bit of coil fouling which will further decrease the coil output.
If you have the time/inclination you could (boiler off) run off the hot water down to say 35c (measured with a household thermometer from a tap if you havn't got a cylinder temperature sensor) or/and run off all the hot water and measure the time it takes to reheat up to 60c, you can then easily calculate the average coil output, also you can observe the boiler gas meter and take a few spot readings as the cylinder is heating up which will give a good idea of the reducing boiler power as the cylinder is coming up to temperature, ensure anyway that you take the gas meter reading at the start and end of your test even if you don't take any readings in between.
 

Reply to hot water cylinder return valve setting in the Central Heating Forum area at PlumbersForums.net

Creating content since 2001. Untold Media.

OFFICIAL SPONSORS

OFFICIAL SPONSORS

Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock