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central heating heat loss

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central heating heat loss

Postby RogerS » 04 Jan 2018, 13:00

Can someone please confirm that I am correct in my thinking ?

The first photo is a rough drawing of how the CH layout used to work. The pump was located in the garage by the heat store. The pump fed a metre into the house where the feed split to two motorised valves. One valve sent the hot water down a dedicated pipe to the hot water cylinder. The second valve fed the CH feed and about another metre after this valve it split to feed the upstairs rads and downstairs rads. Within feet of where the split occurred, this feed pipe was hitting the radiators that it was designed to feed. Assuming that the system was reasonably balanced then my argument is that the temperature in this feed pipe is going to be roughly that when it left the heat store...give or take the odd degree.

Wardrew ch pipe layout.png
(117.22 KiB)


NB the diagram does not show the CH split to the downstairs rads.

Now when my plumber put in the new boiler he was very fixated on neutral points and insisted that the pump be relocated up at the hot water tank and the feed and vent pipes connected there to create the neutral point. However this meant that the motorised valves needed to be located here as well which in turn necessitated modifications to the primary hot water feed.

The two original valves were removed and the T between HW and rads removed. The feed to the rads was cut off and blocked. The original and remaining feed to the HW tank became the main feed to the new pumps' location. Two new valves after the pump...the one we're interested in...the CH rad one then feeding a new pipe that T'd in part way down the upstairs original CH feed run.

Wardrew ch pipe layout  modified.png
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With this new arrangement, I am 11 degrees of heat lower at the pump then that leaving the heat store because of the long run now built in to the system due to the new layout. 66 degrees leaving the heat store (don't worry about whether this is high enough etc for the purposes of this discussion) and 55 degrees when it hits the actual CH upstairs rad circuit. My plumber does not accept this. He says that I'd see the same thing on the original system which I think is bonkers. Original system...distance between heat store and first rad ...4m. New system...distance about 30m.

A pressurised system was and is out of the question as there was existing inaccessible pipework underneath the floorboards with joints of unknown provenance and as I'd already discovered one joint that wasn't properly soldered, I think this decision is sound.

Supplementary question. The furthest radiators from the CH feed point are in the kitchen. With every other radiator and HW tank off, the hottest temperature of the incoming CH water to these rads is a meagre 36 degrees. Is this an indication of an under-powered pump or a blockage in the old pipework ?

Second supplementary question. My plumber says that you get different temperature readings if the pipe is vertical or horizontal. EG horizontal copper pipe coming to a 90 degree feeding the vertical pipe into the radiator tail. Doesn't make sense to me.
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Re: central heating heat loss

Postby 9fingers » 04 Jan 2018, 14:16

For whatever reason this smacks of inadequate flow rate.

This could be any of:-

too high flow rate through another part of the circuit
inadequate pump flow
airlock
blocked pipes

11 degree drop along a pipe is too low flow rate giving time to lose heat. If the flow rate were near infinite, then there would be no loss.

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Re: central heating heat loss

Postby RogerS » 04 Jan 2018, 14:44

9fingers wrote:For whatever reason this smacks of inadequate flow rate.

This could be any of:-

too high flow rate through another part of the circuit
inadequate pump flow
airlock
blocked pipes

11 degree drop along a pipe is too low flow rate giving time to lose heat. If the flow rate were near infinite, then there would be no loss.

Bob



Thanks Bob...agree with all that. What is your take on the plumbers statement that you'd get 11 degree loss in the original circuit. Which I disagree with.
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Re: central heating heat loss

Postby 9fingers » 04 Jan 2018, 14:50

The only places for significant temp drop is across the boiler,cylinder and rads.
Anywhere else is loss/waste.
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Re: central heating heat loss

Postby RogerS » 04 Jan 2018, 15:00

9fingers wrote:The only places for significant temp drop is across the boiler,cylinder and rads.
Anywhere else is loss/waste.
Bob



I agree but my concern is that there is more heat loss with the new layout than the old by virtue of the way it's been fed? Am I right ?
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Re: central heating heat loss

Postby 9fingers » 04 Jan 2018, 15:09

RogerS wrote:
9fingers wrote:The only places for significant temp drop is across the boiler,cylinder and rads.
Anywhere else is loss/waste.
Bob



I agree but my concern is that there is more heat loss with the new layout than the old by virtue of the way it's been fed? Am I right ?

Quite possibly but it is exposing the weakness in the flow rate for whatever reason.
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Re: central heating heat loss

Postby 9fingers » 04 Jan 2018, 15:41

9fingers wrote:
RogerS wrote:
9fingers wrote:The only places for significant temp drop is across the boiler,cylinder and rads.
Anywhere else is loss/waste.
Bob



I agree but my concern is that there is more heat loss with the new layout than the old by virtue of the way it's been fed? Am I right ?

Quite possibly but it is exposing the weakness in the flow rate for whatever reason.
Bob



If the pipe run is the same length as before and the flow rate the same then the temp loss across all the pipes should be the same but with a different distribution. But there should be minimal temp loss across the pipework by design.
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Re: central heating heat loss

Postby 9fingers » 04 Jan 2018, 15:51

I wonder if in an attempt to balance the system, you have strangled the flow through all the radiators and hence increasing the pipework loss?

At least one radiator should have its lockshield valve fully open and the others throttled back, if needed, to both achieve balance and maximise flow rate.

Does the temperature difference change much with pump speed? not sure what that would tell you apart from that a greater flow should reduce the temp drop.

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Re: central heating heat loss

Postby RogerS » 04 Jan 2018, 15:55

9fingers wrote:
9fingers wrote:
RogerS wrote:

I agree but my concern is that there is more heat loss with the new layout than the old by virtue of the way it's been fed? Am I right ?

Quite possibly but it is exposing the weakness in the flow rate for whatever reason.
Bob



If the pipe run is the same length as before and the flow rate the same then the temp loss across all the pipes should be the same but with a different distribution. But there should be minimal temp loss across the pipework by design.


That's the crux of it. Initial system, only a few metres between the heat store and the first rad in the system...So any heat loss is that in those few metres. New system...there is about 40m of pipework before we get to the first rad. So surely there will be a lot more heat loss - given the same flow rate as before.
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Re: central heating heat loss

Postby 9fingers » 04 Jan 2018, 16:04

So the loop length is now much greater than before?
If so presumably the bore has been increased to reduce friction? Has the frictional loss in those fashion radiators been taken into account. :lol:

ISTR that you had airlock problems when filling. Now that dissolved air will have come out of the circulation water, are you convinced that no further airlocks exist?

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Re: central heating heat loss

Postby Deejay » 04 Jan 2018, 16:36

Afternoon Roger

The furthest radiators from the CH feed point are in the kitchen. With every other radiator and HW tank off, the hottest temperature of the incoming CH water to these rads is a meagre 36 degrees. Is this an indication of an under-powered pump or a blockage in the old pipework ?

Does your boiler have an automatic bypass valve to maintain a minimum flow through the boiler?

Under the above circumstances, might it be opening and diverting some of the flow away from the heating circuit? The boiler will still reach the set flow temperature but the flow to the heating circuit will be reduced by sending some of it straight back to the return side of the boiler. The speed of the CH water may be reduced to a point where losses in the pipework become excessive.

P.S. +1 to Bob's suggestion re bleeding.

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Re: central heating heat loss

Postby RogerS » 04 Jan 2018, 17:38

OK....plumbers have spent 4 hours here and just left. Lots of air in the system and they spend a long time getting rid of it. hoses, suction pump, the works. Some of that air was limiting the flow in the main return we think. I could get one or two rads really toastie but fire them all up and they all became virtually cold.

We've left the system to settle down for a few days and then he's coming back to do a final balance. Think that or two of those lousy Italian Cordivari radiators are Friday Night jobs. Time will tell.

We had the heat loss discussion. I came at it by describing a system where there was 500 yards of copper pipe between heat store and the first rad. Eventually he agreed with me. But we then had the further discussion about the discussions we'd had at the outset. There were the three options - as he reminded me.

Option A - keep the pump at the heat store and run two pipes down to it to create the neutral point. Trouble was that given all the steel etc in the building getting a clean run back - especially for the vent pipe that needed a gradual upward slope - was nigh on impossible.

Option B - the one we went for. Move the pump up to the area of the HW tank (where the feed and vent pipes were), make the tweaks to the circuit as outlined above. The option we went for. What I am a bit annoyed about is that he didn't point out to me the implications re heat loss in the changed circuit - especially as it was he who suggested the use of the heat store to run the boiler fully in condensing mode etc. This heat loss may mean that I can't run the boiler as efficiently as I wanted to and I've wasted all that money. Time will tell. He suggested that with the improved flow rate that it might not be as bad as it was. Fingers crossed.

Had I known about the heat loss then I guess I could have tried a bit harder to find a way to go the Option A route. Hard call. I do know that I tried damn hard the first time round (it was possible pump noise that was the driver as LOML has the hearing of a barn owl).

Option C - a pressurised system. Which, as previously mentioned, was ruled out due to unknown existing pipework provenance. We shall see...may have to bite the bullet and go for it downline.

Many thanks for all the advice, chaps
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Re: central heating heat loss

Postby Andyp » 04 Jan 2018, 18:08

Stupid question perhaps. How is all the air getting into the system being as you bled the system just a week or so back? and IIRC only one rad was a cause for concern.
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Re: central heating heat loss

Postby RogerS » 04 Jan 2018, 19:56

Andyp wrote:Stupid question perhaps. How is all the air getting into the system being as you bled the system just a week or so back? and IIRC only one rad was a cause for concern.



Bleeding rads only bleeds what's in that rad at that moment in time. Here, a lot of the airlocks were in the main pipework. You need to replace the bleed valve with a drain cock and open it up good and proper and hope that the head in the tank pushes out the air...as the path is now freer and not restricted by the bleedvalve. Even that didn't work and they had to stick a suction pump on the rad to pull out the airlock.
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Re: central heating heat loss

Postby Andyp » 04 Jan 2018, 19:58

Fairy snuff
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Re: central heating heat loss

Postby RogerS » 04 Jan 2018, 22:31

Based on a couple of comments that Bob made I think I will ring up Grundfos tomorrow to see if they have a faster circulating pump.
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Re: central heating heat loss

Postby RogerS » 04 Jan 2018, 22:42

:eusa-doh: If the water runs faster then the rads wont have time to radiate... :eusa-think:
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Re: central heating heat loss

Postby RogerS » 05 Jan 2018, 17:31

Aaaaarrrghh! And it's stopped working. Well, not 100% stopped but back to its old bad ways. Nothing downstairs and upstairs rads tepid at best. Pointing to the return having another airlock or partial airlock in it.

I temporarily turned the CH pump off - nothing out of the ordinary ..it is usually cycled on/off by the room thermostat) but I am guessing that there must still be some air in the system and that it had been pushed to one side as it were while the pump was running, pressurising the system to keep the air lock back in its box.
Switching the pump off let the genie out of the bottle and so we're back to square one.
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Re: central heating heat loss

Postby RogerS » 06 Jan 2018, 17:23

Who's the Daddy. Hosepipe out...bled again...now working. Let's be clear about this...bled as in not the wimp bleed from the standard bleed vale but yer full-on bleed using a hose pipe directly connected to where the bleed valve used to be.
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Re: central heating heat loss

Postby 9fingers » 06 Jan 2018, 17:32

RogerS wrote:Who's the Daddy. Hosepipe out...bled again...now working. Let's be clear about this...bled as in not the wimp bleed from the standard bleed vale but yer full-on bleed using a hose pipe directly connected to where the bleed valve used to be.


Every time you add fresh water, you are adding air too which will likely build up over time to cause a problem.

Seems to me that it needs sorting perhaps with an extra bleed point where ever the air is collecting.

Not what you want to hear I'm sure. :D

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Re: central heating heat loss

Postby 9fingers » 06 Jan 2018, 17:54

When I did my solar thermal installers course, one of the tips the lecturer gave us was to install a deliberate "dead leg". This is a vertical pipe installed off the circuit with a bleed valve at the top. Air progressively collects in the dead leg harmlessly and every now and then it can be let out as part of routine maintenance.

The reason it is used in solar thermal system is that you can't usually get to the highest point as this is outside on the roof! But is seems that you have some in-accessible points with pipes running in walls ( absolute 'kin madness in my view but anyway you are where you are)

I also understand why you don't want a standard pressurised system but having it pressurised to slightly more than the head provided by the top to bottom height should help with bleeding.
The circuit becomes mains water, non return valve, pressure reducing valve, expansion chamber and then onto the current circuit (anywhere convenient) with the header tank and overflow replaced with a pressure relief valve with its output connected to the overflow pipe to the outside world.

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Re: central heating heat loss

Postby RogerS » 06 Jan 2018, 18:31

9fingers wrote:When I did my solar thermal installers course, one of the tips the lecturer gave us was to install a deliberate "dead leg". This is a vertical pipe installed off the circuit with a bleed valve at the top. Air progressively collects in the dead leg harmlessly and every now and then it can be let out as part of routine maintenance.


That's a nice idea. How high does it need to go ?

9fingers wrote:I also understand why you don't want a standard pressurised system but having it pressurised to slightly more than the head provided by the top to bottom height should help with bleeding.
The circuit becomes mains water, non return valve, pressure reducing valve, expansion chamber and then onto the current circuit (anywhere convenient) with the header tank and overflow replaced with a pressure relief valve with its output connected to the overflow pipe to the outside world.

Bob


That's worth considering. I'm guessing that the output pressure relief valve could be connected in anywhere ?

What sort of expansion chamber do I need ?
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Re: central heating heat loss

Postby 9fingers » 06 Jan 2018, 19:01

Dead leg height? enough to house whatever volume of air you anticipate between bleeds. You can use larger bore pipe instead of excessive height?

Expansion kits - ebay search for "expansion vessel kit" Maybe fit that where the header tank is so the pressure relief valve can discharge into the over flow pipe. The braided hose and NRV can go wherever you are connecting to the mains

These can include everything you need apart from the pressure regulator ( pressure reducing valve) Toolsatan supplied mine including gauge. 1/2 bar = 15 feet head to 1 bar (32 feet) should do nicely.

hth
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Re: central heating heat loss

Postby 9fingers » 06 Jan 2018, 19:19

DOH! just thought that you can take the water feed from the header tank ball valve too so put the whole lot along side the header tank!

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Re: central heating heat loss

Postby RogerS » 07 Jan 2018, 06:29

Just a thought. Haven't I already got a deadleg ? The vent pipe ?
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