thermal store temperature low when CH on & limited HW

[matt1e]

just had a quick read through so sorry if its been mentioned (on the wine too) but are you sure that the store is actually full of water?
has the cistern been checked at all?

Matt

 

[Dan Robinson]

OK... Here is the charts from a little testing. You owe me two stores worth of water, gas and salt.

From a cold start, we were on after 30 seconds and switched off at 17 minutes.

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• Dan Robinson
• 13 Jan 2012
• 2

Bear in mind, (I assume) due to the way stratification works, my store always shuts down fairly quickly form a cold start.

After this period, I let it settle down and then, ensuring no heating could come on went for a long shower.

Water off the plate was blended to 53 degrees. The shower runs at 11 l/min (thought it was higher actually. Must get a bigger shower head).

The shower lasted 13 minutes, but started 60 seconds after the graph started plotting. After 10 minutes I cranked the shower up to about 46 degrees, not sure what the cold inlet temperature of the plate is though sorry. Boiler shut down after 26 minutes:

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• Dan Robinson
• 13 Jan 2012
• 2

Then the missus had her shower. For this I cranked all the rads up to max at 30 seconds. She started showering at 2:51. The boiler kicked in at 3:40. She finished in the shower at11 minutes and I switched the heating off at 15 minutes. At 17 minutes, for some womean reason she blippe dthe hot water. The boiler shut off after 23 minutes.

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• Dan Robinson
• 13 Jan 2012
• 2

Now, the Hot water draw off to the plate is at the same height in the store as the boiler flow and my measurement point. The plate itself is seated just above the centre point of the store. the rad flow is somewhere between the two. In theory, with no draw on either heating or hot water, my plate draw off static temperature should be the same as the centre store temperature.

The boiler return is a little above the heating return; and the store temp sensor is a little above those.

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• Dan Robinson
• 8 Oct 2011
• 3

No stupid coils involved. As advised... get the boiler taken out of the equation. It is killing your performance and efficiencies.

If you want the original log files feel free to email me. BTW not sure what version of Pico log you have but the new version is pretty handy compared to the one that came of the CD (release 5.22.2)

 

[thermostorebore]

Dan,

I can only thank you for the effort you have put in to record your thermal store operation. I do appreciate it . Your graphs are nothing like mine and prove how the thermal store should work. Your thermal store looks more like something destined for the ISS space station than something residing in a garage. No wonder it works, it looks like its been built by NASA .

Anyway back to business. The guy from Ideal boilers was in. After showing him the data and him having a root around he was pretty adamant that it was crud covering the thermal store coils. The boiler is just protecting itself when the return is nearly the same as the flow. He has seen something similar with the same setup. Although he did say we could reduce the flow, it was more of a band aid. Best course of action was to put some x400 in and let it act for a while then put x100 in.

Matt1E, Cistern is definitely full.

I think everyone is convinced its a thermal loss problem. I am away to get some x400 to give it a go.

Thanks to everyone for their input, knowledge and time. I'll be back in ~3 weeks after it has had time to work to let you know what happened.

C

 

[Norcon]

Best option is rip this thing out and fit an unvented cylinder.

 

[EddieBailey]

having looked at the graphs it shows that the coil only has about 8 degree differential.

This would indicate that the flow arond the primaries is fine but there is a problem with heat transfer in the thermal store.

I often see the reverse in gledhills with poor hot water performance.


I would recommend that you take a sample of the stores water to see what the ph value is.

I am guessing that this will be much more alkaline than you would expect and because of the store generally being above 65 degrees has scaled on the outside of the coil preventing heat transfer.

If this is the case then you have 2 options if you want to keep the store.

1, try to descale the store
2, have the boiler arrangement changed so that the boiler heats the store directly. ( luckily this boiler can be open vent aswell as sealed)

A good thread. It is just a boiler heating a coil that heating a cylinder of water, which applies to millions of systems. The boiler may still be at fault. Many cylinders heated indirectly from boilers act the same inefficient way, but because the water heat up eventually people do not notice. The boiler may prematurely fail because of the cycling, because it was not setup right in the first place.

This is a strange thermal store setup. It is best to clean out the store just in case, as in 1. above, using about three cans of X400. The suggestion at 2. above is the best, as the boiler can be open vented. This is assuming the boiler is OK of course. Connect the boiler directly to the cylinder, probably using the existing CH connections. Use the boiler coil as the CH take off coil, it is big enough to cope with the CH demands. Fit a sealed system kit to the CH circuit. Then no sludge will enter the thermal store from the rad circuit. The thermal store water will only be in the boiler and store and very clean, so it only needs one can of X-400 in the store and one in the rad circuit. It should work faultlessly for years on end after doing these simple updates.

Take no notice of the jokers who wrote saying to expensively rip it out and put in expensive unvented cylinders. If the boiler is at fault then the same problem will occur heating any cylinder through a coil.

 

[Cowan84]

I have just moved into a house in december and ive have what sounds like the exact same system only mines is worse!
I can get the store to 80C (as advised by Macdonalds) but as soon as the CH comes on the store drops to 40C and doesnt go above this till the CH goes off. (macdonalds suggest scaled up coil)
As the information suggests that the HW is to be on all the time i have set the store stat to 60C as this gives good hot water and the boiler is not always running as mines is inefficeint at the moment.

I found that there is a large amount of sludge in the rads which will of course mean that this sludge is passing over the coil and possibly reducing the efficiency. I have put in fernox F3 and noticed a couple of DegC rise within a week, ill let you know if it gets any better.

I was thinking that if the immersion heater was to be removed you might be able to see if the coil is scaled up... is this recomended?

where about are you based? im in dunfermline.

 

[thermostorebore]

Cowan,

I think we are probably in the same estate. Mcdonald Engineers told me the same thing that the coil was probably furred up. They said that they believe the installation of our systems did not include adding inhibitor to the thermal store. I think everyone in the estate will probably have dying systems soon.

I have had x400 running round my system for the last 3 weeks and to be honest I haven't seen much improvement despite whacking the pipes and thermal store on a daily basis.

I had to get the immersion heater replaced once because it was leaking. It only leaked when the tank was cold. I did think about getting someone to unscrew the heater so I could have a look in but after my one leaked I am reluctant to touch the interface again.

Eddie came up with a possible solution about swapping over the boiler and central heating pipes so the boiler directly heats up the thermal store. I have to check whether this is possible or not with the HE24 boiler as I believe my one is a pressurised system and you need to have open vent.

If this is not a possibility then I am going to put a flow reducer in the return back to the boiler to slow it down. 1 step at a time.

C

 

[simond]

I dont think you need a flow reducer if the Ideal has isolation valves, just partially shut them.

 

[John67]

forgive me if someone has already said this but you should really have a stat to operate the central heating pump so that the temp in the store cant drop below 60-65 deg,this would make sure you always have enough heat for DHW. I do know this does not solve the problem with the boiler but it is good practice.

 

[Cowan84]

haha ill need to get you round to compare.

the system might need drained and cleaned a couple of times if there is so much dirt in it, also i was considering jumping out the rads and putting a loop on at the pump then filling with F3/water to see if circulating just the tank would make a difference on cleaning the coils.

Just a note for you that i was told about our boiler, it has an aluminium heat exchanger in it so we should be using fernox as it has additives suitable for this but most other makes dont and you need to add an aluminium additive, someone in the trade would be better to advise on that.

Good idea about using the boiler to heat the store water, let me know how that goes please as i might have to go that route.

My neighbour says that his system has been working perfectly and there is always hot water and the rads are hot but ive not been in to have a good look at his, ill need to go round again.

 

[Cowan84]

C
do you have a red balancing valve on the outlet of the tank? it might be worth a try closing that more and see if that helps to keep the temps in the store. Where is the valve set to at the moment?

 

[thermostorebore]

All,

Right update on the thermal store.

So I had x400 in the thermal store for 4 weeks. It made no real difference.

I had a heating engineer out today to remove the immersion heater so we could have a look inside the thermal store to confirm that it is the coil that is furred up.

The link below shows the state of the immersion heater that was taken out

//www.diynot.com/network/thermostorebore/albums/

It was covered in black oxide. It was really well adhered as well. I put some x800 directly on to the coil. It didn't really move it at all. The picture shows some copper exposed. Inside the cylinder the coil is caked in black oxide and so is the sides. Hardly any copper exposed.

Now that it is confirmed what it is, the next question is what to do about it.

I have tried closing off the isolation valve on the return of the boiler to 3/4 shut. It didn't really make any difference, no wonder having seen the state of the cylinder.

My next port of call is stronger chemicals. Has anyone used Fernox DS40 successfully to treat black oxide like this. This really is the last chance for this cylinder. Does anyone know of a better product.

Any advice is very much appreciated

C

 

[RWhites]

I've been following this thread with interest as I intend to fit a thermal store at some point this year...

I'm no heating engineer, but my practical physics is pretty reasonable. Looking at the pictures you posted, the oxide doesn't look particularly thick, so I'd be surprised if it has a massive impact on the thermal transfer performance, indeed it could be argued that being black is advantageous - as long as the coating it isn't too thick.

My (old and rubbish) heating system has a thin oxide layer inside most of the pipes, but if I touch the outside of the pipes when the heating is on, they are certainly hot - the point being that the (thin) oxide layer is making little difference to the thermal transfer.

My opinion about the problem.... I have to agree with SimonD. It strikes me that the issue is that the flow rate of the water through the boiler, and therefore the 'heat exchanger' coil in the TS. It is going too fast to be able to 'dump' / transfer the heat to the water in the TS, so you need to slow it right down.

Once it is slowed, it will have a chance to transfer heat to the TS, the return temp to the boiler will be lower, and the boiler will keep firing - improving the overall heat transfer.

SimonD did originally suggest opening the return valve 'by only 3/4 turn' (if I remember correctly); that seems to have been re-typed as 3/4 closed in one or two posts. So try it as originally suggested - or variations around there. 3/4 closed might not be restricting the flow enough.

I'm now officially out of my knowledge comfort zone BTW, but first ensure the pump is going as slowly as possible (not the CH pump, the boiler to TS pump), then experiment further with restricting the flow via the return valve. As a last resort (and still relatively cheap to do), why not try installing a radiator as a 'heat dump' on the return side? - If only to test the theory....

Just my thoughts.... good luck!

 

[Dan Robinson]

I agree that from the photos, sludge is not the problem.

I remain convinced the flow/heat transfer is not enough for the boiler - but need to re-appraise myself of the thread from start to finish (which will have to be later).

 

[jackie2321]

What Estate are you on ?