DIY Heatbank - fine tuning of system (incl CH)

Well I have finally implemented my DIY Heatbank system, using the following as a very useful starting point.
http://www.wiki.diyfaq.org.uk/index.php?title=DIY_Heat_Bank
I have a couple of things of which I am currently uncertain in the fine tuning of my setup:
1) Currently I just have a single cylinder stat which is about 400mm or so from the bottom of the tank. I have read elsewhere regarding the use of 2 cylinder stats and a latching relay. I understand this is to allow the tank to work within a temperature range before the boiler re-fires to prevent cycling. I think I get far more cycling from my CH than from the Heatbank though - which leads on to an additional question below. But first: what height does the 2nd stat go on the tank? How are the 2 stats wired together? Can you buy an off the peg latching relay to do this? If not what components are required? Is it worth doing?
2) The CH does not go via the heatbank; this is currently just for DHW. I have a programmable Horstmann roomstat for the CH, but am very dissapointed at how much cycling this causes. It switches in and out very regularly. Can anyone recommend a better unit? I have seen the Honeywell CM67 highly spoken of, but am not sure whether it is still made; screwfix don't list it.
3) I am aware that CH can also be fed off a heatbank, but am not sure exactly what additional plumbing I'd need to do - would I need new tappings into the tank, or could I just tee of the existing output from the top of the tank and use a flow valve after the CH arm of the tee? Currently the flow from the boiler goes system pump > 3port valve -> 1 leg to CH flow // 1 leg to top essex flange/diffuser on the cyclinder .
4) One curious thing I have noticed whilst becoming more intimate with my system is that when the system pump is running a trickle of water is coming over into the FHE tank. The problem originally started when I moved the boiler and pump from the cellar - the flue location did not meet current building regs when the boiler was replaced, so no option but to relocate to 1st floor. Now the pump is obviously much closer to the FHE tank. There is no actual overflow, but hot water is flowing round and back down the fill pipe, which isn't ideal as the tank is acting as a small inefficient plastic radiator in the attic, and causing condensation to boot!! The only way I can prevent it is to close the valve on the fill pipe, once I'm sure the system is devoid of air. This stops the flow around through the tank, effectively making the enclosed, but not unvented as any expansion can still occur up the overflow pipe towards the FHE tank. I don't think running the pump on it's lowest setting is viable - it is a 4 storey house. The boiler and pump (and the heatbank) are all on the 1st floor and heat has to go down 2 storeys and up 1 storey and return obviously, so the pump needs a bit of grunt. What on earth can I do to prevent this occuring?
5) And finally what are the optimum flow and return temps of a condensing boiler - I have a Worcester Bosch Greenstar 24Ri (24kW). I have seen 70/50deg C, but the heatbank needs 75-80degC, so I'd need to go a bit higher than this. If I set the boiler too high, it tends to cut in and out too, even if the system pump is set high, so it's a delicate balance I need to find.
=====
For background and if anyone is interested, the following specific info may be a useful addition to what is on the DIY Heatbanks wiki:
- The first significant improvement I to the blueprint on the wiki was using a 100kW PHE from GEA which was about 180 incl VAT.
- The second improvement is diffuser pipes within the cylinder. I have a c200litre direct tank [1700x500(dia) mm] double insulated. I had the tank made to spec to fit the space I have available with a single top port - a standard BSP 1" male connector. The top connector takes the flow to the PHE primary and is tee'd off to the FHE tank overflow. I added 2 essex flanges myself through which I passed my own diffuser tubes prior to assembley of the flanges into the tank. The diffusers are 22mm copper tube with a yorkshire end stop soldered on and between 30 and 40 (can't recall exactly now!) 6mm holes carefully drilled and cleaned of swarf as much as poss. The diffuser runs across the full width of the tank from the flange to just short of the the opposite internal side of the cylinder. The bottom flange is 100mm from the bottom of the tank and is tee'd to take the PHE and boiler returns (they actually flow in different directions - check the wiki if that confuses). The top flange is for the boiler flow and is just below the top of the cylinder side, below the join with the dome cap. The holes on the diffuser are oriented to aim sideways and upwards from the top diffuser and sideways and downwards from the bottom to diffuser to preserve stratification in the tank.
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To quote from Heatweb:
"With large or fixed output boilers it may not be desirable for the boiler to fire up when only a very small amount of store water needs heating, as the boiler may cycle. To overcome this we can fit a second cylinder thermostat, which will hold off the boiler from firing up until both thermostats are calling for heat. The boiler will then fire until both are satisfied."
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Yes, I knew that!!! Read the question D'OH
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In article < snipped-for-privacy@w56g2000hsf.googlegroup

LOL, I don't know if it is a good idea (the concept sounds ok) but if you want to try it, here's how to do it:
No need for a latching relay, use a common double pole mains relay and use the second contact to provide the latch electrically.
(Use a fixed width font)
___L | RLY | CONTACT A | __- __-o |----o' o------------------o' o---| | LOW LVL STAT | | __- |-----------------| |----o' o---------| HIGH LVL STAT | _|_ ___L RLY | | | CONTACT B | | | __-o | | |--o' o-----------> RLY |___| BOILER COIL | CALL FOR HEAT | _|_ N
All contacts are shown in the no demand position, stat contacts open, relay de-energised.
Control sequence:
1. Low level stat will close first but cannot energise the relay because there is no direct connection to the coil.
2. High level stat closes, energising the relay and closing contacts A and B. Contact B feeds demand to the boiler.
3. After some time heating the heat bank, the high level stat will drop out but the relay will remain energised as it is now fed from the low level stat via relay contact A.
4. When the low level stat drops out the relay de-energises and demand is removed from the boiler.
Almost any 2 pole mains power relay will do the job, first one to hand is from http://cpc.farnell.com/ , item code SW02489, 5.12 inc vat and a screw terminal holder is code SW02484, 3.50 inc vat. Grab a plastic box to mount it in while you're there.
It may be possible to use a single pole relay to do this job but it is cleaner to separate the functions and 2 pole mains relays are cheap.
HTH
--
fred
Plusnet - I hope you like vanilla
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Fred
Many thanks for the guidance on the latching relay circuitry - looks pretty straightforward now I've had a chance to study it a bit.
Can anyone recommend approximate placements of the low and high stats on the tank itself
Cheers
Mike
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Mike Holmes wrote:

Doing it will increase the hysteresis of the stat a bit, and delay the recovery of the heat bank. Depending on how close you get to depleting the heat bank this may or may not be desirable.

The cycle rate of the state is more an indication of the rate of change of temperature in its current location. Is your in a draft?
(I have a Horstmann Centaur stat 7 day programmable jobbie, and don't find that it cycles particularly quickly).

If you used the existing ones, your would in effect run you rads at a fairly high temperature - a lower top tapping would drop the rad temp a bit and might give more even room temps with less overshoot. This will be at the expense of a slightly longer warm up time from cold.
IIUC, your boiler is non modulating, so imposing the heatbank between rads and boiler may reduce cycling a little. (with a boiler that modulates over a wide range this is less desirable since it just complicates the boiler ability to load balance)

"over into" from where? Up the Feed and Expansion pipe, or falling out of the vent pipe?

Lower return temps are more efficient for the boiler, but you are restricted there by the desire to get sufficient energy into the store. I would have thought with a direct tank, it ought to keep the return temperature reasonably down for a fair proportion of the heating time.
Some sticky tape (as a target) and an IR thermometer is handy for tuning here so you can see what is happening with the various pipe temperatures.
--
Cheers,

John.

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John,
Many thanks for the detailed relies - further comments/clarifications below:

Well I hadn't thought so - didn't appreciate it would be so sensitive. It is on the side of the stairs with open ballustrades (where the old room stat was originally), so I guess there may be a slight draft coming down off the edge of the stairs and causing it to cycle

This is exactly the model I have, so that's reassuring that it works well for you. Will have to relocate mine I think.

No it's fully modulating - here is the spec:
http://www.worcester-bosch.co.uk/index.php?fuseaction=product.techdata&con_id 3702

Why is that? I'm thinking that because the boiler fully modulates that there shoudln't be a problem with having the boiler set high for the DHW, meaning the return temp would be higher than ideal from the CH, if the boiler DIDN'T modulate. However as my boiler DOES modulate does this mean that even if it's on 100% (which equals 24kW and max flow temp of 82degC) that it will actually modulate down to achieve the ideal return temperature?
I think perhaps the problem of my system cutting in and out is possibly purely down to the poor placement of my room stat.

I thought the feed pipe and the expansion pipe were different things and that the expansion is synonymous with the vent pipe. Anyway in your terminology, it is flowing out of the vent pipe and back down the Feed and Expansion pipe. As I said there is no net overflow or actual filling from the mains going on - just a flow through the FHE tank that shouldn't be happening.

It's OK on the DHW demand as you say, but I'm thinking the CH return will end up being too high, unless the boiler's ability to modulate down gets round this even though the boiler output is set way up.

The IR thermometer was a bit pricey, so I got a digital one with a probe from screwfix and some thermal paste from maplin to conduct efficiently.
Thanks again
Mike
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Mike Holmes wrote:

Pretty much the same place I have mine by the sounds of it.

http://www.worcester-bosch.co.uk/index.php?fuseaction=product.techdata&con_id 3702
ah ok, the sedbuk database has it wrong then!

OK, with a modulating boiler that changes things. Usually you set the desired flow temperature, and the boiler will adjust the power input to achieve its desired return temperature, so it should run closer to maximum efficiency for longer.
If you stuck the thermal store between boiler and rads then you introduce a big energy store and damper into your closed loop control system. The effect of which will vary depending on the details. It may still work fine, or you might find there is enough phase lag in the control feedback that you get erratic temperature variations.
So unless you have a very well insulated house that usually requires significantly less than the minimum 8kW output of the boiler, you will usually get better results letting the boiler drive the rads directly. Note that 8kW is relatively high, so you would expect some cycling whatever you do once the house is up to temperature.

It could well be. How is the temperature control in the rest of the house? Depending on the answer, you might want to play with the balance of the uncontrolled rad in the same room as the stat.
I noticed when upgrading from a fixed output boiler with a conventional stat, to a modulating one with prog stat the general level of comfort and consistency in the house improved. The boiler runs for longer periods at low output and there is less temperature swing in the house.

No, you have a Feed and Expansion pipe - this will be fed from under the water line in the header tank. It will allow water to feed into the system, as well as allowing expansion to push water back into the tank as the system heats up.
In addition to that you have a vent pipe that dispenses back into the top of the tank, and it serves as a safety feature to prevent dangerous pressure build up. Normally nothing should flow from this pipe.
It is also possible to have both these pipes combined. In this case the vent runs as it currently does, and the F&E pipe tees into it a couple of feet above the highest part of the system pipework.

That is pump over, and you are right, it is not good. Not only will the tank contents get very hot, you will also be introducing fresh air into the water which will accelerate corrosion in the system

The modulation ought to help, but once the house is warm and the TRVs on the rads start to close, the return temp will start to rise. There will come a point where the boiler will have to cycle off on its own internal stat since it won't be able to maintain the output temperature below your selected maximum while still heating at 8kW. Usually in this case the boiler will keep running its pump and then just kick in when the return shows a drop again (assuming the prog stat is still calling for heat).
Note that this type of cycling does not have the same inefficiency drawbacks as you would get with an old high water content boiler with external controls and pump. There each time the boiler cycled off the retained heat would end up being lost from the flue. On yours it should keep most of the heat in the primary circuit, and not much is lost from the boiler anyway.

Yup, that should do it...
--
Cheers,

John.

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You didn't try Ebay then for your IR Thermometer ? A tenner (from HK !!) and seems excellent.
Rob
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On Thu, 13 Dec 2007 03:47:38 +0000, John Rumm wrote:

Or possibly, with a programmable stat, proportional control feature. This is feature where the stat tries to balance the heat loss with heat input from the boiler by pulse width modulating the firing of the boiler. The effect is of "very regular" firing. Not recommended on systems with motorised valves as they have to cycle far more frequently than a traditional bang/bang system.
--
Cheers snipped-for-privacy@howhill.com
Dave. pam is missing e-mail
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I explained a better approach, but you used this one. Ignore the ridiculous electronics. The heat web site has the circuitry of the latching relay. The circuit has been given anyhow on this thread.

The Drayton electronic is fine.

Study the heatweb site. You can use a Surrey flange in the cylinder top and take the CH off the immersed pipe. Best to extend the inner pipe of the flange to the centre of the cylinder. The Ch return to the bottom on the cylinder. The room stat controls the Ch pump. The 3-way valve can go and the boiler "only heats the cylinder directly".

Take the cold feed to the DHW port away from the boiler flow influences. One of the reason why I recommended not to use this drawing.

A DHW only heat bank can operate at 65C, depending on cylinder size. Plate heat exchangers are so efficient at heat transfer.

These can be got for 80 to 100. Get a Gledhill one for a Systemate from a dealer. DPS sell them cheaper than that too.

Best to have the boiler and PHE returns in separate tapings. A 200 litre cylinder will just about do CH and DHW.
It may be worth having circuitry that brings in the boiler immediately when DHW is called (the flow switch activated) and CH is called. This eliminates the lag. When DHW is being called this can be switched out.
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DD - thanks for the suggestions esp clarification of plumbing electrics mods to operate the CH off the heatbank too.

This occurred well before I rigged the heatbank - as explained in the OP.

Perhaps I should try dropping the temperature and see how it performs

I can't vouch for the Gledhill PHEs, but I quizzed GEA about the DPS PHE, which you may or may not have realised they supply. Personally, what I could find out wasn't conclusive, but I wouldn't trust the data quoted for the PHE on the DPS website. I don't pretend to fully understand the ins and outs of the PHE head load/output calcs, but for the input params I gave GEA their model equivalent to the one marketted through DPS was definitely not 100kW, but needed a bigger beast - the one I now have. Sorry to be a bit vague, I don't have the model no and calculation sheets with me at the moment, so I can't check and include full details. Suffice to say that my PHE is definitely 100kW working under the conditions I supplied GEA. I decided I'd rather spend a bit more and get a PHE that definitely did the job rather than having to upgrade and waste money later on.

I don't see any advantage in this - it works fine. When there is a demand for DHW from the PHE primary pump and boiler has demand (ie the system pump is running) the return from the PHE just cariies on straight down the boiler return rather than going through the tank (give or take any differences between the respective pump flows generated - the return tapping into the bottom of the tank will balance the flow at the tee in this scenario, to prevent any resultant presure diffs).

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- The second improvement is diffuser pipes within the cylinder. I

Hi, very interested in this thread. As wanted to build my own heatbank too.
Latest diagram I came up with was:
http://aycu27.webshots.com/image/38786/2000028197462625313_rs.jpg
..but it still comes in expensive!!
If you have plenty of time on your hands, you can read my thread at http://www.screwfix.com/talk/thread.jspa?threadID=63364
Couple of questions, 1.Where did you get your cylinder from and approx what price? 2. Where the essex flanges straight forward enough to fit? 3. What cylinder stats did you use and where these external or pocket type? 4.Have you or do you intend to run Ch from heatbank or leave as is?
Cheers
Ian
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- The second improvement is diffuser pipes within the cylinder. I

Hi, very interested in this thread. As wanted to build my own heatbank too.
Latest diagram I came up with was:
http://aycu27.webshots.com/image/38786/2000028197462625313_rs.jpg
<<<<<
Nice!!!
Have the open vent to the F&E tank separate to the flow to the Plate heat exchnmager or use a Surrey flange at this point.
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That is supposed to be a surrey flange on top, forgot to indicate.
Think you were part of the original screwfix thread and provided a great deal of input so credit mainly to you!!
Just need to source a cheap 200L direct cylinder and find out if it is straight forward enough to fit essex flanges. (I generally fit basic kitchens and bathroom suites for housing associations so basic plumbing OK. Used Surrey flanges before but never Essex which obviously means cutting a hole in new cylinder!
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- The second improvement is diffuser pipes within the cylinder. I

Hi, very interested in this thread. As wanted to build my own heatbank too.
Latest diagram I came up with was:
http://aycu27.webshots.com/image/38786/2000028197462625313_rs.jpg
<<<
Put the check valves on the CH circuits "after" the pumps.
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Also interested as it seems an excellent post-fix to a system with a newish boiler.
Advantages over (say) a combi would seem to be flow rate but the biggest combi advantage is infinite capacity (at whatever flow rate one can argue they deliver).
Web sites for thermal stores don't seem to quote capacity of drawn off hot water. Let's say (hypothetically) one has a thermal store with a tank of water of 300l at 80degC and a plate heat exchanger set to deliver DHW at 65degC. At a flow rate of (say) 30l/min, how long does the hot water last (at that flow rate, boiler heating will be too slow to be significant). Or, in non scientific units, how many full baths can you run one after the other? I should be able to work it out,from the physics I guess, but someone should know!!
--
Bob Mannix
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http://aycu27.webshots.com/image/38786/2000028197462625313_rs.jpg
This heat bank system has many advantages.
Look at the flow pipe from the boiler into the cylidner. Take this pipe inside the cylinder and bend it up towarads the DHW draw-off at the top, terminating inside the dome pointing up. The flow would be sucked into the plate heat X by the DHW pump. When the store is exhausted of heat the flow rate will revert to what the boiler can give, which will be a good shower at least and "never" run out of hot water. This principle is applied to stored water combis which are two stage: high flow using stored water and low rate using only the ouput of the boilers burner.
You could use the flow switch to always bring in the boiler always when DHW is called. This "combines" the output of the boiler and the heat of the stored water, giving greater capacity - or the cyldiner culd be dopwnsized using this method. A high limit stat would need to be on the cylinder to cut out the boiler if the temperature was above 95C.
The diagram uses two CH zones off the store using Wilo Smart pumps. This is a great advantage, as only 0.5 kW will be drawn-off if need be. All rads can then have TRVs on them. This increases comfort conditions.
The boiler only heats the store so running without cycling and to maximum effciency and flow throught it at all times. One of the big problems with boilers fitted these days, is that the auto-by-pass is incorrectly set, allowing too much flow through a condensing boiler reducing its efficiency or in many cases too little flow and reducing the lifespan of the heat exchanger. A boiler heating a store has no such problems and will outlast boilers coupled directly to rads with TRVs on the rads. Also a simple, cheaper, more reliable non-modulating boiler can be fitted.

Later...
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Bob Mannix wrote:

Depending on the sophistication of the boiler and control systems you may get better results with the rads driven directly rather than from the store.

That is not really any different with a store - at some point once you have used any stored heat, the rate at which you can draw further heat from it will be governed by how fast the boiler can replenish it. However this will equate to a significant drop in delivery rate since you can draw energy from the store at a rate well above that which you can get from your gas supply.

Say you are drawing 30lpm of water at 60 deg that is being heated from (say) 5 deg, you require a heating power of (60 - 5) x 4200 x 30 / 60 115.5 kW

Not necessarily... if the boiler is kicked in by a flow switch at the start of the draw off, a 20kW boiler would be replacing a 5th of the energy used in real time. So you reduce the effective energy transfer rate from the store to 95.5 kW

Lets say you want 200L of 45 deg water for the bath, that means you want 145 l of hot water. It will take 4.8 mins to fill the bath. You have had 33.5MJ out of the store, so at say 20kW (1.2 MJ/min) input it will take about half an hour to replenish fully. So you can have baths every half hour indefinitely, or faster for a limited time. Obviously smaller baths and more powerful boilers can change that picture dramatically.
--
Cheers,

John.

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Thanks for the calcs- surely it may be slightly better than that though. The requirement for 145l of hot water might be met twice over by a store (depending on its size) so one might get 2 baths then wait an hour, which is, of course the same on average, but practically may be a lot better! It was the gearing between the size of the store and DHW run-off I was looking for. Looking at the websites on the PHE, for 80degC store, it would seem that 150l flow of hot water out of the PHE would need about 100l of water from the store flowing into it - a 210l or above store would then give two baths (roughly) before the system is cold (ie not able to perform at full spec). Does this sound right?
--
Bob Mannix
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