Green Building Forum - External destratification pump on CH cylinder Sat, 18 Apr 2026 11:09:49 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307907#Comment_307907 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307907#Comment_307907 Sat, 08 Mar 2025 16:44:22 +0000 owlman External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307910#Comment_307910 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307910#Comment_307910 Sat, 08 Mar 2025 20:21:48 +0000 djh External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307913#Comment_307913 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307913#Comment_307913 Sun, 09 Mar 2025 10:00:40 +0000 owlman External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307914#Comment_307914 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307914#Comment_307914 Sun, 09 Mar 2025 16:31:39 +0000 GreenPaddy
My first thought would be to make use of the exiting CH pump, additional pipework to short circuit the CH system immed back to the cyl with a simple control set up based on your particular energy timings.

If you can do a simple sketch of the cyl and connections, or a labelled photo, maybe we could advise further.]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307915#Comment_307915 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307915#Comment_307915 Sun, 09 Mar 2025 17:24:32 +0000 djh External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307916#Comment_307916 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307916#Comment_307916 Sun, 09 Mar 2025 17:29:36 +0000 owlman Last year I re-used a spare 2" BSP port about halfway up the tank ( which is about 7' high ). I had made a 6kW immersion split into 2 x 3kW for flexibility. Each is wired onto a timer to take advantage of cheap tariff.

This set up has allowed me to cut down on log firings. I've found another 1.5" BSP port that I can also repurpose on the other side of the tank. My thought was to fit another 3 or 6kW immersion at the other side.
Obviously this is only heating the top half of the tank, albeit very effectively.

My idea was to use a couple of 1/2" BSP, unused stat ports, at the top and bottom of the tank and simply link them with a small circulating pump, either manual or on timer too. This is simple enough to do and I'm currently looking what would be the best type of circulating pump to use, esp. the flow rates. Not too high I would have thought.

It'd be inconvenient to use the current CH pump, plus a bit of copper and a low capacity pump I thought would cheaply do the job and give me a few hundred liters of usable CH water each day in winter, and maybe even ditch the log burner.
I know the principle is used in other storage vessels often for potable water.]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307917#Comment_307917 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307917#Comment_307917 Sun, 09 Mar 2025 19:04:22 +0000 Peter_in_Hungary
Note - The stat ports on my TS are closed, that is they are just pockets and the stat sensors don't get wet. If yours are the same then you will have to run a drill through them]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307918#Comment_307918 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307918#Comment_307918 Sun, 09 Mar 2025 19:51:59 +0000 owlman External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307919#Comment_307919 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307919#Comment_307919 Mon, 10 Mar 2025 09:54:39 +0000 WillInAberdeen
1) connect an external electric heater in series with the boiler, use that the same as you do with the boiler so the laddomat heats the whole tank volume while maintaining stratification . Then the hot tap still runs hot even when the tank is nearly "empty" of heat.

2) use the pump in the laddomat to pump cold water from the base of the tank round through the boiler and return to the top, where it will sink past the internal immersions and be heated. This might need a mod to the laddomat thermostat plunger, sure I saw instructions for this online somewhere.


If you do connect a new pump then no need to use the stat ports, just tee into the cold water feed pipe and the hot water outlet pipe, near to their respective connectors to the tank.


FWIW the Mixergy hi-tech tanks use their external PHE pump to destratify when connected to a heat pump, so the HP 'sees' a colder flow and runs more efficiently. Not relevant for an immersion tho.]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307922#Comment_307922 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307922#Comment_307922 Mon, 10 Mar 2025 12:36:34 +0000 owlman I had considered this but the downside would be the need to install a shut off on the main return to prevent pushing hot water back up that return. In itself not too difficult but the likelihood of forgetting and also having to mess with shut offs each day.
I still feel that simple is best and cheapest, and if it goes pear shaped then its not too costly. I have 22mm copper and fittings already, then it's only a couple of iron tank couplings plus a pump and unions. Downside is having to drain the tank which has inhibitor in it.
I still can't figure out though what would be best regarding pump flow direction. Hot top to cold bottom, or cold to hot. That's where the whole plan may go awry. I also have to consider pump types especially impeller material in view of potential high temperatures, plus as I mentioned flow rates.]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307924#Comment_307924 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307924#Comment_307924 Mon, 10 Mar 2025 20:03:42 +0000 Peter_in_Hungary Posted By: owlmanDownside is having to drain the tank which has inhibitor in it.
I've seen an immersion heater in the bottom of a tank changed 'hot' that is without draining the tank. The vent was blocked up (the only place that air could get in) and then the immersion heater was unscrewed and the new one put in - quickly. I was surprised at how little water came out, a couple of blubs and that was it, nothing that could not be managed with a towel. In your situation you could make a fitting to replace the plug e.g a male threaded piece of pipe to replace the plug with a tap on the end - turned off -, put sealant on the fitting then remove the plug and quickly replace with the fitting.once done you can put what ever you want on the outboard end of the tap. (ditto the top connection) If you get it wrong then the you will get drips which may need a drain down but you were going to do that anyway. (you are a brave man if the TS is upstairs with a carpeted landing and stairs)



Posted By: owlmanI still can't figure out though what would be best regarding pump flow direction. Hot top to cold bottom, or cold to hot. That's where the whole plan may go awry. I also have to consider pump types especially impeller material in view of potential high temperatures, plus as I mentioned flow rates.

If you use a standard cheapo CH pump with the usual 3 speeds then you can experiment with flow rates and for direction - well the CH pumps have ball valves each end to facilitate changing so you can turn the pump around easily. Flow rates can also be varied by throttling the ball valve on the pump. CH pumps will also stand heat up to the old fashioned CH flow rate temps of about 80 deg. Its probably easier to experiment than to try and calculate or over think the project.]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307925#Comment_307925 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307925#Comment_307925 Mon, 10 Mar 2025 21:52:35 +0000 Mike1 Posted By: Peter_in_HungaryI've seen an immersion heater in the bottom of a tank changed 'hot' that is without draining the tank.Then I'm guessing that a Hungarian immersion heater boss is rather smaller than those normally used in the UK. You definitely don't want to try changing one of them without draining it down - I've seen someone try :)]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307926#Comment_307926 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307926#Comment_307926 Mon, 10 Mar 2025 23:44:52 +0000 Peter_in_Hungary https://heilmann.hu/spd/3033-14/Heizer-Hajdu-bojler-STA-kompakt-futobetet-6-4-3000?utm_source=olcsobbat&utm_medium=shopping&utm_campaign=olcsobbat_shopping_heilmann_haztartasi_gep_alkatresz&utm_source=olcsobbat&utm_medium=shopping&utm_campaign=olcsobbat_shopping_heilmann_haztartasi_gep_alkatresz&gad_source=1

Pipe threads here are quoted in multiples of 1/4" so 1 1/2" = 6/4"

So yes a bit smaller than what I remember going into the top of a copper cylinder in the UK]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307927#Comment_307927 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307927#Comment_307927 Tue, 11 Mar 2025 08:45:42 +0000 WillInAberdeen
Then detect when its hot with a stat near the top. That starts the pump to move some hot water down to the bottom, to mix the tank a little (not too much or you'll have no hot water available!), so the immersion can keep running.

Don't pump cold water the other way into your lovely hot reserve at the top.


If the immersion can't keep up and the top of the tank gets too cold, the stat will stop the pump, until the immersion catches up again.

Finally the tank will be hot all the way to the bottom, so a second stat down there detects this and stops the pump.

For this, you'll need two stat pockets, near the top and the bottom...!



A 2000l tank heated from 40 to 80 DegC holds 93kWh.

Say you run your 6+3kW immersions for 7 hours cheap, that's 63kWh.
So you probably aren't going to heat the whole tank in one go, even if you need that much heat. Better to keep it a bit stratified, with usefully hot water in say the top 2/3.]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307928#Comment_307928 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307928#Comment_307928 Tue, 11 Mar 2025 09:46:33 +0000 owlman
@ WiA Thanks, yes, hot to cold is what I'd instinctively go for, your calcs. broadly reflect what happens in practice.


I'd looked at a couple of low flow circulating pumps that only move about 1/2 Cu M per hour, this type of thing:-

https://www.mrcentralheating.co.uk/dab-vortex-bwo155-r-zm-blueone-1-2-domestic-hot-water-pump

https://www.anchorpumps.com/pump-type/hot-water-service-circulators/grundfos-comfort-hot-water-service-circulators-240v

There are others of this albeit expensive ilk. I think they were mainly designed for re-circulating DHW systems hence the low flow rates.]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307929#Comment_307929 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307929#Comment_307929 Tue, 11 Mar 2025 09:54:47 +0000 Cliff Pope When you draw off hot water from the top it is replaced at the bottom, the cold layer gradually creeping up the tank until finally the hot runs out and at that point you know to stop trying to top up your bath and you get out.

So you want minimal agitation of the water so as to preserve the stratification. The cold inlet pipe should 25mm bore so as to minimise the flow speed and prevent swirl, and there should be a horizontal baffle on the inside of the tank to encourage the cold water to spread out horizontally.

Externally the hot pipes should have long horizontal runs to minimise heat transfer via thermosyphon action, likewise the vent pipe, rather than straight vertical from the top of the tank.

Economy 7 tanks have two immersion heaters - the lower one on a timer to heat the whole tank, the top one to give a limited boost heating only the upper portion during the day.]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307930#Comment_307930 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307930#Comment_307930 Tue, 11 Mar 2025 10:09:57 +0000 owlman Worst case doing that each night on cheap rate would be 12 kWh for 7hrs @ 6.7p would be £5.63 per night .I think, it would be a lot less than that as often we don't use that much for the zoned CH. Also because the 4 x 3 kW immersions are wired independently on 4 timers I could mix and match the input according to demand.]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307934#Comment_307934 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307934#Comment_307934 Tue, 11 Mar 2025 17:46:39 +0000 Peter_in_Hungary Posted By: owlman@ PiH The risk taker in me would love to have a go at the full tank method you describe and I can understand the principle. However, I have visions of me with a finger in the hole reaching for the spanner I've dropped, I shudder to think. On the plus side it's all on the ground floor in the integrated garage with drainage to hand.
Man or mouse ??? Go for it.........]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307935#Comment_307935 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307935#Comment_307935 Tue, 11 Mar 2025 18:43:43 +0000 owlman Buckets at the ready:wink:]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307937#Comment_307937 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307937#Comment_307937 Wed, 12 Mar 2025 07:45:29 +0000 Peter_in_Hungary Posted By: owlmanI think for the methodology to work you would to have the malleable iron fittings made up beforehand with a lever type ball valve in order to fit the copper afterwards, as you suggest. Maybe with taper fit rather than straight fit too.
Buckets at the readyhttps:///newforum/extensions/Vanillacons/smilies/standard/wink.gif" alt=":wink:" title=":wink:" >
Yes taper fit and I would close the tap and remove the handle 'cos it will be easier to fit without the handle sticking out at a right angle.]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307939#Comment_307939 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307939#Comment_307939 Wed, 12 Mar 2025 10:30:08 +0000 djh Posted By: Cliff PopeI thought the principle of an "ideal" tank is that it is heated at the bottom and the hot water rises to the top.This thread isn't discussing an ideal tank, but the realities of owlman's tank.]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307941#Comment_307941 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307941#Comment_307941 Wed, 12 Mar 2025 11:11:06 +0000 WillInAberdeen
kW x hours = litres x 4.2 x (temp rise) ÷ 3600

Eg 1500 litres x 4.2 x(50 to 80)÷3600 = 53 kWh

= 7.5kW for 7 hours

@6.9p = £3.68


Maybe off topic but guess you have thought about the electric supply side and consumer unit wiring overload ratings etc etc?
4x3kW = 52A without diversity, doesn't leave much for other circuits sockets/cooker/shower/EV
Each 3kW immersion likely needs its own rcbo and radial wiring out from the consumer unit, sparky will advise]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307943#Comment_307943 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=307943#Comment_307943 Wed, 12 Mar 2025 13:26:53 +0000 owlman The first 2 x 3 kW didn't have stats on them because with such a large water volume There was no chance of overheating and the system is well protected with pressure relief valves. However if I do go for another 2 x 3 kW those will have inbuilt stats, for flexibility as well as safety.
When we fitted the storage batteries we split the tails and took off a separate feed just after the meter via a double pole fused isolator. this is the dedicated immersion feed. We also wanted to eliminate any risk of the immersions draining the storage batteries, and it was easier to monitor that way.]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=308832#Comment_308832 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=308832#Comment_308832 Thu, 17 Jul 2025 16:50:20 +0100 Bryan_Rendall
My input on the type of pump needed is that it needs to be non corrosive, i.e. stainless steel, if its in potable water. I tried similar to stir up the water between my solar pre-heat tank and the main DHW cylinder, for times when the preheat tank had become hotter than the main tank. I arranged a standard CH pump to take cooler water from the top of the main tank and re-circulate it to the inlet of the pre-heat tank, thus pushing hotter water forward closer to the taps. It works a treat, but with a standard CH pump, the hot water had a permanent brown tinge, due to ongoing corrosion in the pump. Swapped it out for a similar pump with a stainless impeller for potable water, and problem solved.]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=308833#Comment_308833 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=308833#Comment_308833 Thu, 17 Jul 2025 21:41:44 +0100 djh Posted By: Bryan_RendallMy input on the type of pump needed is that it needs to be non corrosive, i.e. stainless steel, if its in potable water.I believe it's a central heating tank, so not potable water.]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=308834#Comment_308834 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=308834#Comment_308834 Thu, 17 Jul 2025 22:18:28 +0100 Bryan_Rendall Posted By: djhI believe it's a central heating tank, so not potable water.

Indeed, that is what the OP started off with. But within about 10 posts, it got a bit blurry, and not clear if everyone else was still on CH or DHW. So thought I might pitch that bit if for good luck! :bigsmile:]]> External destratification pump on CH cylinder https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=308836#Comment_308836 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18272&Focus=308836#Comment_308836 Fri, 18 Jul 2025 12:02:24 +0100 owlman