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What are you using to limit the tank hot water temperature? Does your immersion element have a thermostat fitted, and if so what is it set to?

Paul

I know that solar thermal sometimes gets very hot so I wonder how they resolve this?

Once the bottom of my tank (the coldest part of the tank) reaches 70C, mine stops circulating water to the solar collector. You can see in the attached plot,  once that happens the water in the collector boils and is released as steam through a relief valve on the collector.

dBC

Think you should have a heat dump, as its not a good idea at all to let the system stagnate.  I dump mine to the woodburner, then if that gets to 65 degrees the heating pump comes on. Then as a further protection I have 2 large radiators externally. In the summer i can set it to go straight to the external rads.

Where are you measuring the 75C - is that what the tank thermostat is set to?

I suspect you may be getting some boiling in the HW tank which causes the bubbling over from the HW vent pipe into the cold storage tank.

Since the level of water in the vent pipe is at the same level as the water in the cold tank, it doesn't take much to push water from the vent pipe into the cold tank; even the slightest trace of boiling will do it!

>What are you using to limit the tank hot water temperature? Does your immersion element have a thermostat fitted, and if so what is it set to?

Paul<

I have only got the internal thermostat working as yet but have bought an external one in case the internal turns off too soon.

Yesterday as my first day with the mk2 was a bit of a test so I had the thermostat up to maximum.

the top of the cylinder peaked at 75c and never went above that but the bottom was coolish till mid afternoon when it started to warm up and the at 4:30 something happened, Not sure what?

have a look at this chart - don't take any notice of a drop in temp just after 12 as I had pulled off the sensors by accident.

it's the sudden changes as 4:30 that puzzle me.

I can't see your chart details clearly but if the red line is the tank top temp, yellow is the mid & blue is the bottom tank temp then it looks like hot water was drawn off at 4:30ish hence the drop in the red line.

This drawn off water was replaced by water from the cold tank but if that cold tank water was at a higher temp than the existing water at the bottom of the HW tank then this would account for the sudden rise in the blue line which then stabilised as more hot water was drawn off

>dBC

Think you should have a heat dump, as its not a good idea at all to let the system stagnate.  I dump mine to the woodburner, then if that gets to 65 degrees the heating pump comes on. Then as a further protection I have 2 large radiators externally. In the summer i can set it to go straight to the external rads.<

I could switch the central heating pump on and open the zone valves this would suck heat from my tank to the rads,

I did intend to set this up for the winter but have not done any real work on that yet.

I was hoping to get the top of the tank to more than 75c without this sort of problem though as I need to get the middle of the tank to at least 55c so that my hot water last's me till the morning.

what I think is happening is that as the water expands it is rising in the vent until it overflows into the cold tank.

I can raise this vent by 500mm. Do you think that would be worth trying?

thanks

what I think is happening is that as the water expands it is rising in the vent until it overflows into the cold tank.

I can raise this vent by 500mm. Do you think that would be worth trying?

 

Since the cold tank, HW tank & vent pipe are all connected, the level of water in the vent pipe will be the same as the level in the cold tank.

The purpose of the vent pipe is to vent air, the purpose of the feed pipe from the cold tank to the HW tank is to act as an expansion pipe as well as feed (used to be called feed/expansion pipe).

As the HW heats up, the expanding HW will raise the water level in the cold water tank & the vent pipe the same amount ie the water level in each will stay equal

Raising the vent pipe may alleviate the problem but only if the vent pipe is already too low relative to the cold water tank water level.

Also, the recommended diameter of the vent pipe is 22mm (although I've always used 15mm with no problem)

There is a formula for calculating the height that the vent pipe needs to be above the level of the water surface in the cold water storage tank (CWST).

This is 150mm + 40mm for each meter of system height.

System height is defined as the height from the coldwater inlet at the bottom of the cyclinder to the top of the water level in the CWST

Michael, I'm not sure that this is the best approach IMO

Many of us suffer from the same dilemma about stratification in the hot water tank, but to heat the tank this way will cause you all sorts of problems, such as drawing water off at 75deg could cause serious scalding. For example a 2 second exposure to 65deg heated water would create a partial thickness burn in just 2 seconds (Charted Institute of Plumbing & Heating Engineering), unless of course you are using a thermostatic valve to combat this risk.
Even though your thermostat recorded 75deg, in localized areas this could be considerably more - even boiling point, which I suspect has caused the issues you have highlighted.

Would in not be better to intelligently circulate the water in the tank from top to bottom using a pump, which could be triggered when the top of the tank reaches say 50deg, so you end up with a full tank of 50deg water, instead of a third at 75deg, another third at 50deg and the final third at 25deg. Some others in the forum have had a lot of success with this already.

Paul

Hi Mike, and welcome to the Mk2 Club :)

We had a similar experience here very soon after installing my first PV diverter (which wasn't even a Mk2!)

Although the heating element of our original immersion heater was working OK, the thermostat wasn't.  Consequently, the water over-heated and belched its way through the vent pipe into the cold tank immediately above.  A new immersion element + thermostat solved the problem.

If possible, it's worth getting an element that's as long as the tank will take.  We have a standard 27" one, but I intend to fit a longer one soon, and ordered a suitable looking one  from PlumbWorld.  Never again - they sent me the wrong item, which was damaged in transit, and then another package arrived that was nothing to do with me, and soooo many emails!

In most cases, the amount of diverted power is less than the immersion would take if it were to be simply switched on manually.  But many immersions are never used from one year to the next - until a PV Diverter appears on the scene.  This is probably an all-too common problem.

Thanks Paul, 

This Mk2 unit is actually a pre-built one that I have supplied, so I'm pleased to learn that it's working so well.   I'm also grateful that my name is now not so prominently associated with this commissioning problem that is not of my making :)

...so I'm pleased to learn that it's working so well..

The problem is that it's working too well, you've made it too efficient!!

It's very tempting for the consumer to hike up the thermostat, but I can recall this incident which made me reconsider that option, and I hope Michael does likewise.

Paul

I have sitting next to the tank a whole house TMV ready to be installed ! I just couldn't wait to test the Mk2 ;)

the TMV is causing more me more headaches as I am not sure how it should be fitted in relation to the vent Pipe

ie should vent pipe go straight to tank in loft with the TMV tee'd off to supply the taps

or should it be TMV first with and tee  off to the taps and the vent? I've  had no answer from the manufacturer yet.

Dominator my system height would be just over 4 meters so I would need 310mm above the cold tank water level.

at the moment I think I have 250 mm and the max I could go would be 750mm !

Pipe is 22mm BTW

Thanks for your explanation I had thought the vent was the expansion pipe as well.

Do you think the Hot water is going up the cold feed then?

Hi Robin yes every thing about the mk2 seems good!

I have just fitted a dual element 16/36'' immersion but to be honest I think the destratification would have been better as you can see from my graph I need very high temperatures at the top to get a reasonable store of water.

I'm not sure if I should send the TVM back to be honest as I am worried I won't be able to get over this problem of the cold tank getting hot! if that's the case I will have to lower the temperature at the top of the tank back to what it was before.

thanks for replies I do appreciate them.

Mike

Paul You have now got me thinking about a destratification system.

Do you have personal experience of it?

I would like to know where to get a pump as I understand they must be potable water pumps made of Bronze?

I can only find bronze pumps costing several hundred pounds.

There also seems to be an argument about where to fix the pipes some say its ok to fit them between the inlet and outlet, some say you should fit to new essex flanges fitted to the tank.

dominator99 I have been reading lot's about this and you are right that the feed is also the expansion pipe. It seems not many people understand this (including me) so in a way what is happening is perfectly normal as the bottom of my tank warms up to approx 35c then this warm water is pushed up into the expansion tank.

I suppose at lower temperatures it is normally not noticeable .

Thanks

mike

My setup is a little different in that I have a 300 litre unvented cylinder.  I have a diverter and we often achieve 86C in the cylinder before the thermostat set at its maximum cuts the immersion heater out. 

I installed a mixer valve set to its maximum of 50C to avoid the risk of scalding, and the only problem that has created for us is that the mixer and the thermostatic shower both always seem to blend a little cold with the hot meaning that when the cylinder was cool enough to be heated by the heat pump (i.e. under 50C), the shower wasn't getting quite hot enough.  A restrictor in the cold supply to the shower solved the problem in the short term, and I've now replaced the shower mixer with a non-thermostatic one.

Shower manufacturers seem to quote a hot water temp to shower temp minimum of 10-15C for thermostatic showers.

86C ! I don't think I will  be able to do that.

I have been hovering near my H/W cylinder all day and working in the loft.

I am now unsure of what is happening again! Because I have just heard my tank start to boil at the top and my sensor is only reading 75 which is where it was thursday when the trouble began.

But..... I felt the cold feed and thats cold and i went to expansion tank and the vent was cold !!! sooo I have left the immersion on experimentaly to see if I can follow the sequence of events.

Does the overflow/vent get hot and overflow  into the tank first or does the cold feed get hot first.

If this wasn't such a pain it could be quite interesting :)

Ah! I have to take back everything I said this morning as the water is definitely boiling over from the vent.

I have turned of the supply now that I know and will be fitting the new tank stat tomorrow.

Well I can only hope this will help someone in the future when they stumble over this thread ;)

ofarm as far as your problem with the showers One of mine can easily be fed straight from the tank as it's just the other side of the wall but the other one won;t be so easy to do. I have two choices don't fit the TMV and go for destratification or fit it and see what happens !

Thanks everyone for listening to me prattling on.

I think you have to fit and see what happens.  The risks from water that is too hot are serious - see page three in this doc http://www.bre.co.uk/pdf/waternews4.pdf.  As my son said, our hot water is nearly hot enough to make a cup of tea - it's been at 87C today!

We may have been unlucky with our aged shower, and we'll try another thermostatic shower in our forthcoming extension.

Andy