iBoost causing tank overflow?

zeupater

evomatt wrote: »

If your immersion is heating the whole tank (ie from the bottom of the tank) you could easily put 16kwh into it. Most are at the top of the tank so will only put about 4kwh into it (enough for a hot 10 min shower)

Hi

Regarding ... "If your immersion is heating the whole tank (ie from the bottom of the tank) you could easily put 16kwh into it." ...

On the basis of a 50C heat rise (ie ~10C mains feed to ~60C DHW), 16kWh of heat input would require a 276 litre cylinder full of 10C mains water ((16/1.16)/50) .... operating the DHW significantly over 60C without a thermostatic mixing valve should be avoided in order to minimise the risk of serious scalding .... the vast majority of UK DHW cylinders are around half of this capacity and it is rare (/abnormal) to have a mains supply temperature (completely cold) cylinder prior to commencing the heating cycle - it is therefore in no way 'easy' to divert 16kWh of generation into a cylinder ....

HTH
Z

JimLad

My immersion is approx 1 quarter of the way up the cylinder and on sunny days i regularly get about 12-14 kwh into it with the thermostat turned up.

zeupater

JimLad wrote: »

My immersion is approx 1 quarter of the way up the cylinder and on sunny days i regularly get about 12-14 kwh into it with the thermostat turned up.

Hi

The relevant questions are therefore ...

1. How are you measuring the energy diverted ?
2. How large is your cylinder ?
3. How much water are you drawing off during the day ?
4. What temperature has the thermostat been turned up to ?
5. Is there a whole house TMV installed ?

... Of course, if the answers to 4 & 5 are significantly >60C and no, then there are further questions regarding sanity, love for your family, understanding of the relative immersive timescales to severe burns at increasing water temperatures and, of course, personal liability insurance ...

Seriously, 14kWh is enough energy to raise 12tonnes of water by 1C (14/1.16) which is the equivalent of raising ~133litres (12000/90) from a 10C mains temperature to boiling .... the equivalent of around 80kettles full of water .... to place 133litres into context, it's pretty close to the size of cylinder which most people would have in their homes ...

We have a large solar thermal ET setup which is dedicated to DHW, accurately metered, operates in conjunction with a multi-coil cylinder which is larger than the size calculated to absorb 16kWh in the previous post and we operate it at well over 60C due to having solar grade whole house thermostatic mixing so, according to the calculations, should be well able to take 16kWh on board .... but believe me it's not 'easy' even with that setup ...

HTH
Z

zeupater

JimLad wrote: »

Unvented (I think....Its fed by the mains water and has pressure cylinders?)

Hi

Okay, so you have a pressurised system which includes an expansion vessel which has likely been sized for water expansion at standard operating temperatures. At a standard DHW 60C water would be expected to expand by ~2%, at 75C it's 3% and by 90C it's 4%, therefore it's likely that what's happening is that the expansion vessel is undersized to cope with your higher temperatures, has reached maximum pressure and a pressure release valve is simply preventing your system from rupturing and showering everyone below it with near boiling water ... of course, if the water is >100C and the pressure causes the tank to fail you'll then have a steam induced catastrophic failure, in which case you'd better be elsewhere ...

If it's a pressurised system, talk to an expert and/or turn the temperature down .... if your pressure release valve is set to a typical value and is working correctly it's likely that you will be operating at a temperature of around 90C, which is ridiculously high .... example for details .... http://www.megaflo-unvented.co.uk/megaflo-description.php

HTH
Z

zeupater

JimLad wrote: »

My immersion is approx 1 quarter of the way up the cylinder and on sunny days i regularly get about 12-14 kwh into it with the thermostat turned up.

Hi

Now, if the above two posts are correct, here's probably what's happening ....

As you've already stated ... "Yesterday was pretty sunny and only put in 5kw after i had adjusted it down a bit." ... which suggests that if the day in question represented typical usage the 5kWh is the normal energy replacement budget. However, with the temperature turned up in order to maximise diversion, your system is reaching a predefined safety pressure and hot water is continually being expelled from the top of the cylinder, being replaced by the cold water feed at the bottom .... in effect, if "on sunny days i regularly get about 12-14 kwh into it with the thermostat turned up", then of that 12-14kWh of diverted generation you're simply running somewhere around 7-9kWh of it down the drain ....

HTH
Z

evomatt

i got lost with all those calculations but it is easy to feed 16kwh into a family sized tank with an immersion at the bottom. The iboost will tell you how much it has put in during the day. As you use the water during the day it will begin to heat it back up to the set temperature. So you might get 4 hours of good sun in the morning and feed 10kwh of that into the tank, have a couple of baths then feed another 6kw or more as it will heat it back up as you use it.

pinnks

Just out of interest, how much water do you use in a year? We only get through about 40m3 but then there are only two of us and I can't remember the last time the bath had water in it...

zeupater

evomatt wrote: »

i got lost with all those calculations but it is easy to feed 16kwh into a family sized tank with an immersion at the bottom. The iboost will tell you how much it has put in during the day. As you use the water during the day it will begin to heat it back up to the set temperature. So you might get 4 hours of good sun in the morning and feed 10kwh of that into the tank, have a couple of baths then feed another 6kw or more as it will heat it back up as you use it.

Hi

Again, how's that .... ?

If a typical bath is 80-100 litres (~150litre=overflow) mixed to 40C (in recommended range) with mains at 10C and DHW at a high 70C to maximise generation storage, then the bath DeltaT to both mains & DHW are equal at 30C ((40-10) & (70-40)) and therefore the mix would be 1:1, so 50litres of each DHW and mains ...

To replenish the DHW used in the (100litre) bath you would need to raise 50litres of water from 10C to 70C, which would consume around 3.5kWh (1.16*0.05*60), so 2 baths would be about 7kWh .... still well short of 16kWh.

As already stated, to place in context, 16kWh would raise something like a 276litre cylinder from cold to hot. In this household it's very rare to have a completely cold cylinder before heating and that's probably true for most homes, the other relevant point being that 276litres is roughly double the capacity of the average household cylinder ....

Your own figures would suggest a situation where a mains water temperature cylinder at the start of day with 16kWh of energy diversion and usage (baths etc) resulting in a hot cylinder at the end of the day ... what happens in day2 is that starting with already heated water, even if it's really sunny all you'll be able to divert is the energy required to replenish losses and usage that day, which would be nowhere near the 16kWh achieved in day1 ...

Would it be possible to let us know what size your cylinder is ?

HTH
Z

JimLad

Found some on my phone from a few weeks ago before the iboost was installed. Looks like the tank is 150L and the immersion goes 1-5.

I did some googling of the model and it appears the numbers mean the following...

1 = 10°C, 2 = 25°C, 3 = 40°C, 4 = 55°C & 5 = 70°C

Would should I have it set at to optimise my free hot water but minimise danger?

evomatt

I have mine set at 5 (70 deg) my kids are 8 and 9 and know the water comes out hot and to use the mixer tap. If the kids were younger id set it lower prob on number 4 to avoid scalding.
Also it looks like your stat is at the bottom of the tank so you'll be heating the whole tank (maximizing excess generation) im guessing your iboost will feed about 12kwh into it if youve used all the hot water the day before, then as you use the hot water during the day it will keep topping it up so you can easily feed 16kwh into it.
If youd fed 16kwh into it, not used any then the next day the iboost said it had put another 16kwh into it youve got a problem somewhere.

my kids cant wait till the summer as they can have warm water in the paddling pool instead of freezing cold hosepipe water