Choice of intelligent switches ?

pinnks

I have been wondering about a de-strat pump but thought you had to use one suitable for potable water, not a normal CH pump?

Dave_Fowler

pinnks wrote: »

I have been wondering about a de-strat pump but thought you had to use one suitable for potable water, not a normal CH pump?

Pumps specified for potable use are usually made from bronze. There was a Grunfoss bronze version available when I was looking at pumps - but a bargain 'normal' pump on the 'bay couldn't be resisted. Plastic pumps wouldn't be suitable for de-strat pumps because of the high water temperature.

I don't drink much hot water from the tap - and what's wrong with a bit of iron in the water!? Seriously, I don't really see any problem as the water in the tank is changed on a daily basis by normal house usage.

Dave F

orrery

EricMears wrote: »

Immersion thermostats are usually installed inside a metal sleeve attached to the heating element and it's very hard to see how it's not going to be at exactly the same temperature as the element itself.

No sleeve on mine but that's not the point. If you have a hot tank of water and then draw off half for a bath, you end up with hot at the top and cold at the bottom. As the immersion cuts in,there is cold at the bottom, but the hot at the top may well be near boiling locally in contact with the element. At this point you don't want the destrat pump to run as you can end up with a tank full of tepid water.

I took the power from the solarImmersion for the pump (the second channel set up for this purpose) and ran the power through x2 thermostats - one at the top to ensure that we had hot water present and one at the bottom to ensure that it wasn't already up to temperature. So, to run there needed to be:
- diversion above 10%
- water at the top of the tank > 60C
- water at the bottom of the tank < 60C

That way you get de-strat when you need it, rapid heating at the top of the tank (on poor generation days) and the destrat pump isn't on all the time when the tank is already hot.

And, no the controls weren't the problem - I bypassed them to check.

orrery

Dave_Fowler wrote: »

Pump is Grunfoss Selectric Type UPS 15-50
Non-return valve is 22mm double check valve DZR, Screwfix 26003
Never had any problems with air intake, Pump is set to minimum power setting.

NRV is in the recirc loop, presumably, and nothing in the cold feed?

I think the pump power may be part of the issue - I had a baby pump which was probably having difficulty overcoming the NRV and clearing the gas from the pipe.

EricMears

orrery wrote: »

No sleeve on mine but that's not the point.

Actually, it could be highly relevant. A thermocouple within the immersion unit itself is very responsive. If you're relying on an external thermocouple clamped on side of tank there's a terrific amount of delay in the control loop.

The circuit described seems pretty logical and ought to have worked OK - although the 'gassing' (which I still believe indicates inleaking air) will have prevented. If you're convinced it's caused by dissolved air 'un-dissolving' you might like to include something like this in the circuit :
https://www.bes.co.uk/product/108a~PL~1972~-Air-Separators.html

Dave_Fowler

orrery wrote: »

NRV is in the recirc loop, presumably, and nothing in the cold feed?

I think the pump power may be part of the issue - I had a baby pump which was probably having difficulty overcoming the NRV and clearing the gas from the pipe.

Here's a picture of the de-strat pump

From the left:-

Cold water feed from tank in loft to bottom of hot water tank, NRV, pump isolate valve, pump, pump isolate valve, hot water feed from tank and expansion pipe to loft (pipes to domestic hot water taps branch off just below the wooden slats.

Water goes through the pump from right to left.

The sheet of paper near the pump is only for the photograph and is there to show there is no connection to the middle pipe - this is part of the central heating system.

Dave F

orrery

Dave_Fowler wrote: »

Here's a picture of the de-strat pump...

Great - many thanks - very useful.

orrery

EricMears wrote: »

Actually, it could be highly relevant. A thermocouple within the immersion unit itself is very responsive. If you're relying on an external thermocouple clamped on side of tank there's a terrific amount of delay in the control loop.

No, the thermostat is inserted into a sleeve in the immersion assembly - a replaceable component in the immersion assembly itself. It is hanging 18" down into the tank whereas the immersion is 30" (apparently, the longest thermostats are 18"). I have x3 external stats, the usual one for the gas c/h and the other two for the destrat pump control as described. None of the external ones control the immersion.

phsci

One of the recurrent issues on this thread is that many people don’t know how much they export to the grid or how much energy is diverted over a full year to their immersion heater.
I installed an Intelligent Immersion Mk II device in October 2013 and took the opportunity of installing at the same time a couple of extra standard revenue type meters (Landis & Gyr E110) to measure both export to the grid and energy sent to the immersion via my proportional device. I record meter readings weekly and have data for the complete calendar years 2014 and 2015. I’ll stick to 2015 for simplicity; 2014 wasn’t a lot different and the amount diverted so far this year is slightly (695 kWh against 654 kWh to date) ahead of the curve.
Last year, my 3.8 kWp PV system generated 3219 kWh. I self consumed 1636 kWh (51%), another 1054 kWh (33%) went to the immersion, and the remaining 530 kWh (16%) went off to the grid. I have also been recording gas consumption weekly since 2012 (monthly before then). The installation of the Intelligent Immersion seems to be saving significantly more gas annually than the 1000 or so kWh diverted. In fact, I’m now using around 4000 kWh less of gas than before the device was installed. There are obviously some contributory explanations unrelated to the device. I don’t live in a lab and there are a couple of other people living here (my wife and one of my grown-up sons), so human factors cannot be controlled. The weather (and therefore the temperature of incoming cold water) varies from year to year. However, modernising of our bathrooms took place several years before the I2 was installed so I’m confident that the DHW requirement hasn’t decreased significantly over the critical period. Our condensing system boiler has a SEDBUK seasonal efficiency rating over 90% but a comparative hot water efficiency of only 58%. Pipe runs (22mm) from boiler to hot water cylinder are about 10 metres. My guess is that firing the boiler up just to maintain a supply of hot water was very wasteful!
The Intelligent Immersion is currently on sale for £190.80 and mine is just 10 weeks off its third birthday. It has proven reliable over that period, responds instantaneously to changes in production or load, and has a very small deadband (adjustable) of just 10 Watts. I’m paying 3.02p per kWh for my gas and I don’t think it unreasonable to guess that I’m saving at least 2000 kWh of gas per annum or £60. Payback period therefore, not much more than 3 years and clearly better than the period for the overall PV system. I’m happy with it.

orrery

phsci wrote: »

My guess is that firing the boiler up just to maintain a supply of hot water was very wasteful! .....
....... Payback period therefore, not much more than 3 years and clearly better than the period for the overall PV system. I’m happy with it.

I suspect that you are right about the boiler - in my case the boiler feeds the bathroom radiator and towel rail as well (these bypass the h/w and c/h control valves). My saving on gas is remarkably high - too high to be credible, so I attribute a significant part of it to other measures, like boiler replacement.

The payback period you quote is good, but it wouldn't work for anyone who needs professional installation.