Solarimmersion installed - Really need some help installing a de strat pump

[j2d]

I recently installed a Solarimmersion unit to work with our 4kw PV system.

The hot water tank is 1050mm high and I have just fitted a 27" immersion heater (as the old one was only 11")

I have also added an immersion jacket to the existing foam covering over the tank.

I've moved the cylinder stat up to just above the bottom of the 27" immersion heater, and the hot water is set to come on for 30 mins early morning and 30 mins late afternoon.

Cylinder stat set to 50 degrees, immersion set to abt 65 degrees.

The idea is obvious - I won't need to use the gas boiler while the weather is nice. But this still only means 2/3 of the tank will ever get heated unless the cylinder stat kicks in.

The solar immersion is designed to be used with a de strat pump, which I would really like to make use of, but I haven't got an idea of what I need and how to do it, even after searching google for 2hrs im none the wiser..

I have been reading a washing machine pump would do the job, is this correct?

Any help would be greatly appreciated

John

 

[The Solar King]

Are you sure you need to run a destrat pump? You may already have sufficient in the tank for your daily needs. Firstly, do you know what your daily hot water usage is at normal outlet temperature? This is unlikely to ever be above 45degC. Most calculations work on about 40ltrs per person per day. SAP works on considerably less, 25N+36 where floor area is unknown (SAP works on an outlet temperature of 47degC).

At 450mm diameter, the tank will hold about 140ltrs. This means 140ltrs stored at 60degC would give you the equivalent of 200litres, or 2/3 of the tank will give you 132ltrs. The other thing to bare in mind is that when the water in a cylinder stratifies, it does not stop at the level of the heating element, it will also pass below it. This is why when sizing a tank for solar thermal up to 30% of the solar volume (Vs) can be counted as daily volume (Vd). It is possible you could still have an effective volume of 150litres if the water is stored at 60degC.

Incidentallly, is this a vented or unvented cylinder? If it is the latter, please be careful what you do with the thermostats, or get a qualified person with an unvented ticket to look at this for you.

 

[The Solar King]

One other point. When storing water at 60degC, (or at any temperature at which there is a scald risk to infants and others), it is important to fit a thermostatic mixing valve either at the point of exit from the tank or at each point of use.

 

[j2d]

Are you sure you need to run a destrat pump? You may already have sufficient in the tank for your daily needs. Firstly, do you know what your daily hot water usage is at normal outlet temperature? This is unlikely to ever be above 45degC. Most calculations work on about 40ltrs per person per day. SAP works on considerably less, 25N+36 where floor area is unknown (SAP works on an outlet temperature of 47degC).

At 450mm diameter, the tank will hold about 140ltrs. This means 140ltrs stored at 60degC would give you the equivalent of 200litres, or 2/3 of the tank will give you 132ltrs. The other thing to bare in mind is that when the water in a cylinder stratifies, it does not stop at the level of the heating element, it will also pass below it. This is why when sizing a tank for solar thermal up to 30% of the solar volume (Vs) can be counted as daily volume (Vd). It is possible you could still have an effective volume of 150litres if the water is stored at 60degC.

Incidentallly, is this a vented or unvented cylinder? If it is the latter, please be careful what you do with the thermostats, or get a qualified person with an unvented ticket to look at this for you.

Thanks for the info Solar King, I feel I could definitely benefit from a de-strat pump, 2 reasons, first being during this time of year once the immersion has heated 2/3s of the tank there is still plenty of excess PV available which at present will just flow back into the grid, whereas if I fit a de-strat pump the solarimmersion can be set to turn the pump on at a certain level which will then eventually give me a full tank of hot water at no extra cost, instead 2/3s of a tank.

The problem of only having 2/3s of the tank heated, is that with the kids having a bath, me and maybe the mrs both having showers, there isn't much hot water left, and if the wife decides to have a shower before work it will be before the solar has kicked in to reheat the water, and may possibly not be as hot as she wants it, whereas a full tank of hot water should be plenty.

Re which system we have, I believe it is a vented system as we have tanks in the loft.

So can anyone point me in the right direction of where to start with this de-strat pump installation?

John

 

[Gavin A]

a destrat pump should be pretty simple to sus out if you know what you're doing, but IMO it's really one of those things where if you have to ask the question then it raises serious doubts about your competence to carry the work out.

so for that reason, I'm ooot.

 

[Worcester]

A de-strat pump would either need to be bronze (usual in pumped hot water systems) stainless steel or high tmeperature plastic, it simply acts as a 'short circuit' betweeen the top (hot water outlet) and bottom (cold water inlet) of the cylinder. - It needs to be able to be able to be used in a 'potable water' installation.
Grundfos Comfort | Grundfos
or
Grundfos make a model PU.120 for example which is plastic bodied for potable water applications. You wont find it on the Grundfos website, the info is on the Euro Boiler Parts website. PU 120 Grundfos Pump 1 Plastic Body Replacement Grundfos Bronze Pump (It's not officially available in the UK as Grundfos UK don't talk to Grundfos EU )

Important note: You need to be a competent plumber to install and a competent electrician to wire in.

 

[Digon]

I fitted a 36" immersion and whole house TMV to my 150L tank and despite running at higher tank temps my system was still shutting off at around mid-day. Note the higher temps will give scale problems in hard water areas.
I looked at fitting a de-strat pump but found that fitting a second immersion (11") via essex flange to the bottom of the tank was a much easier and cheaper route in my case. Fitting the flange, although straight forward, is not something I'd recommend a diy'r try unless very competent.
I'm switching my home made diverter manually between the two immersions at the moment until I get the software to do it for me, but I think your unit could be set to change to the bottom immersion after the top immersion has been off for x-minutes.

With a de-strat pump during the winter you risk stirring the tank after heating the top of the tank when the sun is going down and ending up with a tank of luke warm water. With two immersions you will nearly always have the top of the tank hot enough for at least one bath in the winter and on good days the bottom immersion will increase that.

Despite my bottom immersion limiting at 65C instead of the 75C on the spec sheet it is still making a big contribution.
The attached graphs show two bath draws on the top immersion setup and two bath draws plus a next morning shower draw on the two immersion setup.
The temp sensors are located 8" from top, middle and 6" from bottom of my tank.

 

[Worcester]

Hi Dijon, looks like someone has a bath at 7pm and someone a shower at 8pm
I've PM's you with a couple of questions, as I also monitor the cylinder using openenergymonitor.org products.

 

[Digon]

Hi Dijon, looks like someone has a bath at 7pm and someone a shower at 8pm
I've PM's you with a couple of questions, as I also monitor the cylinder using openenergymonitor.org products.

They are both quite large 7pm and 8pm bath draws but they look very different due to the different temp distributions at the start of the first bath. Only the top and btm immersion graph shows the much lower draw from a shower at about 7:45am.

 

[Digon]

Here is another graph screenshot taken a few minutes ago. It shows the tank heated on the top immersion until 12am and then switched to the bottom immersion. I am now going to switch it back to the top to use the remaining sun to top up the top of the tank. Note that it has been limiting since about 3pm even on the btm immersion

.