PV Divert to immersion difficulties

[Radian]

In a another recent thread I noted how I'd corrected the calibration of my immersion thermostat to store more energy using my DIY PV diverter as the factory setting was quite a bit out and switched off the immersion well before reaching the temperature setpoint. It now max's out at 75^oC as promised on the dial.

 

But I'm still very unhappy with the results I'm getting. Evidently due to stratification, the water leaving the top of the cylinder can be as high as 75^oC while the point where my DHT22 tank sensor is located is at 45^oC.

This is a much greater difference than I anticipated. I confess to being totally inexperienced about this issue and it has really made all the effort of building the diverter somewhat pointless as I can only get about 3kWh into the tank at a time. Since getting 3.2kWp of solar PV on the roof late last year, this is the first time I've seen more than 20kWh generated in one day and more than half of that is now pointlessly going out to the grid.

 

 

 

Even this whopping 30^oC difference is being measured statically (when everything has settled for an hour or two). When I draw off water, it continues to come out at a constant 75^oC for quite some time (while being pushed up by the cold feed) which rapidly trips the tank sensor turning on the gas boiler and I don't want that! Here's how it looks: briefly drawing off HW, the lower part of the tank got down to 28^oC even though the water at the top was still coming out at 75^oC (not plotted - the HW trace is the output from the DHT22, I don't have a permanent sensor at the top)

 

The fact that the water coming off the top remains pretty constant at the same high temperature suggests an even slug of water to the depth of the immersion that gradually gets pushed up by the cold, like so:

 

I get that a de-stratification pump would help but it'd be quite a bit of effort to implement and I'm not so sure the outcome would be worth it. I went blindly into building the diverter in the first place. I don't want to put even more effort into the project only to be disappointed once more.

[ProDave]

The issue is you have a top mounted immersion heater.  I would never choose a cylinder with that configuration (I suspect you did not choose it)

 

My own (Telford stainless unvented heat pump cylinder) has the immersion entering from the side near the bottom and does what you want, heats the whole tank quite evenly and sinks all the surplus PV we have ever had.

 

[Radian]

1 minute ago, ProDave said:

The issue is you have a top mounted immersion heater.  I would never choose a cylinder with that configuration (I suspect you did not choose it)

 

My own (Telford stainless unvented heat pump cylinder) has the immersion entering from the side near the bottom and does what you want, heats the whole tank quite evenly and sinks all the surplus PV we have ever had.

 

That's the really frustrating bit. I replaced our old cylinder with this one just a couple of years ago. Hadn't even heard of PV diversion back then and didn't even bother to fit an immersion until this year. Huh.

Reading around other forums I see it's not even straightforward fitting a destratification pump as non-return valves don't seem to work well in gravity fed systems. They seem to double-up as check valves to prevent thermosyphoning and as such need a substantial forward flow.

Plus the pumps are more expensive than regular CH circulation pumps as they have to deal with a constant supply of fresh hard-water.

[S2D2]

I take it export payments are not an option? I'd be tempted to ignore the issue even if not, 3kWh/day is a reasonable input and everything will change if you switch to ASHP when the gas boiler dies, I seem to remember you thought it might not last much longer in another thread?

[JamesPa]

If you want to get really creative (and take some risks it might not work) fit destratification and, then when you swap out your boiler for an ashp, add in a plate heat exchanger.  Then you might not need to change out your DHW cylinder because the PHE will augment the coil area to make it large enough for ashp.  That's how the Mixergy 'heat pump conversion kit' appears to work. 

 

This idea was discussed over in the ashp forum, but is not tested, so far as I am aware, other than by Mixergy.  However if destratification works then, with the right PHE and correctly adjusted pump, it's difficult (at least for me) to see how the idea could fail.  Some will, however, argue that vented hot water systems are an abomination and should be swapped out at the earliest possible opportunity.

 

I think (someone please correct me if I'm wrong) a destratification pump is just a pump designed for domestic water, and is commonly used for recirculation loops eg in hotels.  They are more expensive because they use better materials and meet the WRAS standards.

[SteamyTea]

@Radian

Did I read earlier that you have a vented system, the same as I do?

 

I have noticed that in the summer my DHW energy usage drops considerably, my F&E tank is in the loft, so not surprising.

Now I know this may not be approved of, but if that tank was warmed to say 25-30°C (an insulated metal one is probably best) with a relatively low power heater (fish tank one maybe) run off the PV, then imported energy may be reduced.

I don't think there is a huge health risk as there will be a few billion water tanks in the world that a constantly at that sort of temperature and we don't hear much about it.

[jack]

Look into Willis heaters. They take cold water from low down, heat it,  and return it to the top of the tank.

 

Do you have access to any sort of connector lower down in the tank? You might even get away with breaking into the cold feed where it comes into the tank.

 

Then you just need a return connection at the top of the tank. I don't know whether there's such a thing as an adaptor that would allow you to blank off the immersion heater boss and provide a connector you can connect to the hot water output of the Willis heater.

[JamesPa]

13 minutes ago, jack said:

Then you just need a return connection at the top of the tank. I don't know whether there's such a thing as an adaptor that would allow you to blank off the immersion heater boss and provide a connector you can connect to the hot water output of the Willis heater.

A Surrey/York flange might be useful here.

[Dillsue]

If youve got enough clearance above the HW cylinder you can get long immersions. Quick Google search says screwfix do a 3' one. That's gonna heat a fair bit of your average cylinder.

[Radian]

On 19/04/2023 at 18:50, S2D2 said:

I take it export payments are not an option? I'd be tempted to ignore the issue even if not, 3kWh/day is a reasonable input and everything will change if you switch to ASHP when the gas boiler dies, I seem to remember you thought it might not last much longer in another thread?

We've signed up for Octopus Outgoing but they're taking ages to get it all in place. Gave the old boiler a new lease of life by repairing it's SMPS and gaining full control over it via ebus (most importantly being able to adjust flow temperature for PDHW).

 

On 19/04/2023 at 19:10, JamesPa said:

I think (someone please correct me if I'm wrong) a destratification pump is just a pump designed for domestic water, and is commonly used for recirculation loops eg in hotels.  They are more expensive because they use better materials and meet the WRAS standards.

That's right, brass or stainless steel. Not so much expense in the materials but lower economy of scale due to not being so universal in use. Paying £200 for a pump probably negates any economic advantage in using destratification in the life of this system (or mine come to that).

 

On 19/04/2023 at 19:47, SteamyTea said:

Now I know this may not be approved of, but if that tank was warmed to say 25-30°C (an insulated metal one is probably best) with a relatively low power heater (fish tank one maybe) run off the PV, then imported energy may be reduced.

I don't think there is a huge health risk as there will be a few billion water tanks in the world that a constantly at that sort of temperature and we don't hear much about it.

 

I'm becoming a bit less less dismissive of the dangers of Legionella, as the forum post I linked to earlier said:

Quote

The Legionella Pneumophila species has 4 serogroups, 1 is the deadly end (1, 3, 4 and 6) and 6 usually manifests itself as a mild cold. If a household experiences many cold like infections, almost always conclude there is a legionella infection in the hot water supply. The bacteria is ingested most easily via fine particle sprays during a shower. Shower heads are particularly prone to harbouring legionella.

 

And only today day I read how a woman had all four limbs amputated after contracting Legionella from a mist machine. The suggestion that mild exposure might lead to cold-like symptoms is the kicker for me. Disentangling this from hayfever (which our whole household suffers from) is fraught with difficulty.

 

On 19/04/2023 at 19:54, jack said:

Look into Willis heaters. They take cold water from low down, heat it,  and return it to the top of the tank.

Yes I'm aware of their use for giving a boost to HW flow but plumbing one in doesn't really get me much further than using the internal one I've already got. The same tap points top and bottom would be required, but I guess thermosyphoning might eliminate the need for a circulating pump?

 

On 19/04/2023 at 21:45, Dillsue said:

If youve got enough clearance above the HW cylinder you can get long immersions. Quick Google search says screwfix do a 3' one. That's gonna heat a fair bit of your average cylinder.

And I thought the 27" one I bought was pretty long! Good suggestion though.

 

[Quoted text from spammer deleted by mods. OP's following answer left in case of interest]

 

To investigate suitability, I've been range-rating my gas boiler down to the lowest working power level and found 15kW to be about the lower limit before the house is to slow to respond. Insulation can't be taken much further without a major ripping out of vaulted ceilings and replacement of all glazing with 3G. But any savings gained would be equally applicable to gas. The only change in the equation would come with a significant re-structuring of the energy market to favor electric over gas. I see it as being pretty much on parity at the moment.

[jack]

45 minutes ago, Radian said:

Yes I'm aware of their use for giving a boost to HW flow but plumbing one in doesn't really get me much further than using the internal one I've already got.

 

I disagree. Your current immersion is unable to heat more than the top of the tank because of its position. Putting a Willis heater low down would allow much more of the tank to be heated.

 

47 minutes ago, Radian said:

The same tap points top and bottom would be required, but I guess thermosyphoning might eliminate the need for a circulating pump?

 

Exactly. You were complaining about the complexity of a destratification pump. This is a potential solution.

 

I don't actually know whether a Willis heater addresses all of your problems, but I can't see why you wouldn't at least look into it further. They're pretty cheap and there isn't much plumbing involved.

[dpmiller]

thermosyphoning is exactly how a Willis works. It's why we love'em here in NI- piping-hot water to the top of the tank in minutes for a quick basin of dishes, or a whole tankful for a bath  in an hour.

 

You don't need the stat set too high to get the flow going either, so I'd opine it would match a diverter well.

[Miek]

8 hours ago, jack said:

 

I disagree. Your current immersion is unable to heat more than the top of the tank because of its position. Putting a Willis heater low down would allow much more of the tank to be heated.

 

 

Exactly. You were complaining about the complexity of a destratification pump. This is a potential solution.

 

I don't actually know whether a Willis heater addresses all of your problems, but I can't see why you wouldn't at least look into it further. They're pretty cheap and there isn't much plumbing involved.

I think this is probably your best solution to heat the whole tank.  Also has no moving parts so no pump needed. Often used to retrofit solar thermal heating to an old tank, so it's a proven solution. 

 

As with anything relying on convection use big bore pipes and swept bends to reduce pipe friction hindering the circulation 

[S2D2]

9 hours ago, Radian said:

We've signed up for Octopus Outgoing but they're taking ages to get it all in place

There may just be a good reason for this but you might find a quick call/email will hurry things along, Octopus are quite good for that. Think they estimated five weeks when I set up a new export and it was done in five days.

 

15p export skews the sums significantly, I couldn't really justify a few hundred for the divert so you're probably just looking at one of the good cheap suggestions mentioned in the thread rather than anything more significant.

[Radian]

11 hours ago, jack said:

I disagree. Your current immersion is unable to heat more than the top of the tank because of its position. Putting a Willis heater low down would allow much more of the tank to be heated.

 

Bear with. I'm trying to get my head around this.

Like this? So the cold water entering the bottom of the Willis heater will 'float' up once heated due to it's lower density (the basic principle of thermosyphoning).

But will this continue to displace the hot water at the top of the HW cylinder until it's all 'pushed down'? I'm not finding it easy to picture the equilibrium point of the whole system.

[dpmiller]

^ yes.

whenever the water entering the willis from the bottom of the tank is hot enough to trip the stat, the whole tank is hot and no more energy will be requested.

[Radian]

1 hour ago, dpmiller said:

^ yes.

whenever the water entering the willis from the bottom of the tank is hot enough to trip the stat, the whole tank is hot and no more energy will be requested.

 

So it can't get 'plugged' with a buoyant slug of hot water at the top of the cylinder like in the picture I posted above?

Hopefully then it sinks ending up like this...

 

Edited April 20, 2023 by Radian

[SteamyTea]

What sort of power heater is used in this sort of installation.

I would have thought 3 kW would be too large.

[dpmiller]

no it can't get plugged. the tank would need to be hotter than the output of the willis for that to happen

 

aside/ you can however if you're careful use the willis as an instantaneous- once it's on for a minute or two, opening a hot tap gently will take the flow *through* the heater...

[dpmiller]

standard 3kW element @SteamyTea

[SteamyTea]

Just now, dpmiller said:

standard 3kW element @SteamyTea

That is quite a bit of power to pump the water around with.

Does it cycle a lot?

One would not fit a 3kW water pump.

[dpmiller]

yes, they cycle on and off, kettling away happily. And have done for decades. Elements last a very long time as the base-down orientation keeps the electrics cool...

[jack]

33 minutes ago, Radian said:

 

So it can't get 'plugged' with a buoyant slug of hot water at the top of the cylinder like in the picture I posted above?

Hopefully then it sinks ending up like this...

 

 

Ignore the fact that the Willis heater is external - the two situations are almost exactly the same (albeit there might be slightly more stratification in the Willis heater example due to how/where it introduces the heated water to the tank - an internal immersion is more likely to cause some turbulence/mixing).

 

In both cases, buoyancy causes hot water to rise and displace cold water downwards, and also in both cases, the distance that the cold water will be pushed down is limited by the vertical position of the heater.

[Radian]

1 hour ago, SteamyTea said:

That is quite a bit of power to pump the water around with.

Does it cycle a lot?

One would not fit a 3kW water pump.

 

My power control into the immersion is proportional so I can adjust it as required. The advantage of having a powerful heater is that it can absorb excess energy when available. I'd like to 'fill it' as fast as possible - you never know when the Sun's going to go in or someone decides to bake a cake. Currently the top half of the cylinder (almost exactly half: 1.2m high cylinder, 0.6m high immersion heater) has 60l which will increase in temperature by 1000/(60*1.16) = 14.4^oC for every kWh put in. A 3kW heater doesn't exactly sound crazy.

 

1 hour ago, dpmiller said:

aside/ you can however if you're careful use the willis as an instantaneous- once it's on for a minute or two, opening a hot tap gently will take the flow *through* the heater...

 

It would be great if that was the case. At the moment, the slightest draw-off from the top of the cylinder brings fresh cold up to the thermostat around 1/3 the way up the cylinder and invariably trips the boiler on even though there's a vast amount of 70oC water up at the top (when sunny).

I'm trying to work out the best pipe layout to help this happen. Literally what I've drawn would seem to be a good start as the first thing the cold flow going in sees is the Willis, but I think the take-off point could be improved like this:

[dpmiller]

I wouldn't overthink it tbh, they just... work.