markharro Posted July 13, 2022 Share Posted July 13, 2022 Hi I am sure I have read somewhere that it is essential or at least highly recommended to have a water tank that is capable of having immersion heaters fitted at the top and the bottom to offload extra generation from solar PV? I have just had my second response from a potential ASHP installer. The first insisted that they can only fit one make of tank that was only compatible with one immersion only and the second installer has just emailed me to respond to my query about immersions by saying "There is one immersion heater located in the middle of the tank. I don’t believe you would find an unvented heat pump cylinder with 2 immersion heaters.(Safety thing)" surely this is rubbish? Is it not possible that this is simply the installer's laziness and them just wanting to use a tank that they are familiar with? Thanks Link to comment Share on other sites More sharing options...
JohnMo Posted July 13, 2022 Share Posted July 13, 2022 I think your diverter will only be directed on one immersion. Otherwise you need to divert into another controller that firstly heated one immersion then the other. Mixology cylinder has I believe two immersion heaters and a controller for PV. They use an external PHE to charge from a HP. Link to comment Share on other sites More sharing options...
Carrerahill Posted July 13, 2022 Share Posted July 13, 2022 It is not essential or really a rule as such. Just, helpful. Depends on many factors, work out what spare you will have first, load analysis would help. Also, do you consider a small battery. Plenty of 6kW (dual 3kW) tanks exist, very common commercially. I have a friend who has a 9kW tank. Link to comment Share on other sites More sharing options...
ReedRichards Posted July 13, 2022 Share Posted July 13, 2022 (edited) My water tank has one immersion heater in the middle. But it has an integrated buffer tank at the base so that did not give me much choice. It turns out that there is a safety cut-out fitted to the tank at about the same level as the immersion heater. That is triggered when the measured temperature (as reported by the heat pump controller) is about 75 C and stops the heat pump from heating the tank. It's easy enough to reset but I don't want to keep doing that so even with a single immersion heater I have to make sure that the water at that level in the tank does not get too hot. A destratification pump would help but I doubt that a lower immersion heater would improve matters significantly. Edited July 13, 2022 by ReedRichards Link to comment Share on other sites More sharing options...
Wil Posted July 13, 2022 Share Posted July 13, 2022 Despite asking for two, I was persuaded to have a single immersion for PV dump at the bottom of my tank. This means the 300l tank can sometimes take up to 14kWh of PV dump until it’s up to 80degrees (stat 2/3 of the way up) and the immersion stat cuts out on overheat. This works great for dumping loads of heat in summer and in winter I find that what little diversion I get tends to heat the top of the tank by stratification anyway. The lower the immersion, the more capacity it has to go at unless you have some way of overcoming the stratification (I’ve not tested that theory so maybe it doesn’t matter where your immersion is, it all gets to the same temp in the end for a given input). But having a second immersion at the top allows you to heat the top section quickly for boosting HW for additional guests etc. Link to comment Share on other sites More sharing options...
joth Posted July 13, 2022 Share Posted July 13, 2022 Hmm sounds like a "fact" of convenience. Pretty sure Peter will be along soon to say speak to Telford cylinders or someone at cylinders 2 go and they'll make one in any configuration you want. Eddi supports 2 immersion heaters. That said, I also have a limit of just one immersion, in the centre of the 300L UVC, and it does indeed limit the amount of PV divert I can achieve. This was not entirely by choice but a limit of my UVC (Oso Geocoil) does not come in any other option. My hair-brained idea to solve this is, at some point, to install a secondary return circuit, and connect the return (via NRV) into the main water inlet, that way it will agitate the entire tank when sec return is in use, which means on a sunny day I can just run the secondary return pump to increase effective capacity. O/c that does mean increasing unintended internal heat gains from the sec return circuit running on hot days, so it may not be the world's smartest plan. I could use a diverter valve to switch the pump from sec return to the UVC agitation short-circuit route, but that that's all need to be WRAS approved thus pricy. Hence why I've left this whole idea on the back burner. When it's really sunny, I just put on the ASHP to DHW and crank up the target temp to 55ºC which is arguably better due to the >1 COP. (And when I say "I", what I mean is my home control system does that). Link to comment Share on other sites More sharing options...
JohnMo Posted July 13, 2022 Share Posted July 13, 2022 Another option is to heat via a Willis immersion, they connect to bottom and top of tank, supposedly can heat just the top or the whole tank, no external pumps needed.. 1 Link to comment Share on other sites More sharing options...
kommando Posted July 13, 2022 Share Posted July 13, 2022 (edited) My solar diversion immersion is a 3KW Willis (developed in NI I believe), it pulls water in from the bottom of the tank via thermosyphon effect and put it back in at the middle. I have temp probes at the bottom feed pipe, exit from the Willis and at the 3/4 tank point so I know what is going on. With the Willis at full bore the water goes in at 20C and exits it at 58C. The 3/4 tank probe goes up during the day and somehow on a hot and sunny day ends up in the high 60c's with only the Willis heating the water. Edited July 13, 2022 by kommando Link to comment Share on other sites More sharing options...
ReedRichards Posted July 13, 2022 Share Posted July 13, 2022 10 hours ago, markharro said: Hi I am sure I have read somewhere that it is essential or at least highly recommended to have a water tank that is capable of having immersion heaters fitted at the top and the bottom to offload extra generation from solar PV? This fundamentally is not true. You just need one immersion as low down the tank as you can get it. The second immersion at the top is just for circumstances when you have run out of hot water and need a bit more. This probably isn't going to happen very often and when it does the chances are the sun won't be shining and you will use the ASHP. Link to comment Share on other sites More sharing options...
Tommytipee Posted July 14, 2022 Share Posted July 14, 2022 (edited) Just get a bigger immersion. Baxi do a 9kw one. https://www.heatingspareparts.com/boiler-spares/partno/94110302 Edited July 14, 2022 by Tommytipee Link to comment Share on other sites More sharing options...
dpmiller Posted July 14, 2022 Share Posted July 14, 2022 I've two immersions in my TS, and an iBoost diverter that runs both. Upper one is at a low-ish temp and when it is satisfied the diverter swaps to the bottom one with it's stat maxed out... Link to comment Share on other sites More sharing options...
ReedRichards Posted July 14, 2022 Share Posted July 14, 2022 3 minutes ago, dpmiller said: I've two immersions in my TS, and an iBoost diverter that runs both. Upper one is at a low-ish temp and when it is satisfied the diverter swaps to the bottom one with it's stat maxed out... So that gives you some warm water quickly but does it ultimately get the cylinder hotter than if you just ran the immersion heater at the bottom? Link to comment Share on other sites More sharing options...
dpmiller Posted July 14, 2022 Share Posted July 14, 2022 There's only a certain about of excess to divert tho, so I'm trying to make the most of it. You ask a good Q tho... Specifically, I've a DHW coil at the top and an ASHP coil at the bottom, and the cylinder is sized such that the top is big enough for a good shower and the bottom can act as a buffer for the heatpump. Link to comment Share on other sites More sharing options...
joth Posted July 14, 2022 Share Posted July 14, 2022 2 hours ago, Tommytipee said: Just get a bigger immersion. Baxi do a 9kw one. https://www.heatingspareparts.com/boiler-spares/partno/94110302 £1200 RRP!!!! 🤯 No thanks! The reason for 2 immersions for PV divert is in winter there's probably not enough sunlight in the day to heat the whole tank, so if you heat the top half first and when (and only when) that is at target, switch over to heat the bottom half, you have the best odds of achieving some usable DHW from it and possibly avoiding the need to use fossil fuel to top it up. A single immersion at the bottom of the tank, regardless of its size, might only get the whole tank to 40°C on a cloudy day. More useful to have half a tank at 60°C than a whole tepid tank. That's the idea. Link to comment Share on other sites More sharing options...
HughF Posted July 14, 2022 Share Posted July 14, 2022 36 minutes ago, dpmiller said: There's only a certain about of excess to divert tho, so I'm trying to make the most of it. You ask a good Q tho... Specifically, I've a DHW coil at the top and an ASHP coil at the bottom, and the cylinder is sized such that the top is big enough for a good shower and the bottom can act as a buffer for the heatpump. Interesting, a TS heated by an ASHP. What do you heat the TS to in order to get a decent DHW temp? Link to comment Share on other sites More sharing options...
dpmiller Posted July 14, 2022 Share Posted July 14, 2022 ASHP takes the tank to 48c, then the top immersion pushes it up farther. But 48 is enough for the tempering valve on the tank to be active and the DHW to be verging on uncomfortable Link to comment Share on other sites More sharing options...
HughF Posted July 15, 2022 Share Posted July 15, 2022 On 14/07/2022 at 10:55, dpmiller said: ASHP takes the tank to 48c, then the top immersion pushes it up farther. But 48 is enough for the tempering valve on the tank to be active and the DHW to be verging on uncomfortable Interesting. Do you have a DHW coil or do you use a plate, flow switch and shunt pump? Link to comment Share on other sites More sharing options...
Bramco Posted July 15, 2022 Share Posted July 15, 2022 On 14/07/2022 at 09:58, joth said: That's the idea Exactly. Our plumber ordered a 300l tank from Newark with 2 immersions and an ashp sized coil. The PV diverter goes to the upper immersion until it is satisfied and then to the bottom immersion. So ridiculous amounts of hot water in summer when you need a cool shower BUT in winter an optimum solution giving half a tank on a duller day and 3/4 on a sunny day. ASHP folks are so pushed, they can simply take the mugs money and not bother learning about real eco houses. Simon Link to comment Share on other sites More sharing options...
dpmiller Posted July 15, 2022 Share Posted July 15, 2022 28 minutes ago, HughF said: Interesting. Do you have a DHW coil or do you use a plate, flow switch and shunt pump? Coil. It's brave size, 3m2/ 42kW. The heat pump coil is the same size. 1 Link to comment Share on other sites More sharing options...
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