Tank in Tank Combined Cylinders

[mike2016]

Hi,

I was over talking to a company that I plan to use to do my MHRV, DHW, underfloor Heating and Part L obligations for a new self build. I have been angling away from the need for a hot water cylinder as there's only me in a 2 bedroom house, with the possibility of a second person the odd time (as a guest or to help pay the mortgage). I've been reading the forums a lot this year and one valued contributor mentioned the excessive heat caused by their hot water cylinder which I found concerning. I was strongly thinking of a Sunamp PV and this is what the same person installed to good effect, plus it's smaller. 

I'm planning a relatively small house (75m2) so space is at a premium and any unbalancing of heat in the house could make things uncomfortable in a larger proportion of the house. The system suggested was a tank in tank system - the main tank is charged by an ASHP with a pressurized hot water cylinder inside the main one. I think the main tank water can be used in the ASHP defrost cycle and to speed heat recovery of the inner hot water cylinder. 

One way to control the standing losses might be the use of vacuum insulated panels. They are used in the Sunamp PV for this purpose, but are flat and easier to fit I would imagine. Can they be retrofitted to a hot water tank or do you have to buy a specific tank with them installed? I only found one or two examples of them being mentioned online, most were just academic papers. I'm keen to avoid overheating and standing losses from such a system, but perhaps the sunamp would be a better fit for me. I'd only be using 45-50 litres of hot water a day but with visitors etc would need to size larger. 

Any thoughts / advice? Thanks! 

[Alexphd1]

I presume you don't have mains gas?

[warby]

I do hope a certain Welsh wizard of a plumber answers you before the Match between Wales and England start at 2:00pm. I can remember from the old forum that he is a rare commodity in Wales, he has little interest in Sport but he will be insufferable if they win. 

But if England win 

I was always told not to mock the afflicted. So 2-1, 2-1, 2-1, 2-1, 2-1, 2-1, 2-1, 2-1, 2-1, 2-1, 2-1, 2-1

Edited June 16, 2016 by warby
Spreading the good news

[PeterW]

I've found the tank in tank system took up more room than less - they need 150 or so litres in the primary, so anything usable is around 300l in total. If you have decent insulation, a small (150l) ASHP heated tank is tiny, and you can ask Telford (other manufacturers are available....) to add additional insulation. You can also ensure the cupboard it's stored in has insulated walls to reduce the heat loss.

[SteamyTea]

I don't know the details of every TiT, but I think the design is fundamentally flawed.  If you have two volumes of a material next to each other, the temperature will try to equalise, you can't change that.  So I have never considered it as an option.

I like the Sunamp idea as a replacement for my 200lt E7 cylinder.  The clever bit is the vacuum panel insulation, not the phase change material.  I think I worked out that I could fit one in the loft, instead of a dedicated cupboard in the main house (they are not much bigger than a gas boiler).  This would free up some space in my 50m² house (75m² would be luxury).

The one issue I would have, and it would take some research to see if it is a real problem or just an academic one, would be the 'occasional visitors'.  With my E7 cylinder, I can easily and quickly turn the temp up from my usual 50°C to 70°C or more.  This can double the thermal storage.  I run at 50°C as it reduces the losses significantly and was perfectly adequate when there was two of us in the house.

My neighbour, who is also single, uses just the instantaneous shower for her hot water (I assume a kettle for washing up).  Her bills are lower than mine at about 3 MWh,year^-1 (mine around 3.8 MWh.year^-1).  She is in less than me though, so there may not be quite the savings as presumed.  She does not take advantage of the E7 pricing, so she pays about the same a year as I do.

But as usual, it is a case of just doing the sums and the answer just plops out.

 

Re the extra insulation.  I just used Celotex sheet.  Worked well and saves a couple of kWh/day.  If you have room, why bother with vacuum panels.

[mike2016]

Thanks for all the info provided. I have mains gas so this is also an option, I just have to meet Part L but this can be covered elsewhere. I like the idea of diversifying my energy sources a bit so will be taking all this into account. Cheers. 

[DeeJunFan]

12 minutes ago, mike2016 said:

Thanks for all the info provided. I have mains gas so this is also an option, I just have to meet Part L but this can be covered elsewhere. I like the idea of diversifying my energy sources a bit so will be taking all this into account. Cheers. 

If you have it use it!

[Nickfromwales]

With gas available, I have to ask why not fit a combi, and a small buffer to deal with the Ufh? 

Dhw in abundance, and zero standing losses from the water production. 

[mike2016]

That's probably the best option behind door number 1 alright....!! I can take care of part L another way....I wonder if we'll see any interruption in the gas supply down the road but you can't plan for everything!

 

[Alexphd1]

I think some gas combis can be slowed down to 9kw max output for heating and still the full power for DHW, which would still probably be over kill for your UFH.

[Nickfromwales]

Hence the buffer