Have thermal stores had their day?

[Workerbee]

Hello everyone,

I'm replacing my vented direct dhw cylinder in a one-bed all-electric house and I was planning on having an uvc installed. But then I came across the idea of a thermal store. This would still give mains pressure hot water - important for a good shower - but also seemed to provide additional benefits:

- no g3 requirement so I could install it myself

- no annual service 

- some of them can run ufh as well as dhw. So rather than installing electric ufh, I could maybe put in wet ufh which might future-proof me if I ever want a heat pump etc.

 

But:

- I've read people saying thermal stores have had their day and they've been superceded by uvcs

- It sounds (from a different thread on here) like I'd need a larger size compared to an uvc because of the way they lose heat

 

A lot of the YouTube videos I've found about thermal stores seem to be about 10 years old. Am I looking at old tech or might it be a good fit for me? 

 

Thanks!

 

[SteamyTea]

It really comes down to what you want to achieve with thermal storage.

 

There is energy input types.

Input temperatures.

Output Temperatures.

Output Flow Rates (power).

Storage Quantity (energy).

Physical size.

Overall Mass.

Easy of Plumbing.

Acceptable Thermal Losses.

 

I have an all electric (E7), small house.

My DHW is a 200lt, gravity fed, system that is pumped to the shower.  Works fine and has done for 20 years (had to change the old cylinder when it was 20something years old.

 

In all honesty, there is not really going to be much difference, on a small system, whichever type you choose, especially if you only have one energy input type i.e. electricity.

 

[Workerbee]

Thank you for the reply. I will have a look into the specs you mention, particularly things such as standing heat losses, but it is good to know thermal stores are not old tech. The fact uvcs seem far more prevalent made me suspicious that thermal stores must have some major flaw, particularly as they seem to carry benefits such as potential self-installation. 

 

Do you find 200l is more than enough? It would be supplying two of us having showers. There was also a thread I found that mentioned thermal stores need to be oversized compared to uvcs. I suppose for those thermal stores that can supply heating as well, it would need to be even bigger.

 

That's another puzzle: whether to get wet ufh and a compatible thermal store, to hopefully have a future proofed setup I can add a heat pump to in the future, or just focus on dhw and have separate electric ufh, and lean into being a 'dry' electric house and hoping battery tech moves forward, solar becomes more efficient for my small roof etc.

[JohnMo]

  On 13/04/2025 at 20:10, Workerbee said:

separate electric ufh

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Running electric UFH sounds super expensive. Why not a good old storage heater, that least you charge up on cheap rate.

 

A simple direct UVC can be charged to about 70 degrees and should last all day between heating.

 

Run it on a smart tariff or good E7.

 

[Workerbee]

Thank you, yes you're right, but I'm putting a stone floor down in the kitchen so I'm keen to have something to take the chill off it just in that room. So that's why I'm wondering about whether to go electric ufh just as a secondary heat source, or install wet ufh (in combination with a thermal store that does heating) which can potentially be upgraded to a primary heat source if I get a heat pump one day.

 

For primary heating, storage heaters definitely make most sense, but I've actually been looking at oil filled radiators, partly because I haven't found any storage heaters I like the design of (daft, I know, because it'll end up costing me), and partly because I always think of the bad rep of storage heaters of old. But I presume they're a lot better now?

 

You mentioned about a uvc lasting all day - what sort of size would I need? Plumbers I spoke to suggested around 150l for 2 people. So I'm thinking maybe the 210l if I went the thermal store route, to make up for the different way it loses heat. 

 

I suppose you can always increase your stored hot water by increasing the temp, but I'm feeling it would be better to oversize than undersize a cylinder, whatever type it is.

 

[JohnMo]

  On 13/04/2025 at 21:14, Workerbee said:

150l

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We have a 210L heat pump cylinder this is the temperature profile today. 

 

 

Before 7am it was heated to the 53 degs, temp dropped doing wash up etc. At 11am excess solar PV heated to nearly 70. The drop around 1900 was me having a shower.  The temperature sensor is in the bottom third of the cylinder, so loads of hot water left.

 

So a 150L heater to decent temperature should be an issue. But the hotter you heat the quicker it looses temperature so you maybe better a slightly bigger cylinder heated slightly cooler? But if you are thinking heat pump in future, just install a heat pump UVC now and won't connect the heating coil, then do a 180 to 210L.  Thermal store if connected to heating could be in the region of 300-400L to be useful.

[ProDave]

A thermal store sounds like a good idea, just like a combo boiler does, until you drill down into the details.

 

Un vented cylinder every day for me.  Why are people so against a modest maintenance requirement to get something good?

[SteamyTea]

  On 14/04/2025 at 08:00, ProDave said:

until you drill down into the details.

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And find they are governed by the same Laws of Thermodynamics.

 

The real problem is that too many people want to ignore the science.

[JohnMo]

An UVC runs hot until the cylinder is depleted, the thermocline rises up the cylinder as hot water is consumed. So a 200L cylinder has approx 200L of usable hot water. A thermal store comes in two varieties, internal coil and external plate exchanger.  The internal coil requires most the cylinder water to be several degs hotter than the DHW output. As the top half of the cylinder cools to say 45 degs, the coil heat transfer rate drops and you effectively has no hot water. An external coil performs better, but not as well as an UVC.  Add a draw for UFH and you soon run out of hot water for the taps.

 

I have used both and wouldn't go back to a thermal store, to many one way conversations with wife on why has the hot water gone cold AGAIN. Thermal store think big or don't think about them at all. 

[ProDave]

  On 14/04/2025 at 08:08, SteamyTea said:

And find they are governed by the same Laws of Thermodynamics.

 

The real problem is that too many people want to ignore the science.

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No they don't.  See @JohnMo description above.  You have to store the water in the thermal store much hotter than your target water temperature if you want to get a decent volume of water out, which is why they are a very poor match to an ASHP.

[joth]

  On 14/04/2025 at 08:00, ProDave said:

Why are people so against a modest maintenance requirement to get something good?

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Mostly that you can't DIY it, and I've had zero success paying someone to do it in a way I'm happy with. One no show, another looked at me oddly (you want me to do what??), another failed to mark the service book to say they'd don't it. 

 

Don't get me wrong, I'm very happy I have uvc and wouldn't choose anything else, but the difficulty having it serviced suggests virtually no one does (unless part of a wider boiler/heat pump service plan maybe?? That I don't need)

[ProDave]

  On 14/04/2025 at 13:07, joth said:

Mostly that you can't DIY it, and I've had zero success paying someone to do it in a way I'm happy with. One no show, another looked at me oddly (you want me to do what??), another failed to mark the service book to say they'd don't it. 

 

Don't get me wrong, I'm very happy I have uvc and wouldn't choose anything else, but the difficulty having it serviced suggests virtually no one does (unless part of a wider boiler/heat pump service plan maybe?? That I don't need)

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I guess I am lucky to have a local plumber I know well and often worked with.  He was happy for me to run all the pipes after discussing with him, he just came and filled the tank, checked everything and signed the paperwork for a modest sum.

[joth]

  On 14/04/2025 at 13:19, ProDave said:

I guess I am lucky to have a local plumber I know well and often worked with.  He was happy for me to run all the pipes after discussing with him, he just came and filled the tank, checked everything and signed the paperwork for a modest sum.

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Yeah getting it installed was fine, it's the annual service that is an ongoing headache. (Not because I don't want to pay, just faff finding someone to do it). And it's not like I'm in some backwater area. 

 

I mean, I don't even know what to Google for to get semi sensible suggestions - most things come back with Gas Safe boiler servicing 

[JohnMo]

I just did the G3 training, half a day in a classroom. Allowed me do my own install and services.

[Iceverge]

I've done both. 

 

UVC and TS. 

 

From trial and error real life living the UVC will deliver useable 40deg water until the average temperature of the tank is about 30 deg. 

 

The TS is all out of ideas once the average tank temp is 50deg. 

 

Assuming you can store water at 70deg on an immersion then an UVC will give you double the hot water of a TS. 

 

If you have a heat pump you will have to run it very hot for a TS which will destroy your COP unless you have a really giant tank. 

 

That said I really like the TS. It does a nice job of combining heating, oil boiler and solid fuel cooker and pressurised DHW. It could take an immersion and solar too if needed. 

 

Both are pointless for space heating though unless you can hold 2-3m³ of hot water. 

 

For your situation a direct UVC on a TOU tariff would be the cheapest long term. Have you thought about A2A HP for space heating. Ours is proving economical. 

 

I do my own servicing, (no G3 in Ireland). Takes 15 mins a year. Check the safety valves, clean the filter and top up the accumulator. It is dummy level if you have any mechanical competence at all. 

 

 

 

 

 

 

 

 

[Hammertime]

That's really interesting. The idea of being able to DIY a TS is attractive, but the chances of running out of hot water with a TS sounds high, unless you're heating and storing it at much higher temperatures, which would be uneconomical in comparison, particularly when relying on the immersion. 

Edited April 15 by Hammertime

[bassanclan]

660l TS, provides plenty heat for radiators and ufh. Hot water for 5 showers.

Allows it to be heated from solar (via Immersion) or wood burning stove or gas. What's not to like?

[Nickfromwales]

  On 15/04/2025 at 21:10, bassanclan said:

660l TS, provides plenty heat for radiators and ufh. Hot water for 5 showers.

Allows it to be heated from solar (via Immersion) or wood burning stove or gas. What's not to like?

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Open vented? 

[Temp]

  On 15/04/2025 at 20:09, Hammertime said:

That's really interesting. The idea of being able to DIY a TS is attractive, but the chances of running out of hot water with a TS sounds high, unless you're heating and storing it at much higher temperatures, which would be uneconomical in comparison, particularly when relying on the immersion. 

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We have 300L vented TS. Currently heated by an oil boiler. The hot in and DHW hot out are both near the top of the TS so I think when depleted and the boiler fires up you almost have hot water on demand. We've never run out of hot water and have high flow rate showers.

 

The down sides are...

 

Our store has an external plate heat exchanger , pumps and blending valves which all leak heat into the room its in. 

 

They aren't ideal for use with a heat pump. Our store is set to about 55C and feeds both DHW and CH so we would need a flow rate of >55C all the time. The CH/UFH doesn't need such a high flow rate in modern well designed houses and a lower flow temperature would help the COP so it's likely that a tank for DHW only would be better., or perhaps two tanks at different temperatures?

[dpmiller]

^ like @Temp we find that smart placing of the controls/sensors allows a TS to recover before it runs out of heat, allowing continuous drawoff. And we're on an ASHP...

[JohnMo]

  On 15/04/2025 at 21:10, bassanclan said:

660l TS,  What's not to like?

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The size, having to heat 660L of water.

 

Heating an UVC via excess PV is way more bang for your excess kWh. Mostly due to the volume of water having to be heated. 210L compared to 660L, a 210L will add 3x the heat per kWh of energy. If I add 10 degs you would add 3.3 degs, less the approach temperature of the heat exchanger to transfer heat to DHW. So in real terms 1 or 2 degs instead of 10 degs.

 

As said previously UVC are way better (having had both).

[Iceverge]

  On 15/04/2025 at 20:09, Hammertime said:

storing it at much higher temperatures, which would be uneconomical in comparison, particularly when relying on the immersion. 

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Not necessarily, high temperatures are fine for Immersions. It's heat pumps that suffer. The catch with immersion heating is it's often best done in a small time window for the best tariff so you need lots of capacity.

 

If you have lots of space or a fossil fuel boiler and/or a solid fuel boiler I think they are a good solution. 

 

With heat on demand like with an oil or gas boiler they act almost like a capacitor in a circuit to smooth the cycling of the boiler. Storing much energy is not feasible when you look at the physics if it. A 300l thermal store at 70deg will give you about 7kWh of usable DHW, or it might run UFH for under an hour in a well insulated modern 200m2 house. 

 

For the situation in my parents house they can still have pressurised DHW in the event of a power cut as the solid fuel Rayburn feeds the store on a gravity circuit and a small Genny does the water pump. 

 

As alluded to above a TS will have far more copper pipes hanging off it Vs an UVC, especially a direct UVC and these loose a significant amount of the energy, even when lagged when compared to the tank itself. Not an issue in a old farmhouse but may ( and has ) led to overheating of smaller or well insulated houses. 

 

 

TLDR,

 

TS good for old houses on solid fuel or Gas/Oil with space for a large tank. 

 

TS bad for small/well insulated houses on an immersion or HP. 

 

G3 is simple to DIY, don't let it affect your decision. 

 

 

 

Edited April 17 by Iceverge

[joth]

  On 15/04/2025 at 10:17, JohnMo said:

I just did the G3 training, half a day in a classroom. Allowed me do my own install and services.

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Had a quick look - courses are about £200 and need refreshing every 5 years. That's not bad value, but I would need to bend the entry requirements a bit. Examples below.

I think as a Loxone installer I could confidently state I have 3 years experience as a professional heating controls engineer.

 

 

 

https://gretraining.co.uk/product/g3-unvented

Entry Requirements:

Trainees must hold a recognised trade qualification (e.g. NVQ/SNVQ Level 2 or 3 in Plumbing and Heating or Domestic Heating)

OR be working towards one

OR have evidence of 3 years experience in the plumbing or heating industry, including a cover letter.

 

 

https://www.logic4training.co.uk/courses/plumbing/water-safety-courses/hot-water-systems-and-safety-course-unvented-hot-water-course/

Entry requirements

Candidates must be experienced in the installation of domestic hot and cold vented and unvented water systems and hold one Department for Communities and Local Government (DCLG) recognised competency in a conventional fuel.

The initial assessment is designed for operatives who do not have an up to date HWSS certificate.

 

 

 

 

 

 

 

Edited April 17 by joth

[JohnMo]

  On 17/04/2025 at 10:33, joth said:

Had a quick look - courses are about £200 and need refreshing every 5 years. That's not bad value, but I would need to bend the entry requirements a bit. Examples below.

I think as a Loxone installer I could confidently state I have 3 years experience as a professional heating controls engineer.

 

 

 

https://gretraining.co.uk/product/g3-unvented

Entry Requirements:

Trainees must hold a recognised trade qualification (e.g. NVQ/SNVQ Level 2 or 3 in Plumbing and Heating or Domestic Heating)

OR be working towards one

OR have evidence of 3 years experience in the plumbing or heating industry, including a cover letter.

 

 

https://www.logic4training.co.uk/courses/plumbing/water-safety-courses/hot-water-systems-and-safety-course-unvented-hot-water-course/

Entry requirements

Candidates must be experienced in the installation of domestic hot and cold vented and unvented water systems and hold one Department for Communities and Local Government (DCLG) recognised competency in a conventional fuel.

The initial assessment is designed for operatives who do not have an up to date HWSS certificate.

 

 

 

 

 

 

 

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I went direct to local college and they asked questions, said I was building own house had done the plumbing design and install, paid the money, did course. From what I remember seemed cheaper than you were quoted. Did G3 and water hygiene.