Trystan Lea is on twitter @TrystanLea

Thermal Stores are usually open vented and the DHW is mains fed via a plate heat exchanger - so generally they don't need a regular legionella purge... https://www.gledhill.net/five-things-worth-knowing-thermal-stores/

@andygo 

Hi Andy, that was the comparison I was trying to make, whether it is better to use a Thermal store and no legionella purge, or an unvented cylinder with a legionella purge. I think in my new house I will go down the cylinder route.

On twitter it says that Trystan cannot be messaged, which is a shame as I would like to compare notes with him.

Cheers Mark

Can you please expand on the decision to opt for an (unvented) cylinder rater than a thermal store @bontwoody ?

Looking at the tables you published, isn't the most significant factor the capacity of each method? The thermal store is 2½ times the size.

Regarding the new house, are you considering an entirely separate system for space heating? If the energy for DHW and space-heating are to derived from the same source, doesn't this favour the thermal store option?

Personally I like thermal stoors for heat pumps

Advantages

• lower water temperature required, therefor better efficiency
• the entire cylinder is at the same temperature therefor there is no stratisfication and more energy is stored at a lower temperature
• No need for leogeonella controll or risk assesments in commercial or let properties
• No need for G3, pressurised cylinder certification and the asociated leagal requirement for an anual service (it is however a good idea to service anually anyway)
• No requirement for building notification
• Less heat loss as the water is stored at a lower temperature

Dissadvantages

• Higher cost
• The design must be done correctly or it will not perform.

I agree @heacol and I've had a 280-litre thermal store supplying DHW and UFH for the past 17 years.

For the sake of others reading these comments, can I please note that Part-L1a of the current Building Regs states that the Approved Document requirements do not apply where the pressurized cylinder is installed substantially for space-heating.

That's why the certification, authorised installer and annual-check requirements do not apply. In my case it enabled me to install everything myself and gradually add further UFH zones when I got around to it.

Part-L is due for re-publication soon and we will need to check if that guidance is retained. For reference, I'm writing this in April 2022.

Unlike @heacol my thermal store does have some measure of temperature stratification. There's about 5degC difference between top and bottom of the cylinder. Only the top 40% of the cylinder is used to heat up incoming water for DHW requirements. I feed in energy from my solar-thermal array close to the bottom of the tank so that it kicks in at a lower temperature.

My thermal store pipework is somewhat more complex than most because we also don't have mains water. Everything here is run from a well with about 20,000-litres in it 😎 

@transparent 

My current thinking is as follows, but I would really like to get some more information from Trystan to check my facts.

If both his unvented cylinder system and my heat store system both produce the same amount of hot water, (not really established yet) then the choice really boils down to the costs to run. Since posting the comparison initially I did find an MCS guide (

MCSCERTIFIED.COM "https://mcscertified.com/wp-content/uploads/2020/07/Heat-Pump-Guide.pdf"

) which on page 28 recommends a weekly pasteurisation cycle.

If running a weekly cycle then the costs of the two systems really favour Trystan’s (£2.19 per week V £5.43 per week, mine). There are some other factors to consider as mentioned in Brendan’s post such as building notification and annual servicing.

The increased running costs of my thermal store are caused by the higher temperature I heat it to. Trystan heats his cylinder to 42C for his hot water. I heat my store to 51C. The reasons for this are in the way the two systems work.

Trystan uses his water directly and so gets his to the tap close to 42C. To heat my water, I have to transfer the heat in my store to the cold water running through its heat exchanger. Because of this there are various factors at play in the temperature of the water getting to my tap, such as the temperature of the cold water going in and the flow rate as the water takes time to heat up as it passes through the exchanger.

Even though I have a greater capacity as the temperature in my store decreases, so does the speed at which I can transfer heat to the water in my exchanger. When my store reaches 42C, the water at the tap is no longer hot.

Where a thermal store really scores if you have more than one heating source, such as solar water heaters or a wood burning stove with back boiler.

When I designed my current house, I initially thought my UFH input would come from my thermal store, but soon realised that it would be wasteful because the store is at a higher temperature than needed, so now the UFH is directly heated from the heat pump without a mixer valve. This is achieved by diverting the water with a 3 way motorised valve.

In my new house, I am thinking of a similar system with UFH on the ground floor, but am considering other ways of heating the upper floors. It’s an old house so I’m thinking flexibly.

So that’s how I’m currently thinking, I’m more than open to being convinced otherwise. 🙂

@transparent 

Just thought of something that may affect the comparison, I assumed my store would be at 35C when my heating cycle begins. That probably isnt true as the water stops being hot before that happens!

Revised comparison with my start temperature at 40C

@transparent  What temperature do you heat you store to? Do you find a bath depletes it? I also installed my own store and heat pump. Its a nice feeling when it comes together 🙂 

Thanks for the pic @bontwoody 😀 

The top of my thermal store would usually reach 55C but can go higher on a sunny day like today. There's no limit set for how much the solar-thermal array can deliver.

There's a DHW thermal-mixer-valve above the insulated tank, which I've left set at the default 42C.

Yes, a bath will use all the available energy above 42C, meaning that lower temp water then emerges from the hot tap. But we have only one bath here and three showers.

And I'm glad to find someone else who installed their own. It was my Building Regs Inspector who suggested that I went down this route. I'd like to think they he was expressing confidence in my practical skills - but it could equally be that he'd seen the poorly-installed systems from the local professionals! 😥 

@transparent 

Lol you obviously have more room than me to move around your store! I was seriously thinking of fitting a solar water heater when I started, but didnt because of the cost which I couldnt justify when I could pretty much heat the water for free from my PV array. However its a much more efficient system and I still hanker after it.

One problem I have now with my current setup is that I was advised to use a splarge pipe rather than a coil to connect the heat pump to the store. The argument was it was more efficient at moving the heat energy, the downside which I only found out after the fact was the huge amount of expensive anti-freeze I would need to put in. Consequently I use the solar coil now to heat the store from my heat pump!

Given my revised cost estimate, Im now on the fence again 😆