Proposed new system

[Onetap]

"so let me confirm my objectives.
A] CH at 40C – 70C as needed
B] DHW at about 50C
C] Pool, taking as needed when available or timed for off-periods"?

Any stored DHW should be kept at 60 degC minimum, or there's a risk of legionella. If you need TMVs (new build, 'at risk' users) , they should be at the point of use, to maintain the temperature in the pipes.

The thermal store doesn't store much DHW. The DHW in the coil heats up to the store temperature when there's no draw off (80degC, say); the TMV on the outlet is there to reduce this to a safe, non-scalding, temperature. The high temperatures also cause limescale problems (with hard water) as mentioned.

I can't see an advantage to storing, say 600 litres of very hot water at 80 degC (thermal store) when you don't need water at 80degC, compared to storing 600 litres of DHW at 60 degC. If you have the plant space, and it's not a choice between one or the other, you might be able to justify a thermal store (or buffer vessel) as well, for example if you planned to add solar thermal to heat the pool, or if you intend to install a woodburner; then you have a requirement to store heat for later use.

Another application, is in conjunction with a non-condensing, non-modulating boiler. A thermal store allows the boiler a long run time, to heat the store from say 56 to 82 degC. The CH is suppled through a mixing valve set; this arrangement allows the CH to draw say 3 kW, without frequent cycling of a boiler sized for the peak demand 20kW say.

The mixing valve set flow temperature could be controlled by a weather compensating controller and it stops drawing heat if the store temperature falls below 56 degC, to avoid low return temperatures and corrosion of the non-condensing boiler.

Modern boilers are usually modulating and condensing, so the above arrangement has infrequent applications, when the off-peak CH demand is less than the minimum boiler output.

Most thermal stores (as against a cylinder you've installed to use as a thermal store) are intended to be used with open vented CH systems. I would avoid an open vented system, if possible.

As a point regarding TRV’s, the theory is fine but their effectiveness is not frequently possible when the rads are oddly positioned or then doors are left open, as in my case.

Install TRVs; if you have a weather compensated system (variable flow temperature, to match the heat losses), they will be trimming room temperatures and will be less likely to be trying to shut down a lot; less noise.

As a point, the rads max output exceeds the heat losses by about 50%

They will deliver the required heat output at a lower flow temperature.

 

[SimonH2]

Just as a data point, while the boiler was being serviced in the flat last week, I made a note of the boiler return temp (as drawn from the store and before the TMV). I had to turn the store thermostat up (from 60˚C) to fire the boiler for testing, and during the run the return temp was climbing gradually from 20˚C to 30˚C.
Flow temperature was rising from around 65˚ to 70˚ during this time.

Just to show that you don't have to have a store heated to 80˚ for it to work, and you don't have to have boiler return temps of 60˚ or more. But I did deliberately choose a store a size larger than the manufacturer (Gledhill) recommends for the size/type of property.

 

[ALEC1]

Since you are going to have to spend an inconvenient amount of money, you might as well get it right


one of the few companies to supply schematics of heating systems design is viessmann with a vitodens 200 boiler with weather compensation in their installation guides.

Tha takes the educated guess work from the installer, or client, and leaves a good working system which is reliable, efficient and fit for purpose.