Thermal Store or Unvented Cylinder

[kingkevbo]

Hi,

I have recently emigrated to the Czech Republic, into a house that has a relatively old boiler with sealed CH and an 100 litre unvented cylinder for the DHW. The system is great as there is plenty of hot water at a good pressure.

As the system is old (1998), and my water comes from a well (and therefore quite hard) the system needs replacing as I imagine it is fairly clogged up and inefficient.

As I would like to add a wood burning stove and solar panels to the system, my research tells me that an vented thermal store would be a good idea, due to the added complication of adding these things to a sealed system, ie what happens in the event of power cuts, component failures etc, although I believe wood burners and unvented systems are more common in europe and there are solutions to all these issues.

My main reservation with a thermal store is, and please correct me if I am wrong, is that as the hot water is heated on demand, if someone is having a shower upstairs, and someone opens the hot tap in the kitchen, this will increase the flow though the plate heat exchanger and therefore the temperature of the hot water as it leaves the heat exchanger will be lower and the person in the shower will experience a temperature drop. Is this the case?

It seems to me that the advantage of the unvented cylinder is, as the hot water has already been heated, that the temperature of the hot water does not drop if you open two taps... as long as you dont empty the tank! The only disadvantage I can see of the unvented cylinder is the added complication of adding things like wood burning stoves and solar panels, and integrating all the systems so that they can be used to heat the CH and the DHW. This process seems much simpler with a thermal store.

Any advice would be greatly appreciated.

 

[Onetap]

You need the thermal store to store heat from the solar and the wood-burner. You can supply the hot water from an instaneous plate heat exchanger and/ or a hot water storage cylinder, vented or unvented.

The installation instructions, in the UK, usually advise against connecting an unvented cylinder to a wood-burner since the wood burner cannot be shut down in the event of the cylinder over-heating. If your cylinder is connected as a pumped secondary circuit from the thermal store, there's no reason why it can't be done safely.

The hot water froma plate heat exchanger is usually too hot to use safely ( depending on the primary inlet temperature) and they have a TMV on the outlet. If the plate heat exchanger is big enough, it would be capable of supplying two showers. The narrow waterways get blocked in hard water areas and you need to be able to remove it to descale or have valves to allow it to be isolated while it is descaled in-situ.

 

[kingkevbo]

Thanks for the advice, I didn't think of the possibility of having a thermal store AND and unvented cylinder, with the unvented cylinder being heated by the thermal store. I just had in my mind that it was one or the other.

It would take up a fair bit of space though..

This would give me the best of both worlds, plenty of hot water with no need to de-scale a plate heat exchanger, and the simplicity of connecting solar panels and a wood burner.

Would there be any added inefficiencies of heating an unvented cylinder from a thermal store as opposed to using a plate heat exchanger to heat DHW directly from the thermal store?

 

[DeltaT2]

Onetap is correct, but having a UV cylinder from the heatstore is daft mate, just install a heatbank & try to minimize the limescale in the SS HE. Which would be quite easy on the Primary side.

 

[Onetap]

Would there be any added inefficiencies of heating an unvented cylinder from a thermal store as opposed to using a plate heat exchanger to heat DHW directly from the thermal store?

Yes, in the situation where the thermal store was cold and you just wanted to use the minimum power used to heat the cylinder (in winter, if the wood-burner was off or warming up). You'd have to heat the thermal store through before you could start to heat the cylinder. You could get around that, but only with fairly complex controls.

I'd avoid a heat bank, personally.

 

[kingkevbo]

Apologies for my ignorance, but what is the difference between a heatbank and thermal store? And why would you avoid them?

 

[DeltaT2]

A heatbank has an external stainless steel plate heat exchanger(SSHE) & a thermalstore has a internal heat exchanger............Pros & Cons with both, and to be frank given an alternative I'd have neither.

Effectively your thermalstore or heatbank is the neutral point in the system, where all heat sources feed into and they're great for that, but having a large store of primary water has it's problems, one of which is sludge, limescale....etc etc. That's why they are called Sludge Buckets in the trade!!
HTH

 

[kingkevbo]

Pros & Cons with both, and to be frank given an alternative I'd have neither.

So what your saying is, you dont like them, but you believe that I have no alternative, if I want to add a wood burner and solar panels.

Is it not worth investigating the possibility of added these items to an unvented system, especially as these systems are much more common in europe and you think that thermal stores are not that good anyway?

 

[DeltaT2]

In your case mate, I'd use a heatstore or heatbank. But watch that limescale!!

 

[Onetap]

By thermal store, I meant a water container for storing heat, any insulated cylinder can do this. I'd think you need one of these.

Heatstore and heatbank are terms used in the UK for thermal stores sold mostly for the self-build market. They often have internal heat exchangers and integral feed and expansion tanks. I don't like these things. I don't believe they are of any use with a condensing boiler.

The limescale problem is with the cold water supplied that is heated up for use as DHW. The thermal stores will heat the water above the 60 degC at which you'd normally use it and it then mixes the hot water with cold using a TMV on the heat exchanger outlet. The excess heating causes a lot of limescale to precipitate within the heat exchanger and causes the sludge problem.

 

[SimonH2]

I don't like these things. I don't believe they are of any use with a condensing boiler.

Why do you say that ? You can feed the boiler with nice cold water and extract maximum heat from your burning gas. If designed right, I cannot see any situation when you'd not be condensing.

The limescale problem is with the cold water supplied that is heated up for use as DHW. The thermal stores will heat the water above the 60 degC at which you'd normally use it and it then mixes the hot water with cold using a TMV on the heat exchanger outlet. The excess heating causes a lot of limescale to precipitate within the heat exchanger and causes the sludge problem.

Hmm, well your store contents are a finite quantity of water which is not changed except when you have a leak or maintenance. So only a finite amount of dissolved lime to come out of solution. Any lime that comes out of solution in the DHW side (coil or HE) will be carried out with the water flow (and hence can't "sludge up" anything, or scale up the pipe or plates (which I admit may be an issue - I'm not in an area where it's a problem). I suspect it's simply a case of people (plumbers who don't seem to understand thermal stores) blaming the store for the fact that the rest of the system is making sludge and it collects in the store because that's where the water is still and will allow it to settle out.

Back to the OPs question.
Thermal store technically refers to any system for storing heat. However, in the UK two terms tend to mean different things.

Thermal store is often used to refer to a cylinder with a DHW coil inside it. The cold water supply connects to the bottom of the coil and is heated by transmission through the wall of the tube as it passes up through the store. It's entirely passive and required no electrical parts whatsoever to work. As the store is typically heated above the temperature you need for DHW, a TMV is used to control the DHW temperature - mixing in cold water as required. You are correct that these have a finite DHW capacity, but in practice the one I installed in the flat turned out to be capable of keeping the output hot with all the taps turned on full (bath, shower, kitchen sink).

Heat bank is often used to refer to a system that uses an external plate heat exchanger (PHE) with a pumped circuit that takes hot water from the top of the storage cylinder, through the primary side of the PHE, and back to the bottom of the cylinder. Obviously this is not passive, requiring an electrical supply for the pump to work. It has several advantages over a thermal store - the PHE can be of any capacity you want (100kW is easily achieved), and as there is noting inside the cylinder, should anything go wrong then it's fixable. You can also make your own using "any old cylinder" for the store and adding the external components.
One issue with heat banks is that without additional controls, you may pump a lot of hot or warm water back into the bottom of the store. Ideally you only want to put cold water into the bottom of the store so it stays stratified - that allows you to extract the maximum amount of heat from your solar panels. I have come up with a setup that I believe would avoid this, at the expanse of additional hardware (here if you're interested) - but no I haven't tried it !

Both will give you mains pressure hot water, but you do lose the benefit of a storage/backup supply in the form of your header tank. I'm not sure what this means for you as you say your water comes from a well. In theory, both types of store could work with a header tank - but would need designing to suit the lower pressures (and hence need larger pipes etc).

Scaling shouldn't be an issue, there are well established techniques for descaling systems, but it would be an item of periodic maintenance required.

 

[SimonH2]

Oops, double post