Mains pressure thermal store advantages ?

[robkgraham]

I've got a low hot water pressure problem at the kitchen tap due to a low head and SWMBO's request for a mixer tap on the end of a hose (I recommend these by the way but be aware of the high insertion loss). The question of a pump has been discussed in a recent thread "Hot Water Pump" and one of the options is a basically a CH pump from Grundfos which is designed to address this problem. I have my concerns about this pump as it's flow switch and head requirements will only just be met by my set up, plus physically plumbing it in will not be easy due to lack of space under the kitchen sink.

By chance I stumbled on this website - think someone had posted it in this group :-

is a mains pressure thermal store system where the mains is heated by a heat exchanger inside the tank. In terms of plumbing effort and cost this is possibly times 10 in both but what other advantages would I get - it would allow me to get rid of the Triton electric shower and fit a decent pressure one as well as sort out the kitchen hot water flow rate I know, but what are the other advantages? - and disadvantages !

Rob

[tarquinlinbin]

Advantages-bags of hot water at mains pressure whenever you need it. No more struggling with a combi

Disadvantages, it takes up space,needs to be fitted by certified tradesman.

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[Mr Fizzion]

Since when?

Mr F.

[Christian McArdle]

I'd go for a model that uses an external plate heat exchanger. Some are more sophisticated than others.

You can only get the advantage of these systems if you have good mains flow and pressure.

I have a DPS Pandora. It is really good and works entirely as advertised (unlike their clunky website).

[robkgraham]

What's the advantage of an external heat exchanger ?

This is not a pressurised tank - it has a heat exchanger inside the tank it therefore doesn't need a 'certified' fitter.

Space is gained as it's not that much larger than my dhw tank and I get rid of the cold water tank.

I didn't realise that combis were a problem - I'm running off a log burning stove and an oil burner.

Rob

[Christian McArdle]

Faster heat exchange and lower pressure drop leads to higher potential flow rates compared with an internal coil exchanger. External nature of heat exchanger allows descaling and replacement.

The disadvantage is the requirement for a pump, and, hence, an electrical supply. They should also be more expensive.

Christian.

[Antony]

I have an Albion Mainsflow installed in my loft. This type has the coil inside the cylinder (the 22mm pipe splits into four 10mm tubes to increase the surface area). I have a proper ion-exchange water softener, so scaling isn't a concern for me, but Christian is quite right that having an external plate heat exchanger allows for easier descaling when necessary. The external models are much more expensive (several hundred) but marginally more efficient.

Showering is fantastic (Mira Excel), and having potable hot/cold mains pressure water at all taps is great (with aerators fitted in the bathroom). You can remove the 50 gallon water tank from your loft (you may need a small feed/expansion tank depending on design). My unit (140litre direct cased) cost =A3600 from

The roughly equivalent model from DPS was about twice the price.

-Antony

[Christian McArdle]

Yes, it does seem that DPS have put their prices up. I paid 750 for my Pandora, which was a 180L indirect.

Christian.

[robkgraham]

Interesting - both you and Anthony have got smaller tanks than I was advised; it was suggested that I should have a 210 litre with 180l as an option but 140l is a long way smaller. The 210 was so that we could have 2 baths of it but that is a very unlikely scenario with effectively only 2 of us in the house now. Is 140l big enough ?

I'm still not getting good vibes on why I should go the mains pressure way rather than solving my low hot water pressure problem with a pump. I fancy the mains pressure route but the effort and cost do mitigate against it.

Rob

[robkgraham]

Sad person replying to his own mails but a further thought came overnight - if the heat exchanger is external why do you need a specific tank; will the standard dhw tank not act as the store if the feed plumbing is appropriately modified ?

Rob

[Christian McArdle]

A heatbank is little more than a standard HWC modified with the addition of a few flanges, a pump, a flow switch, a heat exchanger and a thermostatic mixing valve. They are certainly DIYable with skill.

The Pandora adds a proprietary expansion system that prevents water evaporation and so does not need a header tank, float valve and overflow. These items are not essential. Some systems use a variable speed pump instead of a TMV. This provides better stratification performance.

Christian.

[Antony]

If you want to run 2 baths at the same time, you need a bigger cylinder. My unit recovers very quickly (with an 18kW boiler), so there's no problem running one bath after another. 140litres is enough for 1 bath + 1 shower in parallel. If your requirements are greater than this, use a bigger unit. Also, if you have mutliple heat sources (e.g. solar, Aga) you would want to go for a larger store.

rather than solving my low hot water pressure problem with a pump.

I was replacing my entire DHW/CH system, so it made sense to install a thermal store. If you're happy with all the other aspects of your system, then just adding a pump may be more sensible as going mains pressure may involve major changes to pipework (going from 15 to 22mm), replacing taps (some can't handle high pressure), etc. Bear in mind that quality high-pressure shower pumps aren't cheap.

-Antony

[Christian McArdle]

To give you some idea of the performance of my 180L attached to a 28kW boiler, it managed to fill a 750 litre birthing pool (approximately 6 full bathloads) to 40C in 41 minutes.

The advantages of a heatbank over pumped gravity feed:

1. More reliable (pumps break down).
2. Cylinder lasts longer (is filled with corrosion inhibitor and doesn't get new supplies of fresh hard water every day).
3. Silent operation.
4. Doesn't require large cold tank in loft.
5. Can supply outlets above cold tank level without fiddly pull switches or expensive and unreliable pressure switches. (Very useful in loft conversions and flats).
6. All taps, including the hot taps, are potable drinking water.
7. Can have multiple primary heating sources, so you can add solid fuel back boilers and solar heating panels, as well as your gas boiler and electrical immersion.
8. Can run the radiators off it, too, particularly when (7) above is implemented, allowing your rads to be heated from many alternative sources depending on economy and availability of energy.

Disadvantages:

1. Needs good pressure and flow from the mains.
2. More expensive to install.
3. No short term backup in the event of mains water failure.

Christian.

[Andy Hall]

You could do this. The water in the cylinder must be fed from an open vented header tank, not pressurised. However, said tank could be a small CH type. Some thermal store integrated products have this built in.

The store needs to be maintained at 80 degrees plus if you want to have the benefit of being able to use a smaller cylinder than would be implied for the volume of usable water at the tap. A side implication is that although plate heat exchangers have a very good transfer rate, (e.g. 100-200kW), if the water in the store is only at DHW temperature (60 degrees), you would not be able to reach this at the taps. An implication of storing water in the cylinder at 80 degrees is that heat losses increase by a third as compared with a conventional arrangement. Although these are not large, it may be prudent to order a cylinder with more insulation.

It would be a good idea to use a cylinder with fast recovery heating coil to the boiler. This means that the boiler can start to deliver all of its output to the cylinder soon after heat starts to be removed from it. The result is of a longer run time before the store is depleted and one is down to the rate that the boiler can deliver. Of course this only matters if the volume and rate of use exceeds the capacity of the cylinder.

[Antony]

appropriately modified ?

It depends on your setup. My Albion Mainsflow has a built in feed/expansion tank (the whole store is in fact a feed/expansion tank...). As the store is direct, it also acts the F&E for the primary boiler/CH circuits. Other stores work differently, where the boiler heats the store indirectly, you might need two F&E tanks, or the boiler/CH might be sealed. There's a lot of variation in design.

-Antony