How much water pressure for a thermal store?

Spinball · May 1, 2013

Perhaps this should be directed to a plumbers forum, but I've had good advice here previously.
We have an 8x4m swimming pool, two old 27kw boilers, 35 radiators and 3 bathrooms.
We have a south facing roof big enough for about 4kw PV and some solar thermal panels also.
We have several large cold water storage tanks in the loft and a 200 litre hot water tank in a bedroom cupboard near the loft.
I'm quoting here an email from a green energy installer advising me not to have a thermal store:

Thermal stores just hold water – the water does not travel anywhere – it simply heats up and cools down. And they do cool down because of heat losses over the entire surface of the store. Effectively you have to keep the store at a very high temperature typically 85°C at the top, all the time, in order to heat the cold mains water passing through an indirect coil in the top of the cylinder and provide hot water at the taps. As the cold mains water passes through the thermal store it takes heat from the stored water which has to be heated up again very quickly to 85°C in order to continually supply water hot enough at the taps. If it is not at this temperature the water at the taps is cooler.

In effect a thermal store operates like a combi boiler in that all the hot water for the house is delivered from 1 cold mains pipe which is also providing all the cold water to the whole house. As a result of which, if for example 2 showers are being used at the same time as a couple of taps the flow of water will reduce. This is one of the reasons in large houses it is not recommended to install a thermal store.

Also pressurising just the hot water supply can cause huge problems with mixer taps and showers due to the different pressures – tank fed cold water and mains pressure hot. The mixer taps and showers may all need to be changed or a new cold mains supply run to all mixer taps, showers etc

With an ‘open vented’ hot water cylinder it can be designed, in accordance with good practice, to have direct connections to showers therefore hot water is not being drawn from just one point, and of course your large cold water storage tanks provide an ample supply of cold water to the cylinder independent of the incoming mains supply which then has time to top up the water level in the cold water storage tanks.

As far a solar thermal system is concerned the high temperature at the base of the thermal store, typically 45°C, significantly limits the performance of the solar system in that under these circumstances the collectors must be at least 49° - 50°C before the solar system will even switch on and begin transferring heat from the panels to the bottom of the cylinder. In a conventional cylinder the incoming cold feed temperature in winter for example may be just 5°- 8°C so the panels need only be 9° - 12°C plus for heat to be transferred to the cylinder. This results in many fewer hours of solar system operation – I believe I mentioned we have examined thermal stores in which the solar system has only operated for 500 hours in a year as opposed to 2000-2500 hours per year.

Can anyone please comment on the above to confirm or deny the thinking?
Many thanks.

SPARTYKUS · May 1, 2013

You might as well be speaking mongolian to me, sorry!

Gavin A · May 1, 2013

can't really fault any of that tbh, it looks like good sensible advice from someone who knows their stuff.

IMO thermal stores are probably best used to combine multiple heat sources, such as solar and biomass, where a lot of storage is used to store 2-3 days hot water heat in of go - so probably 500l minimum, ideally more.

A standard thermal domestic thermal store won't cut it for more than 2 showers, and the installer could well be right about pressure drop with 3 mains showers at once - and you do definitely need both hot and cold taps at the same pressure (ish) for them to work properly, so if you're on a vented system now, then you'd need to replumb the bathrooms entirely to get to that situation.

mains pressure will also use a lot more how water per shower - probably 8-10l a minue vs 2-4l a minute for vented, so 2-3 times the cost per shower.

Billythekid · May 1, 2013

Very difficult to comment, as they have seen your install.

Are your showers currently gravity fed, electric, pumped?

The imbalance of pressures wreaking havoc on showers and mixers is true.

Worcester · May 1, 2013

Thermal stores just hold water – the water does not travel anywhere – it simply heats up and cools down. And they do cool down because of heat losses over the entire surface of the store. Effectively you have to keep the store at a very high temperature typically 85°C at the top, all the time, in order to heat the cold mains water passing through an indirect coil in the top of the cylinder and provide hot water at the taps. As the cold mains water passes through the thermal store it takes heat from the stored water which has to be heated up again very quickly to 85°C in order to continually supply water hot enough at the taps. If it is not at this temperature the water at the taps is cooler.

First part true, second part depends upon good design, and the size of the store.

In effect a thermal store operates like a combi boiler in that all the hot water for the house is delivered from 1 cold mains pipe which is also providing all the cold water to the whole house.

True

As a result of which, if for example 2 showers are being used at the same time as a couple of taps the flow of water will reduce.

And how often does that actually happen? Washing machines, dishwasher etc only take a small amount of water these days.

Taking two showers at once will also have an effect on a gravity fed system, even if multiple take offs (i.e. that means replumbing your hot water) are now added in. - It was common with power showers (before mains pressure hot water) to add in a direct take off from hot water tanks as with loft based cold water tanks you could end up drawing air down the vent tube.

This is one of the reasons in large houses it is not recommended to install a thermal store.

Never heard that said before.

Also pressurising just the hot water supply can cause huge problems with mixer taps and showers due to the different pressures – tank fed cold water and mains pressure hot.

True, however it is very simple when using mains presuure hot water to convert the cold over to mains pressure, - just bypass the tank.

The mixer taps and showers may all need to be changed or a new cold mains supply run to all mixer taps, showers etc

Problem goes away as above

With an ‘open vented’ hot water cylinder it can be designed, in accordance with good practice, to have direct connections to showers therefore hot water is not being drawn from just one point, and of course your large cold water storage tanks provide an ample supply of cold water to the cylinder independent of the incoming mains supply which then has time to top up the water level in the cold water storage tanks.

Supplied at only the pressure of the current feed.

As far a solar thermal system is concerned the high temperature at the base of the thermal store, typically 45°C, significantly limits the performance of the solar system in that under these circumstances the collectors must be at least 49° - 50°C before the solar system will even switch on and begin transferring heat from the panels to the bottom of the cylinder. In a conventional cylinder the incoming cold feed temperature in winter for example may be just 5°- 8°C so the panels need only be 9° - 12°C plus for heat to be transferred to the cylinder. This results in many fewer hours of solar system operation – I believe I mentioned we have examined thermal stores in which the solar system has only operated for 500 hours in a year as opposed to 2000-2500 hours per year.

True with any badly designed, badly comissioned or poorly installed system. If the above was true across the board, then nobody would consider or install solar thermal. Yet it is one of the oldest, proven technologies. The Δt figures are critical in designing and configuring stores. Thats why some stores make use of two solar coils.

[ElectriciansForums.net] How much water pressure for a thermal store?

Worcester · May 1, 2013

As GavinA says thermal stores come into their own when combining multipleheat sources such as solar thermal, excess PV (immerSun), existing boilers etc, and if you are looking to heat the pool, it is diffcult to beat the cost effectiveness of solar thermal.

An alternative to a thermal store would be to use a normal solar thermal cylinder, however the majority of those are unvented (I.e.mains pressure hot water). It is possible (though not so common these days) to use vented hot water tanks (as you have present) with solar thermal.

The challenge is the lengths of the pipe runs from the thermal to the hot water tank - these need to be as short as possible else the losses would mean that the temperature diffenrential (Δt - Between the output of the panels and the bottom of the tank) would be lost so the solar thermal would be innefective, anything over 15m and the losses tend to be to high.

You may find these of use to you:
http://www.energysavingtrust.org.uk...nology/Thermal-stores/How-thermal-stores-work
"A thermal store overcomes a problem highlighted by the Energy Saving Trust solar thermal field trial where, although solar thermal heat was available, it had nowhere to go as the boiler had already fully heated up the hot water cylinder in the morning. This is especially true in spring and autumn, when it is sunny but the air is cold. "
and
http://www.energysavingtrust.org.uk...-a-field-trial-of-solar-water-heating-systems

BruceB · May 3, 2013

Spinball, Whilst the points made in the email you quote are all correct in a certain context, they neglect to mention that the problems can easily be designed out and then you have the advantages of a thermal store which are many, many of which are covered above. Given a free choice I would normally install a thermal store rather than a 'standard' hot water tank. To mention a few: combine many heat sources, reduce boiler cycling, mains pressure hot water the quantity of which is limited only by number and size of heat exchangers and mains pressure/flow rate (which can be augmented if required) and no legionella issues.

Picking up the point above ".....it had nowhere to go as the boiler had already fully heated up the hot water cylinder in the morning..." there is a way around this which is covered in the Gledhill Torrent GreenHeat SOL installation manual as an example, which you can get to from this page:
Torrent Solar Thermal Stores | Gledhill Solar Thermal Stores
When the conditions are right for solar input then only allow the back up boiler to heat the top half of the solar tank.

Spinball · May 3, 2013

Thanks, all. This is such a complex subject that I want to entrust the job to someone and let them get on with it. The problem is when there are conflicting approaches as to the best solution and I don't know which is best.
And this isn't something I can afford to get wrong. When there is a significant financial outlay, I don't want to make that investment and then have the wife complain to me that the showers keep running hot and cold or that the hot water is only luke warm. Never mind not bringing the energy bill down to a tiny fraction of its current £500 a month figure.

The Solar King · May 3, 2013

Ref, use of solar thermal. Worcester is right. One thing to watch with a twin solar coil tank such as the Akvatherm (which his pictures relate to) is what kind of bypass/mixing valve is fitted and that the correct Resol controller is specified. Best to use a solar rated power open, power closed electric valve rather then a simple thermostatic unit with an insert. This can be configured automatically to supply heat to both top and bottom coil, or bottom coil only depending on collector temperature and tank temperature.

You really do need to find someone who knows exactly what they are doing with these things. I spent a great deal of time rectifying one of these a builder who thought he knew everything had installed. They are not straight forward but work well as Gavin says with multiple fuel sources.

How much water pressure for a thermal store?

Spinball

SPARTYKUS

Gavin A

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Billythekid

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Worcester

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Worcester

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BruceB

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Spinball

The Solar King

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