1) can be solved by installing a well insulated "secondary loop" like thay have in hotels.

2) How about fitting an electric boiler in the primary side of the store eg between the biomass boiler and the thermal store. Most of the time the water will flow straight through it, but you could modify the existing stat on the store to turn it and the pump on when the biomass boiler is off.

http://www.heatandplumb.com/acatalog/Heatrae_Sadia_Amptec_Electric_Boilers.html

The main problem is that it would take awhile to heat up 1000L from stone cold - but presumably you can put it on a time clock to check the temperature at say 5am and have it topped up by 7am or something like that.

It's also possible your plumber wasn't approved to work on mains pressure which is why he said it wasn't possible. Mains pressure tanks do exist.

Normal copper tanks will not cope with mains pressure, never mind the expansion pressure created by heating water. A mains pressure imersion tank is needed but even this won't cope with the incease in pressure due to heating expansion. This problem is solved here by fitting a pressure relief valve to the input of the tank, this valve then dribbles water as the water heats up allowing excess pressure to escape. It helps to pipe this valve to a drain! The valve also has a non return function to prevent heated water being pushed back up the mains pipe. (Keeps the water authority happy) You could ignore the whole expansion issue if you connect the tank directly to the mains and so allow the mains to take up the expansion but if the mains was ever turnd off and the heater operated then over pressure would result and in any case the water authority would not be happy.

In your situaion I would super insulate the pipe from the thermal store (and use plastic pipe as I have found this to be less of a heat sink than copper or iron pipes), feed this to the super insulated hot water tank and run the DHW from the tank. The immersion heater in the tank would be set at a lowish temperature to provide adaquate DHW and powered by off peak electricity with the water pre heated by the thermal store.

Peter

A plate heat exchanger on the CH circuit seems to me an expensive solution to providing temp controlled CH water. Why not use the PHx for your DHW, ( store one side and rising mains on the other ), and put the CH on a direct circuit from the store via a ACASO type control valve.

Barney wrote
Be slightly careful of adding more (or thicker) insulation to a pipe - you can easily reach a point where the increased surface area actually loses more heat than it saves.

I have a bit of trouble getting my head around more insulation on a pipe could cause greater heat loss. I can easily accept the point of dimminshing returns, but to add insulation and thereby increase heat loss I find difficult to see. If this is the case can someone explain / detail the breakpoint with various types of insulation.

Peter

How about fitting an immersion heater element to the existing thermal store? Perhaps it already has one or a blanked off hole to fit one?

Perhaps my comment was a bit vague - I was suggesting that the "amount" of insulation is a balancing act - any insulation adds thermal resistance to the flow of heat from a pipe but also adds surface area, if you keep adding insulation you will find a point where the increased resistance "wins" over the increased surface area - if you are in a zone prior to the critical thickness being reached then here increased insulation thickness will add to heat loss rather than diminish it.

It's fairly easy to determine this critical point for any insulation given its U value and knowledge of the pipe temperature/diameter

Regards

Barney

Many thanks for the range of ideas, which I have spent some time reading. Physics is not my strong point. There is definitely a design flaw somewhere in the system, and with hindsight I should have had everything professionally designed first, rather than relying on the installer to suggest the arrangement.

I'm not sure how a secondary loop for DHW would work unless a separate hot water tank was installed or some sort of non-return valve was fitted to the mains supply. We are in quite a high water pressure area.

Increasing the insulation of the pipe run seems to be one priority. The 25mm metal pipes have a thin insulation sleeve (approx 5mm) and were then laid in a slightly larger plastic pipe. They are buried about a metre down, but in frosty weather their route can be traced by the line of grass which remains unfrozen.

I had thought about some sort of in line electric heater, similar to an electric shower. However, the temperature control on our shower works by altering the water flow rate rather than electricity consumption. Would it be safe to run an inline heater if the water coming in was already at 50 degrees and the flow rate was kept low by the hot tap being only partially open?

It looks like the two options are to fit an immersion heater to the thermal store, controlled by thermostat and timer and with better insulated pipes, or the expansion vessel with immersion. This would have the advantage of leaving the CH to be solely heated by the biomass.

Once again, thanks for the ideas. It all suddenly looks much clearer.

This is, in theory, very simple. Swallow the fact that a design faux pas has occurred, and rather than trying to engineer around it, which will always be a compromise, move forward. Forget secondary loops at this distance, with their energy-sapping heat losses and parasitic pump losses. My solution would be to Install a second, much smaller slave thermal store with built-in DHW exchanger (mains) and TMV near the point of use.

If I read you right, you already have heating primaries running between the TS and the house. Tap into these in good old 'S' Plan fashion within the building, and re-tap into the DHW loop already running to drawoffs, but just 30m nearer the drawoff point.

Drawbacks: Yes, you will need a bit of space for an airing cupboard, and there is plumbing and wiring to do. Advantages: An Airing cupboard! No lengthy dead legs, ergo quicker hot water service. Removal of white elephant DHWS, no added electricity to consume. I could go on for days, and have frequently been known to, but just suffice to say that heating a slave tank in this way requires just one dose of heat transfer, (til the next time the DHW needs heating) so the heat loss in the pipes is minimised - and in winter, these same pipes are hot much of the time anyway feeding the heating system. Not quite sure what you mean by a heat exchanger for the central heating though?

Your caveat would be how much disruption to the existing arrangement and (finished?) property this will cause. Very annoying at this stage of the game though! If you paid for this work as opposed to DIY, I would suggest significant redress, as somebody should have known much better, or asked.

I have to ask you, Davebrigg, are the CH pipes insulated to the same standard as the DHW pipes because if they are you will be heating the clouds and you should have a strong chat with your installer and then get all the underground pipes properly sorted
Peter

Thanks again for your comments. I'll try to clarify the current position, and ask for an idiot's guide to some of the advice already given.
The DHW comes from a mains water supply (standard blue 25mm pipe) that enters the thermal store and presumably moves around an internal coil to emerge much hotter from a 22mm flexible metal pipe. It then goes under the garden to a shed on the side of the house. At this point the pipe splits to take the hot water to the kitchen and two bathrooms.
The water for the central heating comes from the same thermal store, underground to the house (with the same cr*p insulation). It enters a plate heat exchanger, then returns to the thermal store. On the other side of the heat exchanger is the central heating circuit. One explanation for using a heat exchanger could be that the thermal store and central heating were fitted by different people. It may also be to keep the Fernox, and whatever sediment appears in the radiators, away from the thermal store. The thermal store is an Akvaterm 1000l tank, which seems to be working well.
I now understand how a continuous flow loop could be used for DHW. Aside from the cost of running a pump, there is the added cost of a divorce lawyer after I explain that the few bits of our house that are looking reasonable will have to be pulled apart to fit the extra pipework. This is a three month renovation that has already lasted nearly four years, so anything that involves more dust and chaos will have to wait for the moment. An immersion element for emergencies (or if we go away at Christmas) sounds like a good idea.

I need to ask Owlman, what does a ACASO motorised valve do? Would it replace the plate heat exchanger?

Also, for Crusoe "My solution would be to Install a second, much smaller slave thermal store with built-in DHW exchanger (mains) and TMV near the point of use.

If I read you right, you already have heating primaries running between the TS and the house. Tap into these in good old 'S' Plan fashion within the building, and re-tap into the DHW loop already running to drawoffs, but just 30m nearer the drawoff point."
An airing cupboard is no problem - there is already an empty one in the bathroom from an earlier oil-fired Aga system. There is not a DHW loop though. I would need to provide a return loop and pump so that a steady flow from the thermal store could heat the tank. Presumably the tank would have a feed from the existing cold water supply.
All this is possible, especially if I going to dig up the garden to improve the pipe insulation anyway, but this time I want to be clear that I understand the system before starting.

As for the installer... I can't say too much as they also participate in online forums. There were problems we found at the time that nearly resulted in legal actions, so the less contact we have with them the better, even if it means more work for me. I should have been more careful when he arrived on a horse, carrying a lassoo and six-guns.

Posted By: DavebriggI now understand how a continuous flow loop could be used for DHW. Aside from the cost of running a pump..

That's modest as the pump needed is very small. It just has to circulate enough water to compensate for heat losses in the pipe work. It's not having to boost pressure or supply the flow rate needed by a shower or anything like that. If you really wanted to get fancy you could put a pipe stat on the furthest tap and use that to control the loop pump - so the loop pump only runs when the temperature at the remote tap falls below a set limit.

there is the added cost of a divorce lawyer after I explain that the few bits of our house that are looking reasonable will have to be pulled apart to fit the extra pipework.

One possibility would be to connect the return path to flow as near as possible to the taps without incuring a divorce. For example you could connect the return to the flow in the shed. That way at least part of the run is kept hot. I'd certainly put in a return pipe when you dig up the garden to insulate the existing flow pipe even if you don't use it yet.

Ok. Firstly to clarify: Yes, 1000 litre thermal store is probably too small. We have an old, listed property with a lot of poorly insulated rooms. I have done what I can within the listing rules but running the heating for an hour brings the TS temp from 80 to 40 degrees unless the boiler is lit. I know a second tank could be added, but just not at the moment.

The PHx does not regulate the temperature of the rads. Getting rid of it would be a relief; it slowly blocked on the CH side after being fitted (undetected leak in the CH system drained all the inhibitor) and spent last summer being soaked and flushed with Fernox.

I Googled Acaso Ct and found an earlier post from this site by Owlman explaining its use to improve layering in the thermal store. That's another can of worms, since the current system has nothing to prevent mixing, and it doesn't take long for the tank to reach an average medium temp throughout.

I have checked the S Plan and found diagrams. The only fly in the ointment might be that the boiler works manually i.e. I go out and light it. (We gave up using wood pellets for several reasons. On the plus side, our fuel is now free and diverted from a journey to landfill.) Therefore, if the TS temp falls, the slave tank can call for hot water all it likes and run the pump all day for no result. This could be overcome if an electric immersion was used in the TS for the winter mornings when all the heat has already gone to the rads. Expensive, as has been pointed out. Another advantage would be that a single flow and return would mean that I could use the existing two ducts under the garden, each large enough to take a single 22mm pipe with better insulation. I have looked at Armaflex insulation, but will see if there is anything else better suited.

The biggest problem seems to be the lack of qualified and experienced installers. Four years ago I could only find one person within 50 miles offering to fit a biomass boiler, but he clearly had little knowledge of what to do once the heat had been produced. I recently found one 'expert' willing to look at our boiler but travel costs and fees put the cost at nearly £500 before any work was done. I don't regret going for something greener, but if I had chosen oil there is someone a 30 second drive away who could fit and service a boiler, relying on designs that have had years to reach their peak.

Again, thanks for all the advice. It's a steep learning curve.

Shouldn't be any flies and ointment DB, as the micro-switch on the 'H' suffix (5 wire) MVs can fire both boiler and pump (normal), or, unsurprisingly if you cannot automate your stoking schedule, , the pump alone. Take the switched live from the valve (via timer or preferably Prog Rm Stat) to a break-on-fall immersion stat (note: not immersion heater stat!) in the TS, which will preclude any chance of the slave store reverse-tranferring heat.

A couple of other points: You should clearly consider 1. Insulation and 2. Zoning in your property, either or both of which will make the 1000 litres go further. It's not just about producing heat ecologically, but coaxing that heat you have produced to go as far as possible. Mixing flow temps to rads/UFH helps too, as noted.

As I repeatedly point out to clients, manual biomass needs a degree in stoking. Can you add a heatpump on off-peak into the mix? Or A N Other automatic source, to help when time is tight or wood supply less than ideal?

There is no reason why the slave TS has to be solely on electric back-up. In the recent mild/sunny weather, a client's 1400 litre overall storage (4 tanks) was at 60C from solar alone. Big advantage of a well-specced TS is that you can add other energy sources.