2-Source Water Heating?

[Martin109]

I am wondering whether it might be possible to adapt a 3-way valve such as this to enable either a wood-burning stove or a passive solar heat exchanger indirectly to heat water in a tank.

Currently I have copper hot water tank with indirect coil, in which the water is heated (or used to be when it was working!) by a pumped circuit through an array of solar tubes.  The system needs renovating anyway, and I plan to install a wood-burning stove, so wonder whether there is a sensible way of switching the system between solar heating and stove heating.

One issue I can see is that the solar circuit usually employs a glycol mix, whereas I believe stove back boiler systems use water?

Any thoughts welcome if you have explored applications like this.

[SoreTween]

As you say the  different fluids are your first problem but not the only one.   A stove system relies on the heat expansion of the fluid to circulate, it cannot (in the UK) be reliant on pumping.  This means the tank must be close to above the stove.  Lots of info here:
https://www.boilerstoves.co.uk/

[pcolbeck]

Yes you can its called a heatstore based system. I have a four source heating system:

Oil
Woodburner
Thermal Solar
Immersion coil

Only have the first two connected at the moment but hope to get solar soon.

There are two tricks to doing this:

1) You heat the water indirectly with all of them that is each type of heating has a separate coil in the water tank.
2) A bloody big water tank to buffer everything (mine is 350 litres)

Basically you run your heating using the water tank water and you get hot water for washing etc using a plate heat exchanger.

Each coil is at a separate level in the tank and a low speed pump prevents stratification.
Its not the sort of thing you can molish yourself from existing parts. Well it is because its just a plate heat exchanger and a load of central heating pumps and copper pipe but you need the special big tank with facility to take the separate coils. You can buy them pre built with all the pumps etc and just get a plumber to install it or install it yourself. You need to work out hopw big a tank you need first which will depend on the size of your house and number of showers etc.

[pcolbeck]

As you say the  different fluids are your first problem but not the only one.   A stove system relies on the heat expansion of the fluid to circulate, it cannot (in the UK) be reliant on pumping.  This means the tank must be close to above the stove.  Lots of info here:
https://www.boilerstoves.co.uk/

Not true. Ours is reliant on pumping as we have concrete floors so cant do a gravity loop. The provsio is that if you are reliant on pumping you need a non electrical safety system that can cut in if the pump or electricity fails.
We have a blow off valve in the attic above the stove. If the water temperature in the pipe rises above a preset temperature it opens the valve and lets cold water through a plate heat exchanger at mains pressure. That soon cools everything down.
Not having a safety system would be madness even with an open vented system like ours as the back-boiler becomes a nice cast iron bomb if the thing kettles.

[SoreTween]

Oooh that's interesting, thank you.  I'd given up on being able to have a stove with back boiler due to the position of our tank.  Do you have any links to info about this system or the parts used?

[pcolbeck]

I have an Xcel heatbank from DPS. They went bust in the end but these guys seem to have taken over making and selling the same system:

https://www.heatweb.co.uk/

Loads of info on their website. We have had ours for eight years with no issues.

[Quisling]

Yes you can its called a heatstore based system. I have a four source heating system:

Oil
Woodburner
Thermal Solar
Immersion coil

Interesting. Not sure I fully understand. The heatstore appears to be an indirect cylinder which keeps each circuit hydraulically separate, allowing for glycol on the heat pump or solar circuit and some other water/inhibitor fluid on the boiler or wood burner circuits. So, what is the heat transfer fluid in the cylinder? Is it plain potable water? I suspect not as they talk about providing DHW via a plate heat exchanger. I’m guessing the heat transfer between the various coils could therefore also include an inhibitor for system protection? Presumably the immersion is heating the transfer medium rather than DHW directly so you could potentially (and expensively) use the system as a low power electric boiler if everything else fails.

The key thing with any mutivalent system is your control strategy and how that is managed. Ideally you want a central controller that dynamically takes inputs from each heat source and the demand side and figures out a merit order for running each pumped circuit to take advantage of free heat first and then decide whether it will run for lowest cost or lowest carbon from the other sources. I doubt standard controls are that sophisticated. In a dream world you’d have a mini BMS that estimates flow rates (from pump speed) and flow and return temperatures for each circuit to calculate approximate kWh data and help keep track of the cost of running the system.

[SoreTween]

Have a read of the brochure for the HEATBANK Xcel on the site pcolbeck linked.  Yes the solar circuit is entirely separate and yes the fluid in the main volume is water.  There are some clever controls in it such as two thermostat (section 07) to reduce thermal cycles and economy mode (section 08) so that the boiler only warms the minimum volume on days when good sun is expected.  Somewhere I read that you can link thermal store controllers to an internet weather service to automatically switch in such an economy mode.  A heatbank isn't the same as an HIU based system but the principles are there, I've not yet found schematics for a whole HIU system.

I've found a safety valve that provides the heat dump but that relies on a quench coil in the boiler is seems.  I still can't find the quench by heat exchanger pcolbeck describes.

[pcolbeck]

Interesting. Not sure I fully understand. The heatstore appears to be an indirect cylinder which keeps each circuit hydraulically separate, allowing for glycol on the heat pump or solar circuit and some other water/inhibitor fluid on the boiler or wood burner circuits. So, what is the heat transfer fluid in the cylinder? Is it plain potable water? I suspect not as they talk about providing DHW via a plate heat exchanger. I’m guessing the heat transfer between the various coils could therefore also include an inhibitor for system protection? Presumably the immersion is heating the transfer medium rather than DHW directly so you could potentially (and expensively) use the system as a low power electric boiler if everything else fails.

Its a 350 litre vented cylinder full of plain old water (well with a corrosion inhibitor added). This water is the water that is pumped round the houses radiators to provide central heating. It also goes via the back-boiler of the log burner. The input and output pipes for the log burner and the oil boiler are at different heights in the cylinder with the  oil burner one being the highest.
The immersion heater if fitted would be just like that of an electric cylinder you might have in an airing cupboard. There is a coil near the bottom the cylinder that would be connected to the glycol circuit of a solar heating panel if we had one. There are thermometers at a couple of levels in the cylinder and a pump circuit that can mix the water up to prevent stratification.

The principle is this, solar would heat the tank up a bit even in winter meaning the other fuel sources have less to do. When the tank temperature drops below a preset temperature the oil boiler kicks in and water is pumped through this to heat up. If the log burner is on then pretty soon the tank will be way past the temperature that the boiler would fire at so it never kicks in.

There is a setting for central heating or hot DHW only (basically a summer setting). The DHW only stetting basically only looks at the temperature in the top third of the tank and bothers with the anti-stratification pump. You can also set times when this all should be operational so the oil boiler doesn't kick in when you are at work or asleep.

The plate heat exchanger is for the hot water taps / shower.  Mains pressure cold water is forced through the plate heat exchanger and pulls heat from the 350 litres of water in the tank. So the shower is mains pressure (which is good) but you do need a pressure reducing valve on the normal taps or they are a bit too spirited.

[Quisling]

Perfect, that all makes sense pcolbeck

We have in theory a slightly simpler system with an automatic wood pellet boiler and solar thermal and a large unvented cylinder.  However, ours is a pressurised cylinder and the boiler primary circuit also runs at 1.5 to 2 bar which is a pain in the butt as our utility plant room contains four expansion vessels - a small one onboard the boiler, a larger one for the central heating circuit, one for the glycol solar circuit and another for the 200+ litre hot water tank.  However, this avoids an open vented expansion tank in the loft and a vent pipe right up through the house.

From experience, our solar thermal works well.  However, winter contribution to the hot water is small although it will preheat the bottom half of the cylinder a bit on a good sunny winter day.  Our immersion is half way up the tank so we can run our domestic hot water on either the immersion or the boiler, or solar only if it's sunny enough.  Our boiler has fairly sophisticated controls which manage the solar too so it checks what the panels are doing against the current DHW temperature and programme time scheduled before deciding whether the boiler needs to kick-in which aids overall system efficiency. 

I like the idea of a system than includes a wood burning stove too though for a bit of extra resilience for when the zombies come/post-Brexit (delete as applicable), although we do have a separate woodburner so when the boiler periodically conks out we can keep part of the house warmish and still have hot water from the immersion.  Just need some PV and a Tesla Powerwall now (or at least one of these https://myenergi.uk/product/eddi/ ), and we're ready for anything.

Regarding the OP (just to keep on topic a bit), the issue of differing hydraulic fluids can always be overcome by adding a suitable heat exchanger (plate, or shell and core) to provide separation.

[Martin109]

All very interesting.  Thanks for the comments - a steep learning-curve!