o/t backboilers on woodburners

[Aidan]

http://www.cmxciphexshow.com/docs/CMX_CIPHEX-wood_solar.pdf

I haven't read all this, but the first diagram shows the central heating and DHW storage cylinder arranged as secondary circuits, with the primary circuit being from either the boiler or the solar heated thermal store. I don't believe solar space heating is effective in the UK, though it may be fine in Arizona. The diagram on page 4 shows the wood fired boiler linked through a plate heat exchanger; the arrangement shown wouldn't be legal in the UK since it would stop dissipating heat in the event of a power or control failure. The gravity circulation heat dump and open vent could be added quite easily. Also all the primary flow passes through the gas-fired boiler so this assumes it has a flue damper or fanned flue, to stop it dumping heat to atmosphere if the wood fired boiler is providing the heat.

I'm don't think that a thermal store has any advantage if you are using a condensing boiler.

 

[Flyfisher]

Thanks for that info, I'll read through it. I don't think I fully understand these hybrid systems yet; I certainly don't understand the need for a separate 'thermal store' and DHW tank. Perhaps it's something to do with adding a wood burner to an existing system, but I've got a completely clean slate and can design the best system from scratch. The problem is determining the 'best system'.

 

[Jon Maine]

Thanks for that info, I'll read through it. I don't think I fully understand these hybrid systems yet; I certainly don't understand the need for a separate 'thermal store' and DHW tank. Perhaps it's something to do with adding a wood burner to an existing system, but I've got a completely clean slate and can design the best system from scratch. The problem is determining the 'best system'.

A simple explaiantion of a thermal store as I understand it.

A mass of water within a tank being heated from different sources some of which can be controlled by thermostats with preference, i.e. if the woodburner is lit, the oil or gas is off whilst the wood burner is running hot.

Also as I understand it some work by holding the hot water you tap off directly in that tank.

Others work by bringing cold water through another heat exchanger coil at one end of the tank which then leaves the tank hot at the other end.

It is in theory a brilliant way forwards if though out properly and all of the options pertinent to your situation are considered in the system design.

 

[Flyfisher]

Yes, that was my basic understanding; in practice, a 'thermal store' seems to me to be little more than a normal DHW tank with the addition of extra coils.

Heat sources (conventional boilers, solid fuel back-boilers, solar panels etc) would deliver their heat into the DHW tank through separate coils and radiators would extract heat through a separate coil. The tank itself contains the DHW used for showers, baths etc.

This all sounds pretty simple to me, yet I've not yet found any system design info that describes such a system; they all seem to use complicated-sounding devices like the Dunsley Neutraliser or 'plate heat exchangers' and I'm not knowledgeable enough to know if such things are really necessary or not.

 

[Jon Maine]

It really depends on what you want to include now or in the forseeable future as to what may be appropriate.

 

[Flyfisher]

My original idea was to build the new system around a 'big DHW tank with a number of coils and immersion heater' and connect it initially to a wood burner with a back burner. Then add radiators. Then add a gas boiler, if necessary (hopefully not). Then, if I haven't lost enthusiasm after all that, add solar panels at some later stage.

Basically, a flexible system that can start out simple but be capable of expansion over time. Well, that's the idea anyway.

 

[JohnB]

I'm out of touch with the latest systems, but several years ago when I owned half a solar hot water installation company, the thermal stores I came across held the water that was circulated round the heating system. Washing water was heated by passing it through a heat exchanger.

 

[Jon Maine]

I'm out of touch with the latest systems, but several years ago when I owned half a solar hot water installation company, the thermal stores I came across held the water that was circulated round the heating system. Washing water was heated by passing it through a heat exchanger.

That is one option I looked at, sounded good to me.

 

[FamilyWiggs]

Thanks guys for the replies and the links. It is a little more complex than I had thought - especially as I have a system that was previously connected to the back boiler before I moved in, that doesn't have a separate heat store to the immersion tank.

Do I really need separate circuits for the two boilers and a heat store? Provided that the current indirect DHW and CH system is vented and there is a heatsink radiator for the backboiler to escape to, could I just connect it to the existing boiler circuit (presumably via the manifold I already have)? I'm thinking that it should be linked from and back into the return flow back to the oil boiler, so that the oil boiler stat rarely if ever fires the boiler when the logburner is on? If I was *really* clever ( :lol: ) I'd include a switch between the pump and the boiler so that the oil boiler didn't fire whilst the logburner was coming up to temperature.

Incidentally, how much of an issue is it that the radiator circuit is less of a circuit and more of a tree with branches? The practicalities of extending the rads to downstairs and not destroying too much of the house/having very long pipe runs has meant I added two branches to a system with two branches already. Is it just a case of more careful balancing of the rads?

 

[Flyfisher]

I'm out of touch with the latest systems, but several years ago when I owned half a solar hot water installation company, the thermal stores I came across held the water that was circulated round the heating system. Washing water was heated by passing it through a heat exchanger.

I wondered about that configuration, but was concerned that the washing water might not become fully heated during its single pass through the tank coil. I thought it would matter less with the heating circuit because the water would be circulated through the tank coil many times.

But I'm only guessing.

 

[skier-hughes]

From the people I've had round this is not an issue, the ehat exchanger is sized to meet the demands put upon it. If the hot water in the tank is so hot that the heat exhanger heats up your hot water too much a thermostat tells a mixer to add cold water to enable it to come out the tap at 60' or whatever.

 

[Aidan]

You'd use a thermostatic mixing valve on the DHW to give a flow temperature of around 60 degC. The water in the coil usually heats up above 60; thermal stores are frequently described as sludge buckets by plumbers.

There used to be a boiler called the Range Powermax; I had one. The early versions had a thermal store with an integral pipe coil. Later versions had an external plate heat exchanger with an extra pump to serve the DHW plate heat exchanger, presumably because of scaling problems. The Powermax boiler was then manufactured by Potterton as a condensing boiler and the thermal store was abandoned. The integral cylinder became a hot water storage cylinder. Thermal stores will sludge up the domestic hot water heat exchanger if fed with hard water and they don't make a happy marriage with condensing boilers.

The water from the thermal store could be too hot to use in radiators and would certainly be too hot for use in UFH. You could use a thermostatic mixing valve on the central heating supply from the thermal store to mix return water to provide a reduced flow temperature. The best installation would be a motorized mixing valve to provide weather compensated heating.

 

[Flyfisher]

Sorry to be dim, but what exactly is the key feature of a 'thermal store' and why is it any different to a normal hot water cylinder?

I thought a 'thermal store' was the same as a hot water cylinder but with the addition of additional coils and, usually, a bit bigger.

 

[FamilyWiggs]

FF - if I have understood properly, a heatstore is used to...err... store the heat from the boiler. This can later be used to heat your DHW or your CH via a heat exchanger )ie the heatstore water remains within the system. A normal hot water cylinder stores the hot water that will subsequently be discharged from your taps and replaced in the tank wirth fresh cold water.

Quite why you'd want to store the heat and then exchange it, rather than store your hot water for later use is beyond me!

 

[MdB]

Three reasons I can think of,

1) Efficiency. It is generally more efficient to have a boiler running a full blast for a good length of time rather than on / off cycling and / or operating at below maximum output.

2) Time shifting. They sometimes get mentioned with heat pumps so that you can store the heat generated at cheap rate electric for use when you want it. Having said that, the local heat pump people don't recommend using them.

3) Temperature differentials. If you want to heat your hot water from solar and it only goes to 40 deg C then it isn't much use for your hot water tank which maybe higher than this already. If however, the solar heats a low temp heat store, this can then be used to preheat the water going into the hot water tank so less is required to get it to temperature.

I'm probably wrong on most of that....

 

[Flyfisher]

Interesting points, particularly the solar heating one; I hadn't considered the heat differential issue.

OK, so ignoring solar heating, I'm probably not really thinking of a thermal store at all, just a hot tank with multiple coils. Stove/boiler heats water tank through one coil, gas boiler also heats water tank through a separate coil, central heating is driven from another separate coil and the tank water itself provides DHW.

I'm sure there'll be a good reason why this is too simple. :wink:

 

[skier-hughes]

For us with no boiler to auto turn on in the morning, we get no heat until we light a fire. With a 1500l heat store, all that lovely warmth from the fire the night before will be stored ready to give us central heating in the morning before we get up and light a fire.

 

[JohnB]

The solar coil goes at the bottom of the hot water cylinder or thermal store. If it doesn't get the water up to temperature, the boiler coil or immersion heater higher up the cylinder adds the extra heat. There's some complicated layering that goes on in the tank that's too difficult for my simple brain to take in!

 

[briany]

I'm in the throes of a similar dilemma in my house. It being a fairly prolonged work in progress due to my day job taking me to far-flung and "interesting" parts of the world, I've had plenty of time to plan, internet-search and draw lots of complex diagrams as to how best integrate two (or more) gravity systems into the central heating.
One of the biggest hurdles I found was to eliminate this 'nuisance' heating, i.e. when only one element in the system is on, how to prevent the other element(s) from turning into big radiators.
The solution I cobbled together (and I want to 'fess up right away, it's been mostly installed, but not yet tested...) was to combine several (four, to be exact) 2-port motorised valves with the central heating pump plus thermostats to activate only the circuit that happens to be heating at any given time.
A picture being worth a thousand words, see attached.

Basically, I use a twin coil copper cylinder (these can be made up with any number of coils by the manufacturers, within limits) as my heat store and DHW source. My woodburner and solid fuel Rayburn are on separate gravity circuits through each of these coils. When either of the appliances is fired up, hot water circulates around the loop, heating the cylinder water indirectly, no problems there!

The central heating loop is connected to both appliance loops, though when cold, is isolated by means of the motorised valves. This prevents water circulating around the central heating loop until I tell it to.

On a given appliance loop, a thermostat, set to an appropriate temperature, turns on (i.e. opens) two motorised valves that allow access to the central heating loop when the appliance is heating water. As the central heating pump is in the line, water cannot yet start to circulate (until the pump comes on).

Once the water in the cylinder reaches an appropriate temperature, a second thermostat switches on the central heating pump, which then diverts water from the appliance and drives it around the rads. Hey presto, warm house!

The other appliance loop, when cold, remains isolated as the two motorised valves on that loop remain off (i.e. closed). This prevents hot water from being driven through the cold appliance. In theory.

I hope this makes sense, and if anyone with more nous in this area than I can let me know in advance if I'm going to blow my house up if this thing is switched on, I'd be ever so grateful.
The diagram should illustrate it somewhat better.
N.B. I've omitted heat leak rads, the open vent to the expansion tank, the cold feed and the DHW feed for declutterification.
Brian

 

[MdB]

What have you got in your system to stop it turning into a bomb? Assume that the power fails and all the valves are in the worst possible situation and your heat sources are at max temperature. I am sure that you have thought of this and there are all sorts of safety valves built into the boilers but it is worth pointing out.