Adding a backboiler to the system

[chilled]

Guys,
during the summer I installed a solar hot water system, and the days are drawing in I'm begrudging the heat that is required to keep the hot water topped up now the sun has gone  
I have a 250l Navitron (Newark) thermal store that has blanks for back burner connection and also an open fire in the front room....

I've started looking at woodburners that will fit into a 'standard' fireplace but I'm quite concerned at the specifications.
The smallest one I've seen quotes 5Kw to the room / 12Kw to the tank.
I'm worried that 12Kw continuous into a 250 litre tank will soon give me a tank of boiling water.
The typical usage will be weekends and high days with the gas boiler / rads used for day to day.
I'll be using it to warm the water whenever we decide to have a fire rather than as a primary heat source.

Does a typical installation rely on thermal losses to reduce the total input to the tank and keep everything running smoothly?

I thought I'd do a quick calculation to give me a view,
watt hrs = temp change C * 1.16 * 250l
12000 * 1 = temp change C * 290
temp change C = 12000 / 290
41C per hour.....

I have visions of hundreds of tanks boiling away over the winter, which can't be correct!

Any pointers as to where my thoughts are heading west?

Also, the tank is up and approximately 3m over from the fireplace, is that still in the realms of 28mm convection action or should I be looking at pumped assistance?

Cheers for any insights you can give.
 

Kevin

[dhaslam]

If a stove is multifuel it is rated by  burning coal  and also test  conditions  don't match actual usage.    When you  light a boiler stove  there won't be much heat to the  tank in the first hour and subsequently  something like 50% of the rated  output is good going, even if you pack it with wood constantly.    I would   say that it would heat the cylinder something like a few degrees in the first hour, then  perhaps by 15C per hour.    After a few hours you can just moderate the fire by reducing  the amount of fuel.  The boiler thermostat will also reduce output when the water is hot enough.      The efficiency of the stove improves when the  water in the boiler is hot but heat transfer to  the tank reduces so you can  boil the water  in the  stove  but there should be plenty of warning before that happens.     

[A.L.]

hello,

I have visions of hundreds of tanks boiling away over the winter, which can't be correct!

- the calculation is essentially correct

12000 * 1 = temp change C * 290

- being pedantic the '1' is superflous/wrong as the units of the '12000' are Watt*hrs

[chilled]

Ah, so the paper figures are aspirations / test environment / ideal combustable materials etc.
I thought I was getting concerned over nothing.
dhaslam, thanks for setting my mind at rest.

So, does anyone have a view on the viability of being able to utilise convection with a 3m horizontal run - issues?
I'll be honest I'm thinking of a pumped system with a thermostat close to the stove to allow for a heat build up before moving any water to prevent 'leaching' of hot water from the tank before it all gets going.... Views?

Also, when sweeping the flue is it possible to insert the brush from inside the stove or will a seperate access method be required?
The flue is already lined and functions fine with an open fire so I plan to extend the liner down to the stove, was wondering if I need to factor in a sweeping point.

A.L.
thanks for the confirmation, I'm currently using that calculation in my arduino / .net app to guage the Kwh from delta t in the tank over an hour. So it's good to get a second opinion as to it's accuracy
Spot on with the superfluous '1' it's was my aid memoir for one hour which I neglected to remove before I pasted it.
(I've also been known to use my fingers to help count as well....)

[dhaslam]

I  don't think that a pump is necessary but it is easy  to fit one later if the  differential between boiler and store temperature is too great.    I have the opposite problem,  the water through the stove doesn't  get time to  gain much temperature  even on the lowest setting.  The pump isn't optional because of the way the loading valve is set up.   

You need to have access to clean  the chimney through the flue.  An opening on the vertical pipe makes it  easy to get the brush in.   There  is  probably access to remove the soot  from inside the stove to clean its own  surfaces but  clear access to the chimney is unlikely.     A lot of the soot will collect in the stove itself so  it will need more regular cleaning than the chimney.         

[DonL]

Hi Chilled

I have a system very similar to the one you propose and have been delighted with it. Solar hot water and a log burning stove with back boiler are a really good fit - when you haven't got the sun you want to light the log burner.

But you have to design the system with a heat dump to keep the back boiler at a reasonable temperature when the tank thermostat is satisfied or everything will boil and you have to put out the stove. My back boiler is only 2 kW rated but I still need a large radiator to dissipate the heat when the DHW is up to temperature. Even a small back boiler will heat your water in a few hours and then you need to do something with the heat. With an 11kW back boiler you'd heat your water in an hour or two and then need a fairly extensive system to dissipate the heat.

I chose to have a pumped circuit for the hot water because the route was difficult and with little height difference; and natural circulation to the heat dump radiator so I could use the wood burner in the event of a power cut. A normally open valve allows the flow to the radiator on power interruption and obviously the pump stops.

With your difference in height you should be able to design for natural circulation but I'd check the sums a bit carefully if dealing with 11kW.

Our experience of using the system is that we have used the log burner more and more and the central heating less and less and now virtually never use it. This was because we located the heat dump radiator where it gave some heat to the rest of our fairly small house so the log burner not only heats the room it is in but warms the bedrooms.

Don

[supremetwo]

My gravity is around twice your 3 metre horizontally and it works fine in 28mm.

To avoid excess water temperature, I used a second pair of outlets on the back boiler and connected those to the pumped circuit to the radiators.

It's rated 11kW water, 4kW room but never sufficient to run the water circuit to my many radiators when wood burning.   

You will need a valve in one pipe of each circuit to prevent the back boiler wasting heat from the system when the stove is not in use.

[ecogeorge]

I've started looking at woodburners that will fit into a 'standard' fireplace but I'm quite concerned at the specifications.
The smallest one I've seen quotes 5Kw to the room / 12Kw to the tank.
I'm worried that 12Kw continuous into a 250 litre tank will soon give me a tank of boiling water.

5kw to room and 12kw to water is not a small stove !!!! You're looking in the wrong place !
Most stoves will not output that amount of heat on wood.
All stoves can be modulated , invest in thermostatic air control -worth every penny  IMHO.
Better to go small and run it hot than slumber a hugh beast!
Link here to 4.6kw stove with back boiler option .--  http://www.morsofires.co.uk/stoves2.php
Mods -please pull link if against rules , was going to link to Navitron stoves but could not see any boiler options -please prove me wrong!

rgds George.

[chilled]

All,
many thanks for all the input.

I had only considered whacking the back boiler directly into the tank to heat the hot water,
I'm now (slowly) starting to think that if the heat output is approaching that of a whole house requirement
then I need to consider using the heat as a secondary input for the whole CH system
(which will mitigate the risk of an over hot tank)

I think that my plan of action is two fold,
1. look around for a less powerful stove / back boiler combo to reduce my thermal exposure.
2. consider the option of plumbing the boiler into the core central heating system which would give better control.

option 2 will also allow me to also add an interface for my solar hot water when in high summer overheat conditions
i.e. boiler off / ch pump on to remove heat from the tank. (which I hit this year before I set up my TDC3 controller better)

Again, thanks for the input guys.

Kevin

[Countrypaul]

Surely one of the benefits of using a thermal store is that it can accept heat input from multiple sources and buffer it effectively. Connecting your back boiler to the TS would then automatically reduce the need for your normal boiler to run and heat the TS. This assumes you run your CH from the TS as well as just DHW. If you wan to run it more like a batch boiler you may need a larger TS - but you would have to work out what size. That way when you run the stove you may be able to generate enough stored hot water to avoid the normal CH boiler being ued until the next evening.

Paul

[chilled]

Hi Paul,

the thermal store is a recent addition as part of my solar water installation.
It is used as a method of storing heat from the gas boiler and heating mains pressure water to the hot taps.
It is not used for any ch activities.
I don't want to necessarily make it a centralised energy store so it is focused on hot water demand only.
(which it does very well)

Kevin

[chilled]

Just a quick follow up, I've "grasped the nettle", "girded my loins" and other fine sayings.
I've placed an order for an Arada Stratford 9Kw inset stove which should be arriving next week some time.

I now need to plan the pipe runs and electrics...

I've just dug out Antmans heat dump wiring diagrams and I'm trying to understand the detail of how they work and henceforth how to modify it to add the stove into the circuit.

Wish me luck   

cheers

Kevin

[Greenbeast]

The antman HD works well but does take a little getting your head round the wiring, he's always been very helpful when i've needed some virtual assistance

[chilled]

The antman HD works well but does take a little getting your head round the wiring, he's always been very helpful when i've needed some virtual assistance

Quick update, I spoken with Antman regarding the diagrams and he was good enough to sanity check my stove modifications.
The electrical part is in and working (emulating the system in parts), so I'm good to go with the stove install next.

cheers

Kevin

[Greenbeast]

good stuff, he's a nice guy