Wood burnin stove - heat haze

doningtonphil

Hello


We have installed a Hamlet 12kw boiler stove (output to water 10kw with wood)with twin wall stainless flue pipe (6 inch internal diameter). We live in a bungalow with a low ridge height. The chimney terminates at just over 4.2 metres (and is above the ridge).


We seem to have to burn the wood with the vents all open a lot of the time to get any heat going into the water thermal store and don't seem to get much heat into the room (nominally should be 2-6kw) and we often struggle to get the temperature in the thermal store above 60 degrees c.


When i look at the chimney, there is not much smoke but quite a heat haze, more so than neighbouring cottages.


Is too much heat being sucked up the chimney?


Are we doing something wrong?


Regards


Phil

Robwiz

No smoke is a good sign. It means you're burning wood efficiently and probably losing the minimum possible amount of heat up your chimney.

To produce 12kW will take a lot of wood burning pretty furiously - something like 4 kg of logs loaded in the stove per hour.

I use Verdo briquettes because they are guaranteed under 10% moisture and heat output of 4.8kW/kg. I know that each briquette will give 8kW of heat (my stove is 8kW) so I add one briquette and adjust the air supply so that the briquette burns away in an hour. You could do the same with one and a half briquettes (a 10kg pack of 6 is £2.49 from Home Bargains) to give you an idea of how hard your stove needs to burn.

How long it takes to get your thermal store temperature up depends on how much water it contains. But 10kWh is a fairly low rate of heat input.

doningtonphil

Thanks Robwiz


2.49 a pack from Homebargains. The best I had go was 3.09. Are they always in stock or 'get them while you can' like the UK Heatlogs from Aldi and the like?


Is it the case that 1kg of heatlog will give out 4.8kw of energy regardless of how it is burned (e.g. burned hard with lots of air would give out 4.8kw in say 30 mins but burned with less air will give out the same energy but over longer period) or are there more and less efficient ways of burning them?


Do yours always have a flame or do they glow?


If you enjoy the maths, the tank is 140 litres. The starting temp is 20 degrees c. How many kw would be required to heat that volume to 60 degrees c?


Regards


Phil

matelodave

It takes 1 kCal to raise 1kg (~1 litre) of water by 1 degree C.


1000 kCal = 1.163 KWh

For a 140 litre tank
140 x (60-20) = 140x40 = 5,600 kCal = 6.5128kWh assuming that you appliance is 100% efficient. Only an immersion heater is 100% efficient as it's actually in the water, although there will be heat loss from the tank unless it's very well insulated.

You would need to know how much heat goes up the chimney and how much is used to heat the room to see how much was available to heat your water and you would also have to take into account heat loss from the pipework between the boiler & the thermal store/hot water tank.

Burning a 1kg heat log that has 4.8kwh.kg will produce 4.8kwh however it gets burnt because that's the energy contained in it. I've found that if I burn stuff too quickly (ie have all the vents wide open) then a lot of heat seems to go straight up the flue rather than heating the room. My stove sounds like a gas fire when it's going full pelt so to get maximum benefit mine works best with the air supply turned down a bit so the flames aren't roaring. Have you got a flue thermometer - they do help you optimise the airflow so that it's not too hot nor too cool.

Robwiz

They appear to be available all the time, but not always in-store. I buy online for collection from the store, which takes a couple of days. The price is the same and that way there's no chance of a wasted journey. I buy 240kg a time which is a reasonable load in the back of the car and doesn't take up too much space in the garage.

They need to be burning properly to give out the full energy - just smouldering isn't efficient. With lots of air they may burn faster but you'd get the 4.8kw/kg. They burn with a nice yellow/orange flame. They need a good roaring kindling fire to get them alight but once they're alight and you have a decent bed of glowing embers they are easy to keep stoking.

I don't particularly enjoy the maths, but looking it up online the answer to your last question is 6.5kWh.

Chimney_Sweep_3

what make is the stove and is your wood nice and dry ? they are 2 of the main points, the flue is a little on the short side how much does it clear the ridge by, if its within 2.3m of the ridgeline it must clear it by a min of 600mm

doningtonphil

Chimney sweep. It is a hamlet stove. We are burning Verde heat logs so very dry.

Our chimney terminates more than 60cm from the ridge and is about 20cm above the ridge (excluding cowl) so complies with the description and diagram in part j.

Our fires are burning nicely now (can't remember what wood we were burning for initial post). My main 'concern' is the heat haze out of the chimney. I know that no smoke is a good sign but none of our neighbours have a clearly visible heat haze.

Are we losing too much heat up our chimney?

Chimney_Sweep_3

Do not know much about that particular stove but it only lists 73% efficient, but it does sound like it, how many of the briquettes are you putting in at a time, they burn very hot.
The flue should terminate at least 600mm or 60cm above the the highest part of the roof not 20cm if it terminates within 2.3meters of said highest point.
Is your flue insulated and the neighbours not ?

Alycidon

Hamlet is a divison of Arada, Aarow, Villager and Stratford are the other brands within the stable. I am a stove retailer and act for the latter three, Hamlet is only available through a leading builders merchant but it is to all intents the same as Stratfords, they are made on the same production line I believe.

In short they are good stoves, made in England by a very reputable company. Efficiency figures can be misleading as there are several ways to measure, some take in account the moisture level within the test fuel some dont. Dont read to much into efficiency figures.

Briquette quality does vary significantly, its the density that varies as some are made on plastic industry machines that cannot produce the pressures needed. The Verdo ones that I have seen and tested are of very good quality.

Heat haze from the chimney is not unusual, the airflow from a stove is very fast compared to an open fire so the flue gets quite a lot hotter than an open fire. Smoke, you will usually get a bit when you restoke the stove but once all the logs have carbonated ( gone black) then there will be very little if any smoke.

Lack of heat into the water and the room is caused 95% of the time by burning fuel which is less than ideally dry. Even kiln dried logs are only warranted at 20% by most of the suppliers, that is well above most stove manufacturers requirements. Most want a maximum of 18%, some 16%.

I produce and sell logs locally air dried at 4%-15% depending on the time of year and kiln dried at about 14%. It takes an extra year to get the moisture down to these levels after processing.

Short term try those Verdo brickets, longer term get yourself some logs of a higher quality or invest in a wood moisture meter. Remember the drier the wood the less energy is lost evaporating off the remaining moisture in your logs so the more energy is converted into heat.

A

Greenfires

Air dried at 4%?? That would be way lower than I've ever seen - and to be honest I don't know how you'd get it that low given that the relative hunidity in the UK is a lot higher than that. Given that UK relative humidity is usually over 70%, you wouldn't expect air dried wood to get much below 14% assuming it was stored outside rather than in a heated building.

Alycidon

Greenfires wrote: »

Air dried at 4%?? That would be way lower than I've ever seen - and to be honest I don't know how you'd get it that low given that the relative hunidity in the UK is a lot higher than that. Given that UK relative humidity is usually over 70%, you wouldn't expect air dried wood to get much below 14% assuming it was stored outside rather than in a heated building.

We had a very dry summer, the cord came in to me in early 2010, was stored in bulk for a year then processed into ventilated firewood bags. Then stored in a large dutch barn with a steel roof and one side partly closed, so get lots of air through the stack, the steel roof also acts like a cooker. The barn is protected by trees so not a lot of direct rain coming in. When processed the Ash was 25%, the Sycamore which had some of the bark stripped off it by forestry machines was 21%. After a futher 15 months drying it was between 4% and 7%. However once the autumn mists etc started then the figures do rise up to about 14% by January after which I like to switch to kiln dried tp protect customers against possible woodworm attack if keeping logs in a garage.

In a normal damp summer my MC in Sept is 8% - 12%.

A