Wood Heaters – Radiant or Convection?

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Re: Wood Heaters – Radiant or Convection?

Postby Cherokee Solar » Mon Jul 16, 2012 8:42 pm

Hey offgridQLD,

The Gourmet, Thermolux unit with the 400ltr tank and 8kW wet back is the one I have installed here and have lived with for a couple of years so ask away!

For a start, it is colder here at 700m ASL down south in the mountains of central Victoria than Queensland! hehe! :D

Right now it is 20C inside and 9C outside, so that wood heater punches well above it's weight.

Over summer, the two solar flat panels produce water that is extremely hot (water with drain back not glycol). The only real drama I have with the system is when it is warm outside, but not sunny, I can't run the wood heater to put heat into the system which would heat the house up. I added a Rinnai instant on LPG water heater which kicks in on these occasions.

The 400 Ltr hot water tank is connected to hydronic radiators (with a coil for the hot water) for the front part of the house on a hot water ring main. Unfortunately I wasted some money putting radiators into the same room with the wood heater (75m2) and these have proven to be unnecessary and never used, but not worth the money pulling out and selling.

The oven took a bit to get used to, but I cook bread in it daily when it is running as well as most meals. As it is a steel unit (rather than cast iron) you just don't need to run it as hot as you would think (ie. a gas oven).

An 8kW wet back will only run 4 radiators - don't install anymore despite what the sales guys reckon and for long term heat I have only 3 of them switched on at the valves. If I had my time again, I'd install the 15kW wet back.

Also you have to install the water tank above the heater unit (or near enough) as it uses convection to get the hot water circulating into the tank. You don't want to consider a pump on this type of system (ie. between the heater wet back and the water tank), because if that pump ever fails it could be messy... The pumps to the solar and the hydronics are good though and use stuff all power (I have them set to use 1Ah each at 24v).

If you have any questions drop a note here.

PS: If you are after a wood heater, make sure you have access to a wood lot.

Regards

Chris
Off grid solar + hot water. Heavily insulated + owner built flamezone house BAL-FZ. 300 mixed fruit trees + herbs + flowers + vegetables. Bees + heritage chickens. High up in the mountains north of Melbourne. http://ferngladefarm.blogspot.com.au/
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Re: Wood Heaters – Radiant or Convection?

Postby offgridQLD » Mon Jul 16, 2012 10:00 pm

Thanks for that Chris,

When you mention QLD people start thinking humid summer days, While I admit the winter days are usually sunny mild the winter nights are still cold where iIam. Last night got down to 5 deg and Sunday night got down to 7 deg. the funny thing is Friday nights minimum was 17 deg and sat nights min was 15 deg (lots of cloud cover). The winters are short two - three months of genuine heater weather and mainly at night. So would go through as much timber as down south. The cold nights are the clear sky days.

I was thinking the 15kw wet back also. I only really want to run two perhaps 3 radiators but most likely big radiators 50m2 - 35m2 and perhaps 20m2 room to heat. So potentially 100m2 to heat from the radiator's - not all the time though.

I am debating if its worth boosting the hot water from the wetback as any good vac tube solar hot water system will do well most of the year where I am. I have the option to switch back to what i have now (instant gas hot water) on the odd day that the solar isn't enough.Like you said there might be times when you need a hot water boost but don't want room heating.

I'm guessing you still use a more simple tank in the roof even if your just running radiators to give you a mass of water to act as a open vented heat bank.

What about pipe length to radiators the only issue with purchasing a oven/cooker style heater for me is that its a good 8- 10m from the kitchen(where I would place a wood stove) to where I would want the radiators. If I went for a insert style wetback wood heater the runs to the radiators would be 1-2m (very short)

Kurt

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Re: Wood Heaters – Radiant or Convection?

Postby zzsstt » Tue Jul 17, 2012 9:56 am

offgridQLD wrote:I am debating if its worth boosting the hot water from the wetback as any good vac tube solar hot water system will do well most of the year where I am. I have the option to switch back to what i have now (instant gas hot water) on the odd day that the solar isn't enough.


The water from the wetback isn't boosted, if that's what you are thinking. The booster is fitted to the tap water exit from the tank, such that it is triggered if the water flowing through it to the taps isn't hot enough - which is probably exactly what your instantaneous gas heater does now!

The water in the tank is heated by the wetback, and/or the solar, and the gas heater cuts in if the water flowing to the taps isn't hot enough. The same applies whether the tank is glycol with a heat exchange coil for the tap water, or if the tank water itself flows to the taps.

offgridQLD wrote:the winter nights are still cold where iIam. Last night got down to 5 deg and Sunday night got down to 7 deg


8:30am, bright sunny morning and it's still 0c with a frost on the ground here.

I have to say that the expense of the system we are discussing seems very high for night-time only usage for a couple of months of the year? Our system runs 24x7 for at least three months, and probably 60-80% of the time for another two. The average minimum temperatures are below 7c for 5 months of the year, with days below 0c for 7 months of the year. Between June and August on average there are 35 days with a minimum below 2c, but that's at the Post Office in town and I think we're a bit colder than that. Are you sure you're going to use it enough to warrant the expense?
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Re: Wood Heaters – Radiant or Convection?

Postby offgridQLD » Tue Jul 17, 2012 11:21 am

As for the expense and the heater going to wast. The question of do we need such a elaborate heater. The house came with a (large in size anyhow) LPG - radiant heater (has a fan). It's a fake log fire insert.

From memory The 45kg gas bottles cost around $120 and If we use the heater from 3pm in the afternoon to say 11pm each night 8hrs a day it will use one gas bottle in 7 days. So potentially costing $1400 over the 3 months of winter. It would be more like $1000- $1200 as we don't use it that consistently. Although just running cost inst the only reason to pick fire wood overthe gas.

Fire wood merchant(local suppler is the large farm next door and I have 8 acres of heavenly treed land to supplement what I purchase. So although we will have to pay for wood the costs should be reasonable compared to LPG gas.

Another issue with the gas fire. It's only rated at 5kw output (way undersized for the space). So it runs flat out if you set the remote at 21 deg and it's say 10 - 12 deg outside it struggles to maintain 17-18 deg even when it's been on for a long time. It would make a good size heater for our 35m2 bedroom but not the large living room. To be honest I only really feel comfortable standing directly in-front of it. The 2nd lounge room/home theater room has no heating nether dose any other part of the home. So something needs to change to make the heating adequate.

Sorry about the bad 1st image below. I had to join two pics together as phone camera that i used at the time doesn't do wide angle to get it on one shot. Anyhow the room is about double the size again behind me for the kitchen/entrance. The ceiling is 3m high in the lounge section 1st pic. Then at the 1/2 way point where I am standing to take the 1st pic it rakes up to 6m high ceiling above the kitchen/entrance. So its one very big air space to heat .There is also a full wall of glass to the right (1st pic) facing north). Then intermittent glass windows to the left side facing south and 3 large glass windows/skylights at the top of the 6m section 2nd pic among other large feature windows. The glass works for us in the day time heating the room and works for us in summer as a lot are louver windows to catch valley breezes. Its partly there for esthetics to take in the valley view's. So we don't need heating or cooling daytime but the glass doesn't work in our favor at night.Making the home harder to heat. We plan to get some window dressing's on them for next winter.
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The one thing I am almost certain of is a wood heater is the only viable option to heat the home given its space and the fact we are off grid so cant use AC heat pumps or mains gas style central heating. I feel we need a good 25kw of heating to maintain 20 deg in the main 3 rooms we use. 15kw radiant wood heat for the main area and then at least 10kw of wetback to power 2 or 3 radiators to do other rooms. Its only The fact that I want to heat other rooms that I thought a wetback with radiators would be the only simple cost effective way to do so.

The domestic hot water option isn't essential as I have other options like a separate pure solar hot water system with instantaneous gas switchover valve for bad days.The oven-cook top isn't essential ether as there wouldn't be a big window of time to use it each year.

That's why I mentioned perhaps I am better of with a large slow combustion wall insert heater than the oven style. One with a generous size wet back to power radiators. I could install it in the same location as the old gas unit simply replace the gas fire that's there now.The gas fire actually has a flue so perhaps with some slight upgrading the same flue could be used or at least the penetration hole and some of the layout is in place all ready. I know I would still be up for the cost of a tank -radiators and the heater unit itself. I know it could be expensive and we wont get as much use(value) out of it as some one down south. We plan to be in the home for a long time and need something and I don't see any other viable options.

In the end just because its not as cold as down south 0 or -5 deg but then again when you heat your home down south you get it to +20deg inside not 12-15deg . We have only have +5 to +12 deg outside but i still need to get mine to +20 deg inside to make it comfortable .It takes some kind of heating to add 8 to 15 deg extra warmth over the ambient temp to make it comfortable inside.

I guess its a bit like saying in Russia it gets to -50 deg so you don't need heating in Vic as it only gets to -5deg. I think anything under 15 deg (inside temp) some heating is then needed. I don't need 25 deg and to be walking around in a T shirt. I am happy to put a jumper on if its 16-17 outside and not use a heater. I just want to be able to create a comfortable home. Especially when we have guests over.

Kurt
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Re: Wood Heaters – Radiant or Convection?

Postby zzsstt » Tue Jul 17, 2012 3:20 pm

I fully understand. My concern, if it can be called that, is that the wetback system has a high thermal mass and a low output (for the job at hand). Like the oven for cooking it has a long heat-up time. If it generates 15kw when runnning flat out, that 15kw first has to heat the cast iron unit itself, then it has to heat 400L (just a guess of the capacity of the tank) of water. At the same time is is having to warm the room it is in, and supply heat to the radiators.

Two scenarios exist:

1/ In Victoria that system is running 24x7 for several months. It reaches an equilibrium where everything is hot and the fire is opened up or shut down in a fairly marginal adjustment range.

2/ In Queensland, the fire is lit at 3pm (?). It slowly heats the water in the tank, and the water in the radiators, so at some point in the (late?) evening the house is warm. It is then left overnight, and possibly not relit in the morning. Through the day that large thermal mass is releasing heat to the house, when the outside temperatures are high, possibly overheating the house.

It may work, but it has always seemed to me that high thermal mass/low output systems like wood burners are most suited to a 24x7 system, and that is hard to do if daytime temperatures are warm. Even here, today it is 17c and sunny and we don't really need the fire, it is still going (as a bed of ash) simply to give it a start when I stoke it up for the evening.

The oven and the wetback both add thermal mass and seem to push the system more towards 24x7 use to see the full benefit. Without 24x7 use for a good portion of the year, are the inefficiencies the system introduces to solar water heating going to have a negative impact that outweighs any positive?

Perhaps a small "fast" wood burner might be a better alternative? I have a little Morso heater in my 50sqm cathedral ceilinged toyroom, it is spec'd at 10kw (73% efficiency) but is very small, burns very fast and enables an unoccupied room to be heated quickly without using too much wood (must be chopped small though). They also do a 15kw version. For cool nights and warm days, I wonder whether such a unit might not be better than a high thermal mass "slow" system? Obviously it won't heat the other rooms other than by air movement, but my experience of circulated water systems would suggested that the time lag between lighting the fire and the radiators significantly warming the rooms may mean that a night-time only fire won't work too well either. Keep in mind that a typical gas boiler to heat a water tank and provide warmth to radiators is going to be 30kw or above for all but the smallest of homes. The wood fired unit is much smaller because it cannot be switched on and off, and it therefore relies (to some extent) on being run 24x to overcome it's lower output.
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Re: Wood Heaters – Radiant or Convection?

Postby offgridQLD » Tue Jul 17, 2012 3:57 pm

yes I am now seeing your point.

The suggestion of a high output fast to heat up combustion wood heater could be a good Idea. Its just heating that 50m2 room behind the main living room that needs to be addressed. The heating of the bedroom was just a luxury ( why not situation if we had a wet back already)

We do have overhead electric heat lamps in both bathroom and bathrooms to cover things getting to uncomfortable when getting changed for 5 min and so on.

Originally I was thinking of a dual door (double sided ) wood combustion insert to go where the gas heater is as I could heat both lounge rooms at the same time. Another option was some kind of radiator and fan system that you can purchase that extracted heat from the flue or others just took air from the ceiling level and pumped it to another room that hot air could be pumped e few feet to heat the 2nd lounge room. This option was my favorite as you could simply not turn the fan on if you didn't want to heat both rooms.

I guess its worth going through it all option on paper first before giving money away.

Kurt
Offgrid 2008, Selectronic PS1 6kw/48v inverter, x 2 Midnite solar classic 150 MPPT, 3960w NE PV 24 x 165w BP panels, 4200w NW PV 21 x 200w DAQO PV, 16x400ah lithium.
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Re: Wood Heaters – Radiant or Convection?

Postby Smurf1976 » Tue Jul 17, 2012 9:55 pm

offgridQLD wrote:In the end just because its not as cold as down south 0 or -5 deg but then again when you heat your home down south you get it to +20deg inside not 12-15deg . We have only have +5 to +12 deg outside but i still need to get mine to +20 deg inside to make it comfortable .It takes some kind of heating to add 8 to 15 deg extra warmth over the ambient temp to make it comfortable inside.

I guess its a bit like saying in Russia it gets to -50 deg so you don't need heating in Vic as it only gets to -5deg. I think anything under 15 deg (inside temp) some heating is then needed. I don't need 25 deg and to be walking around in a T shirt. I am happy to put a jumper on if its 16-17 outside and not use a heater. I just want to be able to create a comfortable home. Especially when we have guests over.

I think the point being made is that if a particular heating system is adequate in Tasmania or Victoria then, due to the climate differences, it is safe to assume it will be powerful enough to do the job in Queensland.

I don't think anyone's saying you shouldn't heat the place as such.
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Re: Wood Heaters – Radiant or Convection?

Postby offgridQLD » Tue Jul 17, 2012 10:12 pm

Its all good , it was just a misunderstanding of the need to heat the thermal mass (water) over a long period. potentially not what i would want everyday as it would require running heaters during the day.No one is doubting the ability of wood heaters to do the job ...Boy some are rated at 35kw!!

My initial point was just I needed to come up with something to do the Job as the gas just wasn't a option I am willing to entertain anymore. zzsstt suggestion of a different style of wood heater arrangement is a valid idea for my needs and I will be taking that advice as i can see his point.

Kurt
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Re: Wood Heaters – Radiant or Convection?

Postby Cherokee Solar » Wed Jul 18, 2012 10:29 pm

Hi Kurt,

I hear you man.

A couple of years back whilst owner building the house I'm in now, I rented in a housing estate at the bottom of the mountain range I'm now up in.

Firstly, cold air falls. That brick veneer house at the bottom of the mountain range was cold over winter and hot over summer. It was always 4 degrees warmer inside than the outside air temperature. 0 degrees outside, 4 inside. No worries. However, 40 degrees outside = 44 degrees inside! :x I reckon that there was about 8 to 10 weeks per year when the house performed well on internal temperature.

Anyway, the only heating it came with was LPG central heating and the supplier, ELGAS who trucked the stuff in, advised me that if I was to switch it on from 16 degrees and up, I would use the entire gas bottle in under two weeks. I believe them. I think it was 90kg of LPG, but around here I have seen the 500kg tanks.

Secondly, LPG has a lower calorific value than Natural Gas and is also prohibitively expensive as it usually needs to be trucked in.

I'm as green as they come, but those two winters were a real bummer. I slept with two doonas on the bed which was an old camping trick. The rest of winter, I was just cold, but you do adapt. It was very hard on my 17 year old dog though.

People, heating using LPG is a financial no no. ;) Unless you can get the stuff cheaply.

I also happen to have the wood heater in question + 2 x solar hot water panels + 400 ltr hot water tank + 8kW wet back + radiators spread throughout the house.

It is made from sheet steel so heats up very quickly. I cut and season my own firewood here and the oven is usable within half an hour from ignition if run properly. There is an art to it, like any wood heater, but it is not comparable to an AGA or ESSE which are cast iron units, which are massive and have a huge amount of thermal mass to heat.

The wet back takes a couple of hours to get serious heat into the water tank as it circulates the hot water by convection, but even today the 400ltr hot water tank received some solar heating so it all adds up. To heat the water up quickly, you have to run the heater on max settings.

Also the fire box is not that big and it will not burn over night, despite how you set it (I also experimented with red gum which has a slightly higher density than the local timber 750kg/m3 vs 650kg/m3). However, this is not a bad thing as bigger fire boxes use more timber. More timber = more hassle = slower combustion = more work. Everything has a trade off and if I had my time again, I would still choose the same unit albeit with the 15kW wet back water jacket and it is much colder here than in Queensland. A 5 degree night can happen any day of the year. The hottest night here would be 22 degrees over high summer.

If the house leaks heat like a sieve, you are stuffed regardless of heating and cooling systems because once you stop mechanically heating or cooling the house, that houses internal temperature will swing dramatically.

The wood box easily heats 75m2, plus cooks (oven and stove top), plus supplies hot water to the house and up to 3 radiators at a time (8kW wetback).

It is 3 degrees outside right now and 19 inside and I'm running the heater on it's lowest setting and have done so for many hours. You can't ask for better than that.

As with any wood combustion box, you must burn seasoned timber, preferably hard wood. Otherwise your flue will block up and you could have a potential house fire as the unburnt materials in the flue may combust without warning.

Chris
Off grid solar + hot water. Heavily insulated + owner built flamezone house BAL-FZ. 300 mixed fruit trees + herbs + flowers + vegetables. Bees + heritage chickens. High up in the mountains north of Melbourne. http://ferngladefarm.blogspot.com.au/
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Re: Wood Heaters – Radiant or Convection?

Postby offgridQLD » Wed Jul 18, 2012 11:09 pm

LPG heater will be sold that's for sure (its actually a very large modern and expensive imported unit ) Some one with a town house in the city on mains gas could re jet it and be happy with it.

The couple over road live in a little well insulated wooden cottage its tiny perhaps under 100m2 total for the entire house.It has a very small open plan kitchen lounge dining area perhaps 40m2 or less. They are also off grid like every one living on our road.They heat there home with a little modern portable fan forced LPG gas heater from two 45kg bottles. The kind of gas heater with the flat panel that burns with a very tight flame and perhaps consumes 1/4 of what ours dose so they go through perhaps 2 bottles of gas over the 8 weeks of peek winter. At $120 so $240 isn't to bad . While LPG is a expensive way to heat for them it's viable works out ok and they get away with it .

It's bit like owning a 6lt V8 but only driving 5km to work each day. You get away with bad gas mileage as it doesn't matter for 5km. Our place on the other hand is like owning the 6lt V8 and driving 200km to work each day .

We will shiver out the next month :lol: or so of winter this year with what we have. That gives us another 9-10 months to make a decision and get something new installed and sorted before next winter.

Kurt
Offgrid 2008, Selectronic PS1 6kw/48v inverter, x 2 Midnite solar classic 150 MPPT, 3960w NE PV 24 x 165w BP panels, 4200w NW PV 21 x 200w DAQO PV, 16x400ah lithium.
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