When to fire for efficient storage?

I would say that catching the storage before it becomes unusable so you have no lack of heat in the system is a good point to recharge. Only burning enough wood to recharge and supply heat during charge seems most efficient.

Another point we haven't mentioned in this discussion of charging and wood use is controlling the charge. Without proper controls the tank tries to heat during the whole burn cycle and runs down in temp as the burn ends until the circ shuts off. If the tank charges earlier in the cycle, the rest of the energy is not being used efficiently I am building a controller to take care of this but Tekmar makes one. This was discussed recently in another thread.

This is a really good question. I think there are two parts:

First, you want to heat the house as much as possible directly from the wood boiler. However, you only need a certain number of hours of burn time per day (average of 7 in my case). What you want to do is choose your burn time to match up with the highest heat demand - that is, the coldest part of the day. That way, your storage only has to carry you through the warmer portion of the day.

Fortunately, that works out well. My take on it is that you want to figure out your burn time for a full load as well as the burn time that you'll need for the day. Assume that you'll put in a full load at bedtime, and back up early enough so that you get enough total burn time. I start my fires anywhere between 4 and 7 in the evening.

Second, as far as deciding how low to let the storage get, I actually look at it the other way. I take an educated guess at how much I'll need in storage to carry me through the next day, and plan my fire start time with that in mind. I don't think there's any penalty at all for letting your storage get depleted, as long as your house temp doesn't drop too much. Typically, the worst case is that I'll have to start a fire earlier. For instance, I skipped yesterday and ran out of storage sometime in the night. The house was a little cool (68 main floor, 70 top floor). Since I'm alone today, I haven't built a fire yet, but I will by 3:00 or so.

The longer burn issue is complex. If you're getting a longer burn by reducing both primary and secondary airflow in the right ratio, and not enough to reduce secondary combustion, you might pick up some efficiency due to slower flue gas velocity through the HX.

However, longer burns from larger or greener wood are definitely less efficient.

Nofo

I do almost the same schedule. Fire between 4-7 over night charge. I like to put one big log on the top of the load before bed so that there is still something in there in the morning so I don't have to relight. I load it again in the morning to make up for any storage lost in the early morning and vary the size of the load depending on weather for the day. Storage takes over sometime in the afternoon and gets me through until I light the next evening fire. I hope once I build my controller for loading the tank that I can burn a little less wood because I won't have the loss from the tank at the end of a burn cycle.

I will build a fire in the evening when I get home. The house gets first dibs on hot water when a zone calls for heat so it will alternate between heating the house and heating the tank. At 10:00 when everyone goes to bed and the thermostats kick down, I load it up once more to bring the tank up to max. My tank is 1200 gallons which is roughly 10,000 lbs which is 10,000 BTU's per degree roughly. Oak is roughly 100,000 BTU's per cubic foot and I have a 4 cubic ft fire box.
So if I want to bring the tank up 20 degrees I'll fill the firebox half way then add a little more for heat loss and any overnight demands I expect.
Very rough but works for me so far. This morning the tank was still at 163 so I won't build another fire till tonight. The tank will probably be around 140 when I get home.
If I used more overnight and/or I expect it to be colder during the day I'll follow the same formula to bring the tank up to max so it can coast the reat of the day.

I am very new to the storage thing. I have a Tarm 40. Had it for 4 weeks...really liking it now( although it did have a learning curve) . How much happier would i be with 750 gallons of pressurized storage ? or 500 gallons of pressurized storage? Would you say to go with 750 or 500? I would really love to fire twice in a day... regardless of temp outside... do you think i could do this with 500 or 750 storage? Well... not regardless. I mean if it is 30 outside or 15. NOt like 60 . Like winter temps..

My experience with 500 pressurized gallons of storage says go for 750.

I think it depends on what temp you are charging the storage to. If you can charge up to 200* you will need less storage than if you charge to 180*. The 750 gallon tank would give you heat for a longer stretch, but also take a little longer to charge up.

I have a very tough time hitting 180* with my 500 gallon pressurized tank. The boiler is 20' from the tank and the 1 1/4" copper supply and returns are un-insulated. With insulation I should be able to hit higher temps.

Birdman said:

I am very new to the storage thing. I have a Tarm 40. Had it for 4 weeks...really liking it now( although it did have a learning curve) . How much happier would i be with 750 gallons of pressurized storage ? or 500 gallons of pressurized storage? Would you say to go with 750 or 500? I would really love to fire twice in a day... regardless of temp outside... do you think i could do this with 500 or 750 storage? Well... not regardless. I mean if it is 30 outside or 15. NOt like 60 . Like winter temps..

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750 would almost certainly be better. It's more efficient to transfer heat at lower temps, and the larger tank would mean you don't have to get it as hot to get the same storage. Lower temp also means less standby loss.

I fire once a day with an EKO 25 and 880 gallons of unpressurized storage. Skipped yesterday, though.

I have been bulding my fires in the evening when I get home, when the wife gets home at about 5 or 5:30 she will turn the heat up and take a shower. This will start the circulator for the floor heat and the forced hot air, and also pushes the water from storage through a hx to heat the dhw tank, no heat from the wood boiler all storage. When I get home at about 6:00 I will biuld a fire and get the boiler up to temp, when the boiler loop to the oil boiler gets to 180 it will allow the circulator to kick in for the baseboard if I am still calling for heat upstairs. When the themostats upstairs are all satisfied and the boiler loop to the oil boiler gets to about 185 to 190 the circulator to charge the tank will turn on and the boiler will heat the tank. At any time when the upstairs themostats call for heat and the boiler loop is up to 180 the baseboard heat will kick in, however during the day when we are gone and there is no fire, only the storage heats the house, unless for some reason the boiler loop goes above 180 then the baseboard will kick in, but not for long. My oil burner will only kick in if the low low temp is reached, and that is likely not to happen.

Steve

I just built a medium fire at 4:30. I got gasification in less than 4 minutes. top of my unpresserized 806 gallons of storage was at 153, mid at 147, low at 131. no zones were calling for heat, seems the insulation and slab really pays off. I'll burn this medium fire tonight, put it in storage and heat my radiant slab if necessary tomorrow with the storage. One small fire each afternoon is all I need so far. sweetheat

sweetheat said:

I just built a medium fire at 4:30. I got gasification in less than 4 minutes. top of my unpresserized 806 gallons of storage was at 153, mid at 147, low at 131. no zones were calling for heat, seems the insulation and slab really pays off. I'll burn this medium fire tonight, put it in storage and heat my radiant slab if necessary tomorrow with the storage. One small fire each afternoon is all I need so far. sweetheat

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With storage temps like that and mild temps, could you have skipped the fire tonight? I'd be tempted - I almost never build a fire just to heat storage. Don't want those lazy BTUs just sitting around that long.

Sounds like you have a liveable schedule, and all that should happen as weather gets colder is that the fires get longer.

nofo- you are right, I could have skipped it tonight. My wood is short 16"- 4 to 6 inch dry maple, beech and ash splits, I at first thought I'd try to bring my bottom temps up closer to 145, is that a reasonable goal or will the top be over 180? My EPDM's limit. It's to far to walk up there tonight. I' ll look in the morning and see what my 3 temp levels are. It's 33 degrees here now at 7:32 How far back (stored BTU'S) do you go before you put some back? 120 top temp? I thought once you got it hot to keep it there with a smaller daily fire, but with these warmer days, difficult for us new to the world of gassification. It's a very efficient machine with a forgiving learning curve. No major leaks or messes yet. Very nice heat in a radiant slab. sweetheat

sweetheat said:

nofo- you are right, I could have skipped it tonight. My wood is short 16"- 4 to 6 inch dry maple, beech and ash splits, I at first thought I'd try to bring my bottom temps up closer to 145, is that a reasonable goal or will the top be over 180? My EPDM's limit. It's to far to walk up there tonight. I' ll look in the morning and see what my 3 temp levels are. It's 33 degrees here now at 7:32 How far back (stored BTU'S) do you go before you put some back? 120 top temp? I thought once you got it hot to keep it there with a smaller daily fire, but with these warmer days, difficult for us new to the world of gassification. It's a very efficient machine with a forgiving learning curve. No major leaks or messes yet. Very nice heat in a radiant slab. sweetheat

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I'm pretty ruthless about running the tank down when it suits my convenience or desire to procrastinate. Since it also preheats my DHW, I can run it down way below the point where it has no usable heat left for space heating. Today it was 40,000 BTU below the point of minimum usefulness before I got around to building a fire. The main floor of the house got down to 68 - I can live with that.

VtRv said:

I'm trying to get on a schedule so that I burn once a day during this milder weather. It's been in the 20's at night and 30 touching 40 during the day so the demand hasn't been all that high. I came home today at noon and the tank was down to 120 on top and 115 on bottom after being 175 last night at midnight. The house wasn't calling for heat but my Superstore was. I learned that I had it set higher then what I think it needed to be so I turned it down until it stopped calling for heat. I didn't start a fire until around 4:30 and by then it was down to 117. I plan to get my tank back up into the 170's and hope to hit 180, that's my goal I haven't achieved yet. The forecast is for tomorrow to be mild so I hope to let the tank go again until late afternoon. I'm interested to see how it changes when we get colder weather. I hope I'm not in for a rude surprise, I'd like to think that I could get by with burning twice a day on all but the coldest days. I've been insulating my storage tank more and more hoping to get the storage to stay hot longer as it seems that on these mild days it would stay warm longer.

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Based on dropping from 175 to 120 in 12 hours, your heat load is quite a bit higher than mine. Doing the math, it works out to something like 19,000 BTU/hr at about 35 degree days. If I were in your place I'd take a hard look at where you might be losing heat - either drafts, inadequate insulation, windows, or heating system issues.

Average heating season temperature in Burlington is 25 degrees, so you just experienced a day that was just a bit warmer than average for me - you may be in a colder area. If you're getting 6 hours now, it will be more like 3 on a cold day - not enough to be all that helpful.

If you can't find and fix heat losses, you'll want to look at more storage. If that 19,000 BTU/hr number is real at 30 degrees, then 1000 gallons would not be excessive - 1500 might be better.

Of course, you'll be burning more hours when it's colder, so the storage won't need to carry you as far.

nofossil,

I think you just said what I was thinking as I was trolling down through the threads on this subject. I don't think that if 750 gallons of storage is good for one installation and 1000 is good for another you just ca't say that someone should have a prticular size tank unless you know the heating demand of the particular installation.

In my case, I don't want to have 700 or 800 gallons loosing heat or having to heat up a larger amount when 500 gallons will be sufficient to carry me through the night and into the next day.

Fred- Is 500 gallons working for you? pressurized or non pressurized.
The reason why I ask is it seems like most people here with 500 gallons wish they had more.
I'm still undecided if I need more, I hope not.

I don't currently have my 500 gallon tank up and running but the 105 gallon tank I'm temporarily using is nearly enough. I'm heating a 960 sq/ft ranch that I've been remodling since I moved in 5 years ago. I installed radiant floor and in every room that I've reworked, I added 1 1/2' urethane foam board to the outside walls. I'm anal on insulating. The attic got an additional foot of cellulose insulation and the windows are all 3/4 inch insulated with some having movable insulation which get drawn at night. As you can see my heat demand is quite low. Tank temp in the morning is usually about 125 to 135.

I'm presently firing once a day about the time my circulators start running in the afternoon but Idon't fill the firebox. I useally make sure there are 2 or 3 splits about 4 inches in diameter at bedtime. Since it idles during the night because of the lag time between circulator calls, I usually load the overnight pieces while the boiler fan is running to "bake" the pieces. Once baked they don't produce as much creosote. The house usually doesn't usually call for heat until late in the afternoon. Besides having low loss, the floors are about 2 inches thick with the addition of cement board, porcelain tile and brazilian cherry I've installed over the existing floors. Not as good storage as water but it all helps. Again, It is your heat demand that is the critical factor.

I'm not taking any heat directly off the boiler because the company that furnished my tubing for the floor informed me that it wasn't oxygen barrier after I had it in place. The 500 gallon tank will be non-pressurized.

I start a new fire when the tank reaches 86 Fahrenheit.
I fill the whole tank to 210 Fahrenheit.

When the outside temp is around 20-30 fahrenheit I need to start the boiler every second day .

If I let the storage get down to far it seems as if the catch 22 applies. the tarmovar wants water going back to the boiler at 32 degrees Celsius, tarm insists that a valve be in place to restrict the flow of hot water returning to the boiler. can anyone help me understand this better. sweetheat

sweetheat said:

If I let the storage get down to far it seems as if the catch 22 applies. the tarmovar wants water going back to the boiler at 32 degrees Celsius, tarm insists that a valve be in place to restrict the flow of hot water returning to the boiler. can anyone help me understand this better. sweetheat

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What should happen with really cold return water is that the Termovar should mix the 180 degree water from the boiler outlet with the cold return water to get the desired minimum inlet temp. I expect that the Termovar wants a much higher output temp than 32C - that's about 90F. I'd expect 55C as a minimum, or around 130F.

In any event, if the boiler is putting out 180, the Termovar should be feeding it a blend of the 180 and the cold return that provides an inlet temp of around 130 or 140. As the return water gets warmer, the percentage blend will change. You should always be getting some cold return mixed in, though, unless the fire is dying.