Good evening . I am George , from Romania , and one winter ago I made a new heating system by my design . Heat requirement of the house is 12kw . The wood boiler is 35 kw , maximum efficiency 80%. Electric boiler has 18 kw . Solar panel has 30 under vacuum glass tubes end discharge the energy in two coils . The buffer is combined with 200 liters boiler and has 800 liters , per total 1005 liters . All the system work nice , but the wood boiler can not heat well the puffer . Maybe someone can advice me how do I have to change the pipes to get a good efficiency . Thanks a lot . George

What target temp are you running in the storage tank? What is the outlet temp of the boiler? What is the set temp of the mixing valve/inlet temp of the boiler? How are you controlling the outlet temp/heat output of the boiler? How are you gauging the heat input to the storage tank? How many Kg of wood are you burning per hour?

1Kg of wood(fir) is around 18,000 BTU. 18Kbtu X 80% efficiency is 14.4Kbtu into the water. That much heat should raise 1005L/265G/2120LB of water roughly 6.7F/3.7C give or take. So if you are burning say 2 kilo's per hour, the tank temp should increase roughly 12F per hour... This of course providing there is enough temp differential to transfer that much heat that quickly. if it cannot, I think you would probably experience boiling though... At a full 35KW/119,000 BTU.hr it should raise the tank temp 56F in one hour...

Good afternoon .
I do not know if you can see the drawing . I try to sent ...
In the storage tank I put between 70-90*C . Mixing valve open at minim 60* , but the boiler pump start/stop at 75*C . The maximum temperature I can get in middle of storage tank is 65*C . Unfortunately I can not use full capacity of tank because of 200 liters boiler inside . Maybe water volume of the mixing valve obstruct the good transfer between the boiler and storage tank ? If I understand well , in one hour I get almost 14*C , that men in 4 hours , if I start from 35*C , I will get in storage tank ...90*C , but in some time I am using..., for example 10 kw/h for heating home . The quantity of wood in a hour is ...10-11kg with 75% efficiency .Volume of fire box is 86 liters .So I can not feel fire box with 40kg of wood , will not remain space for burning .

Yes, I saw the drawing, that is quite a system. Missed the 200L inside boiler so around 800L/211G/1688LB storage tank.

What kind of wood are you burning? IT shouldn't make a really large difference, but I am basing my numbers on 8000 BTU/LB or 18,000 BTU/KG for soft wood...
If I understand you correctly, you are burning 10Kg/HR of wood and @ 75% efficiency that should be around 134,000 BTU/HR into the water. IF you are using 10KW/HR of house heat at the same time that is about 34,000 BTU/HR. SO 100,000 BTU should be going into the 800L storage tank. 100Kbtu into 1688LB of water should raise if 59F or 32C in ONE hour... So where is all the heat going I wonder. the funny thing about boilers, especially wood fired boilers, if you are burning 10Kg of wood and not pulling 134,000 BTU/HR out thru the water, you would probably be making a LOT of steam, so the heat is going somewhere...

EDIT: I just looked at my numbers again, and something dosn't add up. 134000 BTU is 39KW, 4KW greater than the boiler's rated output... How dry is your wood? Depending on species dry wood is between 7500 and 8600(roughly) BTU/lb, but add some moisture (weight) to that wood and the BTU per pound can drop significantly. You can take an educated guess, but it will only really be that, a guess.

With that mixer in the circuit to maintain boiler temp/limit thermal shock, it is also going to be very difficult to get an accurate count of BTU output into the water without a flow meter/sensor in either the supply line or return line at the storage tank measuring how much water is moving into the tank from the boiler circuit...

If you think that all connections to the storage tank are OK , it remains only the possibility that the mixing valve obstruct the transfer between the boiler and the buffer tank . I will check also non return valve after pump .
Usually burn between 15 kg hr and 20 kg of wood per hour , sometime mix with ..8 kg hard petroleum coal . The hard wood I use is hornbeam , I hope is correct in english.
Of course , a part of heat goes to chimney , up to 230*C .
Thanks a lot .

What do you think about this way of connections you can see in the diagram below?
Do you think that the efficiency will be greater?

Ge0rge your diagram didn't link/upload. There is a size limit on attachments, maybe it was too large.

Well of course some goes up the chimney, but the numbers I was using for BTU were corrected for the 75 percent heat harvest/conversion efficiency from the burner into the water that you mentioned.

Ok, that ammount of wood burnt per hour really dosn't make sense. Are we missing a decimal point somewhere, or is that wood burnt per day? Burning 15Kg-20Kg of wood per hour dosn't make any sense. at 2.2LB per Kg That is 264000-352000 BTU/HR! or 77-103 KW/HR worth of wood. That energy input is 2-3 times the rated output of the boiler, how is this possible? even at the 75% conversion rate that is 198-264Kbtu. IF your 800L tank was at 56F/13C, that is enough energy to push it above the 212F/100C boiling point!

Something isn't adding up...

No , sorry .This Quantity of wood is for ...about 3 hours . I will try to send again a the diagram .

Still not load the diagram ???

OK, I see the diagram. My concern would be with potentially "shock cooling" the boiler by pushing cold water directly into a hot cast iron boiler. The purpose of the thermal reg valve in this instance is to only admit water above a certain temp into the boiler to prevent possible damage to the cast iron structure.

Ok, those wood burn numbers make a lot more sense, and come closer to the running circumstances you described earlier.

5KG of wood/HR should put about 66KBTU/HR into the water at 75% boiler efficiency. IF you are using 10KW/34KBTU of heat in the home, that leaves 32KBTU/hr into the tank which should raise it's 1688 LB of water roughly 10.5C/hr

With 32KBTU per hour into the storage tank, but the tank will not exceed a certain temperature(65C?), perhaps you have a loss point somewhere along the way. How much insulation is around the tank? What are the conditions around the tank(in a closed shed with10C room temp, out in the weather, ect...)? How is the plumbing insulated that runs between the systems? An un-insulated tank wall or pipe out in the weather can shed a heck of a lot of heat with hot water pumped thru it...

What is the highest temperature the boiler will output to the tank?

Is the tank water pumped directly thru the boiler or is there a heat transfer coil or heat exchanger in the tank for the wood boiler circuit? Heat transfer is about exchanger surface area, temperature difference and time. IF the wood boiler uses a heat exchanger in the tank, the surface area might not be large enough to transfer all the heat as the tank temp approaches the boiler output temp. The same would be true if the tank water is pumped directly thru the boiler. You cannot heat a tank above the temperature of the supply... But boilers that don't get adequate heat removed from the water tend to make steam, and you havn't mentioned any boiling, so i am inclined to think it is a loss issue, and not a transfer issue

Since heat transfer hinges on temp difference, the hotter the temp the greater the transfer per hour thru the tank wall. if the insulation is insufficient, or conditions are harsh around the tank or plumbing, you could be hitting a tipping point where the heat loss from the tank directly, or other parts of the system exceeds that 32KBRU/HR supply available to the tank from the wood boiler...

Is the wood boiler inside or outside. How automated is it? Will it self dampen to reduce heat as the return water gets hotter?
Is the plan to use the wood boiler only when it gets really cold or is it the main heater with the electric being the backup?
Is the buffer tank just an tank? Does it have any baffles or exchangers?