surely this had been kicked around many threads here and there. i get tid bits on info from each but still undecided what would be the best. with no storage and piping into my fossil boiler which has a domestic coil in it, it seemed that the best option was wood supply to fossil return and wood return to fossil supply to wood return. with the posts i have read this brings a couple concerns and seems that i will pay a price with either that i choose. at first i was focusing my design with the concern of keeping my fossil boiler warm so that my domestic coil stayed warm(plans to add a boiler mate at a later date). however i have read of conerns with this design where if i have heat demands i would have trouble supplying hot even temps to my baseboard and radiant without having wood supply to fossil supply.... any thoughts, regrets or experiences?
ADD-ON PIPING
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ewdudley
Minister of Fire
As you suggest if wood boiler (WB) supply goes to fossil boiler (FB) return then hot water from WB mixes with system return. Then part of the mixed flow goes to the system from the supply side of the FB and the rest of the mixed flow goes back to the WB.
If you connect WB supply to FB supply and WB return to FB return then the system will get as much pure hot supply water from the WB as there is to be had. I think this is the right way to do it, especially with systems that have loads that need high temperature water.
With either configuration if system load gets ahead of the WB output then the temperature inside the FB will drop and DHW temperature may suffer. It seems to me that with WB supply connected to FB supply then DWH would recover faster because pure hot water would fill the FB from the top down, which would heat the tankless coil in the top of the FB sooner.
--ewd
If you connect WB supply to FB supply and WB return to FB return then the system will get as much pure hot supply water from the WB as there is to be had. I think this is the right way to do it, especially with systems that have loads that need high temperature water.
With either configuration if system load gets ahead of the WB output then the temperature inside the FB will drop and DHW temperature may suffer. It seems to me that with WB supply connected to FB supply then DWH would recover faster because pure hot water would fill the FB from the top down, which would heat the tankless coil in the top of the FB sooner.
--ewd
Clarkbug
Minister of Fire
I can attest to this piping idea, as thats the way that my installer hooked things up.
Im not really thrilled with it, and Im hoping to change it around as soon as my funding allows. My house is somewhat over-radiated, so its OK that I was able to use lower temps for supply for this winter, but was also amazingly mild, so I might have dodged a bullet so far.
I dont have a domestic coil in my oil boiler, and the piping arrangement is better for a cast iron boiler that needs to stay hot. What got me was that even when I had nice hot water in my tanks, it was always tempered, and the return coming back would screw up my stratification.
If I had to do it again (and I probably will), I think I want to use a Taco twin-tee on the supply side of the fossil boiler. This way the house and storage loops are separate, and it will reduce the mixing Im getting. I should note that I also have a diverting valve installed in the return line to the wood boiler. If the water coming back to the tanks is over 150, it gets sent back out to the house again, instead of back to the tanks.
My thought for your setup would be that your controls would need to know to circulate water from storage to your fossil boiler to keep it warm when there is DHW demand without a house demand.
Im not really thrilled with it, and Im hoping to change it around as soon as my funding allows. My house is somewhat over-radiated, so its OK that I was able to use lower temps for supply for this winter, but was also amazingly mild, so I might have dodged a bullet so far.
I dont have a domestic coil in my oil boiler, and the piping arrangement is better for a cast iron boiler that needs to stay hot. What got me was that even when I had nice hot water in my tanks, it was always tempered, and the return coming back would screw up my stratification.
If I had to do it again (and I probably will), I think I want to use a Taco twin-tee on the supply side of the fossil boiler. This way the house and storage loops are separate, and it will reduce the mixing Im getting. I should note that I also have a diverting valve installed in the return line to the wood boiler. If the water coming back to the tanks is over 150, it gets sent back out to the house again, instead of back to the tanks.
My thought for your setup would be that your controls would need to know to circulate water from storage to your fossil boiler to keep it warm when there is DHW demand without a house demand.
thanks for the replies guys. your theories are spot on with mine, however your come from education and experience! good point too clark on circulating the wood boiler for domestic. i had thought of that and now just how to get online and circulators/aquastats wired accordingly.
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