Adding backup to wood boiler system

[Engineer]

I'm putting this out here in hopes that one of you guys might be able to offer advice.  I am cutting and pasting a majority of this from a post I made on another forum, and got nothing in response.   

"I built my house starting in 2004, over a period of a couple years; we moved in during December of 2007. The house is a 2-story timber frame raised-post cape with full finished basement. About 4300 sf of finished space.

Just after I started construction, I installed a 2004-vintage Central Boiler CL5648, bought new as a Dual-Fuel propane and wood setup. I use the OWB as my only source for both heat and domestic hot water, so it runs more-or-less 365 days per year. I would estimate I use the propane, either as a backup or on purpose, about 20 days of the year. The rest of the time I have wood in it. I have no other backup heating source, the only thing that is in my house is a 40-gallon indirect-fired hot water storage tank with a circ pump, for DHW. All of my flow from the OWB goes through a flat-plate exchanger and is split up into several zones for the house and one zone for DHW.

I have come to realize that I cannot work with this situation forever, for several reasons. If I get hurt or somehow incapacitated; if the boiler breaks down or needs extensive maintenance and I can't use it for more than a day or two; or if I am on vacation and can't feed the system, etc.

I would like to find a way to install some kind of backup system in the basement of my house, so that I can eliminate (or abandon in place) the duel-fuel gas burner on my OWB (it's horribly inefficient anyway) and use the backup when I cannot feed the OWB with wood. I have also realized that my OWB won't last forever (it's already developed a minor leak) and if I replace it, I won't have the duel-fuel option. It will most likely be a gasifier unit and much more efficient."

Here's the conclusion I have come to - tell me if I'm off base?  I'd like to replace the indirect tank with an air-source heat-pump style system (GE Geospring hybrid-electric or similar) and program the heat-pump to kick on if the supply temp is below a certain point. I really like this idea because it would normally be used the most during the summer months, when I'd like to shut the boiler down, and the cooling and dehumidification features come into play.  What I can't figure out easily is how to configure it all.  Obviously I'm going to need an experienced plumber/HVAC guy for this - I do not have the expertise. I'm looking for ideas and suggestions right now.

[Hilltop366]

I'm not 100% sure what a GE Geospring is, I thought it was a heat pump DHW heater but I doubt you would be able to heat your house with one so I could be wrong. 

Probably the most reliable and long lasting solution is a boiler in the basement, hook all your zones to the basement boiler then use the wood boiler and a heat exchanger to heat your basement boiler. When the wood boiler can't keep the basement boiler up to the temperature the burner will be activated.

[hedgerow]

Engineer. You didn't state but I will assume you  don't have AC and duct work so I guess forced air LP  is out. It depends on how much money you want to spend for a back up you could use a water based heat pump or a Lp boiler will probably be your best choice. I am like you and run my Garn year around to heat my hot water. I do have AC so I have LP forced air for back and a Lp hot water heat for back up for my hot water. I check them yearly but haven used them in 10 year. They are in place just in case.

[OneWithWood]

Our setup is similar to hedgerow's. 
We have a heat exchanger mounted just below the A/C coil on a forced air LP furnace.

[Engineer]

I have been doing domestic hot water and heat for ten years with the wood boiler.  The heat is all through radiant tubing, both in the basement slab and staple-up under the wood floors.  I have no ductwork and no central air. 

The Geospring is a heat-pump hybrid electric water heater.  It would be used only during the summer months for domestic hot water and would not be used for heating.  I was thinking of using the water heater as a passive (indirect) device when the outdoor boiler was in use, and then I could shut the OWB off for a month or two for service and let the electric thingy take over.  I also use my OWB for pool heat, but have not done so this year due to some electrical problems with the unit.

[Hilltop366]

Ok I get it, you were referring to the Geospring just for DHW.

I still think a (gas / oil / eletric) boiler in the way to go as far as a complete backup system.

Perhaps adding a hot water solar collector to your indirect water tank would reduce your summer dhw cost if that is a goal.

[doctorb]

As unlearned as I am on this subject, my recollection is that heat pumps are not good sources of heat once you get too far north of the Mason-Dixon line.  You are in a cold climate and I hope that your choice of backup heat can actually supply you with adequate heat if you need it in the winter.  I am sure that there have been improvements on such things, but I don't know many people in VT who heat their house with a heat pump.  So as a backup source, it may not do the job.  I am ready to shut up and learn from others.

[Gary_C]

The problem with your idea is the outdoor Central Boiler unit is a huge heat exchanger and if left connected it will dump the backup heat from your in house boiler or heat pump into the outdoors. You could stop that heat loss with valves (bypass) or shut off the water circulator in the CB when using your indoor heat source.  But that would leave the CB exposed to freezing temps in the winter. Then you would probably have to go with antifreeze in the CB system plus an isolation method when not burning the CB.

There are no easy answers to your problem. Perhaps the easiest and cheapest method is a temp turn down and hire the neighbors boy to fill the stove when you are away. That plus more insulation in your house to lower heating demand. In other words, spend your money on making your system more efficient and easier to operate.

[Holmes]

Recently a friend asked me to pipe a geo thermal system. They will bury the pipes in the ground and they need a plumber to make all the interior connections and follow the schematics. He has a OWB and wants to stop cutting wood. I mentioned his boiler is on schedule to fail, leak, {CB 10 years old}. It will cost 10 to 15 thousand to replace it so he has decided to put that money into what he thinks is a very good geo thermal system. He has already added solar to his house. The circulators may get expensive to run but the solar may help with that.
   Your boiler is already leaking so where do you want to put the $$$ ?

[Engineer]

At my place of employment, I am responsible for maintaining a geothermal system that heats and cools over 140,000 sf of space.  I think the geothermal systems are nice, but I wouldn't want one for my home. 

Where I'd like to put my money is back into a woodburning system that won't have major problems in ten years, or else is easy to maintain.   I would also like a backup that allows me to shut off the wood boiler in summer.  I refuse to add another entire boiler to the system.   I do not want the backup system - the heat pump or whatever - to provide HEAT.  I only want it to provide domestic hot water in the summertime, for about two to three months max.  In the winter I will be using the outdoor wood boiler, whether it's the one I have or a new one.  HOWEVER, If for some reason there is nobody at home for a period of time in the winter, nobody to feed the OWB, I would be fine with the backup system providing some kind of backflow into the OWB, by a manual valve or switch, just enough to keep the water temperatures around 50-60 degrees and not allow anything to freeze until I get back. 

I think maybe I should be looking at this from another perspective?  Maybe we should assume that I already have an electric water heater (hybrid heat pump or conventional, doesn't matter) and it's my primary heat and hot water source.  Not farfetched - during the 1970's and earlier, this was common until energy costs went sky-high.  So now, I want to add the outdoor wood boiler and make IT my primary source of heat and hot water, and only use the electric unit during June and July.  So how do I make the system work?

[Gary_C]

I currently have forced air heat so the hot water from the outdoor CB first passes thru the sidearm heat exchanger on the 80 gallon electric hot water heater and then goes thru the coil in the forced air furnace and then back to the CB. I do not run the CB in the summer so when I shut down I turn off the outdoor circulator, close the valves to stop the water flow to the sidearm heat exchanger and coil in the forced air system. Then I simply turn the breaker back on for the water heater and I am ready for the summer A/C season.

There is no reason why you could not use your hot water heater to keep the house and CB water at a minimum temperature. One winter I forgot to turn off the breaker on the hot water heater but I doubt it ever ran because when the CB is running the water temperature in the domestic hot water if much higher than the setting on the electric thermostat. The only problem to deal with when gone in the winter is to turn off the solenoid on the air damper when there is no fire. I don't know how well it would work with a gassifier when it runs out of wood but that generally takes many days. I don't know either if the water heater would have enough capacity to keep everything warm for an extended time but that would probably take some testing.

We have two sons that live nearby and one is running a CB for heat too so if we ever go somewhere in the winter for an extended time, we can get someone to stop by and throw some wood in the CB. That's the simplest solution. 

[sam-tip]

I use an instant on tank less water heater as backup  for hot water.  I had it before I used wood. I had to add a water tank to use wood for hot water.  I leave the tank less off when using wood.  The wood hot water just passes through the tankless.

[Ikeholt]

I think about a backup plan as well.  What I'd like to try is putting a propane burner inside the OWB firebox for a day or two.  It wouldn't be efficient, but would get me by while gone for the weekend.   Anybody ever tried it?

[Engineer]

I think about a backup plan as well.  What I'd like to try is putting a propane burner inside the OWB firebox for a day or two.  It wouldn't be efficient, but would get me by while gone for the weekend.   Anybody ever tried it?

Yeah, I wouldn't recommend it.  Why not?  Because that's what I have now.  It's a Central Boiler "Dual-Fuel" model and it flat-out sucks.  Efficiency has got to be in the 10-20% range, if it's even that high, and it also means that I have to have the OWB running 365 days a year or I get no heat or hot water at all.  That's why I want to do a totally separate backup system, so that I can actually shut down the OWB for a month or two in the summer, clean it out and do preventive maintenance.

[711ac]

I think the heat pump HWH would be fine for DHW, but only the DHW. I believe the recovery is very slow on these units and would be useless as a heating backup.
FWIW, I have a oil fired HWH made by Bock, it is designed for radiant heating and DHW. They may offer an LP unit.
This is for DHW in the summer and the wood boiler makes it during heating season.
Your indirect tank might make this GE hp unit work for heat back up acting as storage, (circulating to it from the GE unit) but that's a little too complicated for my little brain. A good heating guy with radiant experiance can answer this one for you.

[sandman]

At my place of employment, I am responsible for maintaining a geothermal system that heats and cools over 140,000 sf of space.  I think the geothermal systems are nice, but I wouldn't want one for my home. 

Where I'd like to put my money is back into a woodburning system that won't have major problems in ten years, or else is easy to maintain.   I would also like a backup that allows me to shut off the wood boiler in summer.  I refuse to add another entire boiler to the system.   I do not want the backup system - the heat pump or whatever - to provide HEAT.  I only want it to provide domestic hot water in the summertime, for about two to three months max.  In the winter I will be using the outdoor wood boiler, whether it's the one I have or a new one.  HOWEVER, If for some reason there is nobody at home for a period of time in the winter, nobody to feed the OWB, I would be fine with the backup system providing some kind of backflow into the OWB, by a manual valve or switch, just enough to keep the water temperatures around 50-60 degrees and not allow anything to freeze until I get back. 

I think maybe I should be looking at this from another perspective?  Maybe we should assume that I already have an electric water heater (hybrid heat pump or conventional, doesn't matter) and it's my primary heat and hot water source.  Not farfetched - during the 1970's and earlier, this was common until energy costs went sky-high.  So now, I want to add the outdoor wood boiler and make IT my primary source of heat and hot water, and only use the electric unit during June and July.  So how do I make the system work?

propane or oil fired on demand hot water heater could doo what you need.

If plumbed correctly it could also heat parts of the house and or keep the owb water form freezing.

also solar hot water works pretty good in the summer and fall and depending on your need for hot water can even help heat the house.

Also a mini split ac with a heat pump works really good for heating in the fall and spring.

it also works really good to remove the humidity and cool the house down on a muggy night like tonight.

They heat pretty good down to about 18ºf

add storage to the heating system in the cellar if theres room.

something like 500-1000 gallons of super insulated water storage.

depending on your needs you can even incorporate hot water into this.

i can burn (inside wood boiler) for 12hrs and heat the house for 2-3 days depending on the temp outside.

in the summer i can have a 4-6 hr fire every couple of days to make hot water.

anyway you look at it there are a bunch of different options depending on the size of your house, your budget and how well its insulated.

Jim

[Nova]

You may have researched this already but if you haven't look up NYLE Geyser and see if that will meet your needs for hot water in the warmer parts of the year.  Unlike the hybrid units it is an after factory add on and if the hot water tank craps out the heat pump can be disconnected and then connected to the new water heater.  Can't speak to how fast they produce but if memory serves me they are wired to one of the elements/thermostat and the other is left in place for high demand assist if the tank gets pulled down and then once a certain temp is reached it switches back to heat pump only for the hot water tank.  It is an indoor unit so you will need a way to deal with condensate from the coils but if you have a floor drain that should do the trick otherwise you will need to be emptying containers.  If you run a dehumidifier in your house and your hot water tank is on the same floor as the dehumidifier you may be able to stop with the DH as the heat pump will do the same function.
As for outdoor heat pumps we have a couple that will produce heat down to about -25C (about -15F).  They will produce heat when it is colder but they are not designed to be primary heaters below the design temp and the fall off in heat production is noticeable when it gets that cold.  If you are willing to pay more there are systems that will function at even lower temps but price goes up accordingly.  Our systems are made by LG and we are very happy with them.  I am not an expert on this by any means but I have been told some manufacturers have design issues where the machine could be damaged by functioning below the design temps so do your research if you pick a machine that is not rated for your climate's coldest days.  By the way I am told by our installer this is not an issue for the LG.  If you are looking for more info pm me as my forays here are not as frequent as they used to be.

[brianJ]

For just domestic hot water in July and August and perhaps a decent portion of June and September,  my first goal would be to make it as simple as possible.   As simple as possible also corelates strongly with lower cost for a double win.    So with this midset I would choose a tankless water heater and splice it into the plumbing just after your boiler's heat exchanger.   Dont forget valve shutoffs and all good to go.