Need help with backup for outdoor wood boiler

Jon_E

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 (Amtrol) 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 wood 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.

I am looking for opinions on what might be a good backup system for me, without costing a huge amount of money and how I would configure it to my system. I have considered using the following so far:

1. Tankless-style electric heater, hooked up to the indirect tank supply line, so that when the incoming water temperature gets below a certain setpoint, the heater kicks on and heats the water going into the tank. This would obviously have no effect when the OWB is running, as the water temperature would already be high enough.
2. 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.
3. Install a full blown backup gas boiler in the basement and plumb it in to the main loop. This is not really viable for both cost and space concerns.
4. Something I haven't thought of yet.

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. I posted this on another forum but got nothing, so I'm hoping that a place dedicated to heating help would generate some feedback. Thanks.

Jamie Hall

The first question which occurs to me is... where are you? The relative benefits -- particularly when talking about a backup of heat pumps and LP gas fired units vary depending on how cold it gets.

My own choice, however, would be a high efficiency LP boiler, hooked to supplement the wood boiler. There are a variety of ways to do this, and a variety of ways to control it. I'd just abandon the old dual fuel burner in place...

hot_rod

Consider your various fuel costs. Also it may be easier to run electrical service for a HP compared to vent, lp line etc involved with a boiler.

It's tough to predict a stable LP price, right now upstate MI is at 88 cents a gallon, not too long ago it was over 4 bucks a gallon up there! electricity stays more stable.

A friend just went through a winter in upstate NY with an air to water heat pump, I'm anxious to see how it performed at those climate conditions, driving a panel radiator and radiant system.

The LP cost data is available and we know the efficiencies of LP fired appliances. With HP you need rated efficiencies of the unit, weather data and required SWT to pencil in operating cost. Energy in vs. energy out, at the end of the day.

Jon_E

I'm in southwestern VT, near the MA and NY border. I'd rather not be dependent on the propane companies here, as they are ruthless on pricing with low-volume users. The last time I bought propane, they tried to charge me over $7 a gallon for it, when the average local rate was just shy of $3. I like the relative stability of electricity.

hot_rod

Are you looking for just DHW backup, or the entire heating system?

You would want to perform a heat load calculation to accurately size a backup boiler system. with 4300 sq feet that may require a a large electric boiler. Do you have extra capacity in your breaker panel. It may require 80- 100 or more amp just for the boiler.

Depending on how ling you plan on staying, run some numbers. a new gasifier may run 10 grand or more installed properly with buffer, controls etc.

Do you have AC, or is that needed or desirable in your area?

Jon_E

I'm digging up an older post I made, for an update. I have just installed a brand-new gasification wood boiler (a Heatmaster SS G200) in place of the old Central Boiler that developed too many leaks. Now I truly have no backup. I am looking for a backup for the entire heating system. I just signed on to a new propane dealer as well, and they have a price I can live with. So, my choices for backup are either something electric (like the heat pump system I mentioned above) or a propane boiler. Either one has to be small (compact) and relatively inexpensive. I don't need a great deal of heating capacity, just enough to keep my house above 50 degrees if I happen to be gone during some part of the winter. If I lose power I'm screwed no matter what, but that's what generators are for. I will need DHW for the summer months - generally early May through late September, when there really is no heating demand and the outdoor wood furnace is shut down. I am leaning still toward the heat pump. If it is providing backup heat in the winter, then nobody is home and we won't be using hot water.

Jamie Hall

I'll go back to what I said before: I'd go for a nice high efficiency LP boiler. There are enough days where you live that drop into single digits or below that a heat pump is not, in my not very humble opinion, a viable option. They simply don't work well, if at all, at those low temperatures.

Jon_E

Any recommendations for a very small wall-mounted unit?

hot_rod

I'm not sure how you plan on using a heat pump water heater to provide DHW and heating?

Ideally you would run a heat load calc, determine what size equipment you need then decide on the fuel source. I'll bet you are looking at 100,000 BTU/ hr or more for that size home in your climate

Probably the LP boiler will be the most cost effective, considering the size of your home. You really should size the back up boiler to maintain a comfortable home, not just a 50° space. Someday you may need the back up to supply heat and DHW.

In most areas you can lock in a low LP price for the winter in summer or late fall, avoid buying a tank load mid winter or you pay top dollar.

Also, the heat pump water heater may not supply the temperature you require, even if you do separate heating from DHW somehow..

Zman

The tankless electric is a tricky one. Depending on your demands, you may not have the electrical service to support it. Any idea what your heat loss is?

Air source heat pumps take heat from the space and put it in the water. There is no advantage in the winter as you are just taking from Peter to pay Paul. You will end up with the same efficiency an electric water heater.

I like the boiler option better that the other 2. Could you put it in a remote location (shed) near your wood boiler?

Is the OWB running glycol or water? Does the backup need to heat the OWB to prevent freezing?

Jon_E

The OWB is running water, no glycol. Did not consider heating the OWB with the backup, but I think it would have to. At least enough to prevent freezing. The backup should probably stay inside.

The air source heat pump is being considered because 99% of the time it will be used only for domestic hot water in the summer. 'hot rod' has a good point, though - it's better for me to look at sizing the backup to provide 100% of the total load in winter, as a worst-case scenario.

hot_rod

That is the Catch 22 of OWF. If you let them burn out in freezing temperatures, guess what

Be aware, running your LP boiler to keep the OWF from freezing will cost some serious $$.

You basically turn the OWF into a "cooling tower" for the LP boiler. That large diameter vent pipe on the OWF will suck your energy $$ like a giant vacuum cleaner.

Once you install a water OWF you are committed. Your choices are to drain it or fill with glycol if you plan on letting it go cold.

Ironman

I suppose it depends upon the severity of your climate, but here in the mountains of VA, I've had no freezing issues keeping the ODWB loop on constant circulation and isolating the secondary house loop with an ETC that senses the SWT from the ODWB. If the water temp drops from the ODWB, the house loop circ stops preventing the indoor heat source from back heating the ODWB.

I've used this method for 15+ years, including my own house, with no problems.

For a reference point, we got down to 0* night before last, but I can see where some places would need antifreeze. Especially if the boiler was left un-fired for an extended period in below freezing temperatures. I have a couple of customers that glycoled theirs because they stay gone for days or weeks during the winter. It's just the prudent thing to do in those circumstances.

It is the catch 22.

Solid_Fuel_Man

What are your electric rates? We may be able to help you pencil out some numbers. Here in northern Maine, LP is ridiculously expensive for the small use customers too. I own two 100# tanks and they last me about 14 months.

Look into buying your tank(s) outright and determine if that will outweigh the savings in LP pruce. Then you are not chained to one supplier, like with oil you can now shop around. As well as have a large enough tank to last (possibly) several years of you burn wood the majority of the time. Then suddenly need to burn LP. This is the exact scenario I'm in as well. LP never goes bad with age as a plus.

jumper

Summer back up DHW? Heat pump hot water heater works well when it's warm. Back up heating? Simplest is electric radiant. Also helps main system when it's unusually cold.

Jon_E

So it's been a couple of months since I posted last, the new OWF is running well, but spring is approaching and I'm going to need to install a backup/summer system in April or May. I've decided on a high-efficiency mod/con wall-mount LP boiler, which needs to be sized to provide primarily DHW and very infrequent heating. However, I have no idea how to determine the size of the boiler I need. Online "calculators" seem to be hit-or-miss and I would like to run through some numbers before I start shopping. Can anyone help me?

Ironman

Go to SlantFin and download their free calculator.

Jon_E

Yet another update to my earlier posts. A few days ago I noticed that the outer shell of my indirect water tank (Amtrol) was discolored and bubbling. Turns out there are a couple of pinhole leaks. The tank is ten years old and now out of warranty. This gives me another option, I think. Would it be reasonable to consider installing a large (80-100 gallon) gas-fired water heater in place of the indirect tank, and use it with a heat exchanger from the OWF? That way I can run the OWF from September through May, shut it down for the summer and let the gas water heater take over. All it will be used for is DHW. In winter, in an emergency, I could get DHW and heat from the gas heater, and also backfeed the OWF (I know, big cost, no worries) to prevent freeze-up. Anything majorly wrong with this idea?

hot_rod

Do you have a heat exchanger already?

If not, consider a solar tank, with either gas or electric backup.

It would simplify the piping and put it all in one "box"

Bradford White has a good selection of solar tanks with backup..

Jon_E

I do have a heat exchanger - it's a plate style unit.

hot_rod

The plate HX provides DHW from the wood burner now? Or the Amtrol indirect tank was fired by the wood boiler?

You certainly would not want to use gas fired energy to keep the OWF warm, very often. The OWF is basically a cooling tower when not fired.

The constant circ of the OWF, disconnected from the heating load would use much less $$ energy, as Bob suggested above.

Larry Weingarten

Hello, Another thing to consider is that with a glass lined tank, you want to use the volume of the tank daily. If you don't, odor can become a problem as the tank turns into a breeding ground for bacteria. If you use a tank that does not require an anode, (non-glass lined) you don't need to worry about this so much.

Yours, Larry

Jon_E

My plate exchanger is in between the OWF and the Amtrol indirect tank.

Can I feed a gas hot water heater off the plate exchanger, just like I feed the indirect tank. I am aware of people using "sidearm" heat exchangers with OWF's to preheat water, I'm assuming any heat exchanger can serve that purpose including the one I have.

psb75

Get a Geospring 80 gal. Heatpump DHW tank that receives wood-fired, pre-heated supply water through an x-changer. Wood will heat your DHW most of the year and all summer will be electric heat-pump (cooling and dehumidifying basement air). There is a $600 rebate on that unit in VT. And go for the LP mod-con as backup boiler. Put glycol in your wood-boiler loop.

Jon_E

If I put in the Geospring unit (or any other electric hybrid heat pump), what's the reason for also installing the LP mod-con? I would imagine no more than one week in any given winter where I would not be running the OWF, and if that was the case, I wouldn't need any hot water and would only have to keep the house and the OWF loop about 45-50 degrees. Besides, I can only afford one or the other. Either unit will have to act as my backup. I am not sure I can put glycol in the wood boiler loop - can that be done in an open (non-pressurised) system?

hot_rod

It sounds like, from your first post that at some point you may abandon the wood completely.

If so a mod con would supply heat and dhw at a reasonable operating cost. The WH heat pump would only supply domestic water.

It's not often that electric boilers are cheaper to operate that HG or LP. There are free online calculators to compare various fuel costs.Compare your electric rates to typical LP or NG to determine the best fuel source.

Sounds like the system currently has a plate HX isolating the boiler from the piping in the house. So it could not be used for DHW production without an indirect or additional HX.

It's not advised to use glycol in an open system, as it cannot be exposed to O2, so that is not an option for protecting the OWF, it must hold 150 gallons or more?

GroundUp

Sorry to drag this up, but if you haven't gotten it taken care of already I'd like to offer my 2 cents. Yes, you can easily feed a gas fired WH tank via the plate exchanger from the OWB loop and allow the gas to take over during the warm season. If the occasional backfeed to the OWB is a concern, you would need a second circ on the WH tank to transfer heat from the WH back to the plate and have the constant circulation carry heat back to the OWB but this is not a good idea. Are there heated outbuildings also served by the OWB or just the house? If only the house, I would also lean toward the mod/con as mentioned above to heat the house (and DHW, if you stick with indirect) which will backfeed the OWB during vacation or whatever assuming the OWB circ is continuous. We do them this way all the time. I only got here from a search of the Amtrol indirect lol