comparing a heat pump to a rocket mass heater

On this video



Somebody left this comment  (i got an email about it, but can't find it on the vid comments)

Does this offer any advantages over a heat pump besides cost of installation? Like it's still a bit less efficient than a heat pump no, even in freezing temperatures

All numbers are for an average montana home


mini split heat pump

professionally install 3 mini splits:  $10k
DIY install $4K
annual energy cost  $1326
annual carbon footprint 18.4 tons


in ground heat pump

professional install $26k
DIY install (NOT for DIY)
annual energy cost $606
annual carbon footprint 8.4 tons


rocket mass heater

professional install $7k
DIY install $1.9k
annual energy cost   $15
annual carbon footprint 0.4 tons



A heat pump will heat your home when you are away for two weeks in the middle of winter.  A rocket mass heater won't.


A rocket mass heater will heat your home when the power goes out for two weeks.  A heat pump won't.


Heat pumps will cool your home in summer, for a lot of money (and carbon footprint).  A rocket mass heater won't cool your home as much, but it will a little - and it will do it for free (zero carbon footprint).


Heat pumps last about ten years.  Rocket mass heaters generally last 40 to 100 years.

How does a rocket mass heater partially cool your home?

John C Daley wrote:How does a rocket mass heater partially cool your home?

I feel like that video was the equal to a mike drop.

It's funny that this should pop up as I just recently discovered this by testing it out myself last week when it was in the 90's during the day. I've been sleeping in my 150 square foot strawbale shop that has a 6" J and after I accidentally forgot to close the windows for the day, I came home only to discover it was stupid warm inside! Then the thought came to me, maybe as it cools the rocket stove will draft the hot air out! Not a new concept obviously, but i can attest to its effectiveness. The barrel and mass were exceptionally cool in the morning as compared to other nights when I had only had windows open.

Back to the original post, I'm glad you posted this Paul as we have the green homes tour in the Bitterroot coming up and one house as a heat pump vs another that has an 8" batch. I'm sure this will be a question.

It sounds like your $15 energy figure for RMH might cover costs for minimal purchases of power saw gas and oil by skilled and experienced operators, with no accounting for substantial wood felling and handling labor, even if the RMH wood biomass required is much less than other wood-fueled options.  How would the RMH cost numbers stack up for market rate local cash purchase of firewood for comparable output BTUs relative to those listed for a heat pump?  

I'm not sure if this is how you ran the numbers, but, in many cases, RMH fuelwood harvest costs over time could be accounted as timber stand improvement (TSI) , and thus production costs for future harvests of higher value forest products, so essentially zero cost for associated buildings heated by what might have otherwise been considered waste wood.  Carbon footprints could be near zero (carbon neutral, except for the fossil energy and lubricants to cut and transport the wood) over time with forest management where annual growth equals annual harvest.  Substantial net carbon could be taken out of the atmosphere where TSI or other factors stimulate forest biomass growth in excess of baseline conditions.

Many thanks,

Angus Murdoch
Kents Store, VA

I will be using minimum values.

Let's take 170 mm batch box that would be on the smaller side for the heater to be used in Montana.
It can be loaded with around 9 kg of firewood. Let's assume we fire it only 2 batches per day, so the weight of wood is 18 kg.
Let's keep doing it for four months = 120 days. So it is 2160 kg of wood.
A cord of wood weighs around 5000 pounds which is close to our weigh above. Cord of wood on craigslist in Billings area costs $150.

How much CO2 will be generated by clean burning of 2160 kg of wood? Let's assume that the firewood has 50% of carbon, so 1080 kg.
1080 kg * 44/12 = 3960 kg = almost 4 tons.

The values are exactly 10 times higher than presented in the listing.

In case someone is using more firewood for an average house in Montana then the values have to be adjusted.

I think the energy cost depends on the cost of firewood, efficiency of the RMH, cost of electricity, efficiency of the heat pump. I could do a back of the envelope calculation but it might not say much because the factors vary widely. The one with the lower energy cost may have as much to do with the particular installation and use-case.  

If you have cheap electricity and expensive firewood it may favor the heat pump.  If you have expensive electricity and cheap firewood it may favor the RMH.

Similarly, if you compare an inefficient heat pump to an efficient RMH, the RMH has cheaper energy cost. if you compare an efficient heat pump to a somehow inefficient RMH?, the heat pump could have cheaper energy cost.

A standard RMH seems very efficient though.  While an efficient heat pump may require an in-ground installation with water-to-water heat exchanger.

If you have to spend $26k to get a heat pump with a coefficient of performance (COP) of 5+ but a $4k-$10k mini-split only has a COP of 2.5-3-3.5? or whatever it is… see what I’m saying?  You’d have to consider how many years the heat pump systems would last.

Also have to consider the effects of the Department To Make you Sad.  In a (admittedly dinky) city, I could not possibly get a rocket mass heater installed.  However, heat pumps are a familiar item and easy to get up to code.  If you pair a heat pump with grid-attached solar as I have done, it is pretty smooth.  Alas, not all permies are off grid in a part of the country where being up to code doesn't matter.  Would I like to also have a rocket mass heater?  Of course!!!  But in my house it is not possible.    

Angus Murdoch wrote:It sounds like your $15 energy figure

We break it down here:

https://permies.com/wiki/204531/annual-operation-fuel-cost-types

The general idea is to try to think of montana averages.  Does the average home with a wood burning heat source have a few trees in the yard?  Most probably have wood shops that see some use.  Stuff like that.  After hours and hours of hashing through that sort of thing, we give each home that has a wood burning heat source, a half cord of wood for free each year.  

I'm reposting a link from the first post:

https://permies.com/wiki/204534/creating-heat-infographic

Mud and I (and others) have a couple hundred hours of work into these numbers and we cite our sources.  And we have a bit more work to do.  But there is enough data collected so far to start sharing some comparisons in this thread.

How much CO2 will be generated by clean burning of 2160 kg of wood?

I've seen the math on this performed several ways.  First, you have to eliminate the water in the wood mathematically, because it can vary so much.  In the end, the amount of CO2 is close to the weight of the wood.  Not exact - but close.  So a ton of wood results in, approximately, a ton of CO2.

Of course, the next step is, that if you are concerned about CO2, there is a quality of CO2 with respect to:  is it part of the natural CO2 cycle?  Or is it pulled from deep sequestration (fossil fuels) and then added to the natural CO2 cycle?  And then comes the factor of growing your fuel source in a way that you end up with a negative carbon footprint.  But these factors have not been considered in the numbers I present.

I think the energy cost depends on the cost of firewood, efficiency of the RMH, cost of electricity, efficiency of the heat pump. I could do a back of the envelope calculation but it might not say much because the factors vary widely. The one with the lower energy cost may have as much to do with the particular installation and use-case.  

Which is precisely why we went with numbers in montana - a place known for a cold climate.  And we explore many of the variables and many of the averages.

Erika Bailey wrote:Also have to consider the effects of the Department To Make you Sad.  In a (admittedly dinky) city, I could not possibly get a rocket mass heater installed.  However, heat pumps are a familiar item and easy to get up to code.  If you pair a heat pump with grid-attached solar as I have done, it is pretty smooth.  Alas, not all permies are off grid in a part of the country where being up to code doesn't matter.  Would I like to also have a rocket mass heater?  Of course!!!  But in my house it is not possible.    

We have not yet completed the part of the infographic about building codes, but rocket mass heaters appeared in the building codes for portland, oregon about twelve years ago.  And dozens of other building codes have adopted those bits in the years after.  At this moment, I would guess that more than half of all building codes would have some mention for rocket mass heaters.

The key is that wood burning stoves have been banned in a lot of places because of chimney fires - a problem that rocket mass heaters don't have.  Further, long term power outages can cause death, frozen pipes and massive property damage - something that rocket mass heaters can solve.  Therefore, more and more building codes are embracing rocket mass heaters.

Here is something I wrote about rocket mass heaters in an apartment

https://permies.com/t/174825/live-apartment-good-rocket-mass

Might be a small typo here. No big deal.

masonry heater
1.2 cords
0.5 cords is free
0.5 cords at $150 per cord
$180

Perhaps what was meant was:

masonry heater
1.2 cords
0.5 cords is free
0.7 cords at $150 per cord
$105

Only trying to be helpful, my apologies if it was nitpicky.

Mike Philips wrote:Might be a small typo here. No big deal.

masonry heater
1.2 cords
0.5 cords is free
0.5 cords at $150 per cord
$180

Perhaps what was meant was:

masonry heater
1.2 cords
0.5 cords is free
0.7 cords at $150 per cord
$105

Only trying to be helpful, my apologies if it was nitpicky.

Excellent catch!

PIE for you!

in ground heat pump
professional install $26k
DIY install (NOT for DIY)
annual energy cost $606
annual carbon footprint 8.4 tons

rocket mass heater
professional install $7k
DIY install $1.9k
annual energy cost   $15
annual carbon footprint 0.4 to..

Heat pumps last about ten years.  Rocket mass heaters generally last 40 to 100 years.

I feel there is significant bias with this comparison!
We have a wood burner for the coldest weather but burn very little wood. This is because we have lots of insulation- key for any home build project.
I installed our Ground Source system myself, never done anything like it before but have basic plumbing and practical skills, back in 2009. It never needs anything other than simple checks and I've once topped up the outside loop fluid pressure. So it's done 14 years already and I'm sure it's good for a lot more. All with virtually no maintenance and excellent comfort from the underfloor warmth. We also use "green" electricity so our carbon footprint is minimal.

An aside to this discussion, I want some of that $150 a cord wood. Here  in maine  we just paid $350 a cord for 5 cords of cut and split firewood. That figgute would surely skeew those numbers

A BIG variable:  I live in Eastern Washington. The desert portion.  Wood fuel is scarce, and not. Forests are about two hours away. On the other hand, I am surrounded by thousands upon thousands of acres of apples and other fruits. Every year, someone within five miles of me is uprooting a hundred acres of fruitwood to replace the crop it produced with the next, latest and greatest crop.  All that said, within a few miles of me are two hydroelectric plants (dams). On up the Columbia river are several more.  I have an 1,800 foot shop, which is heated and cooled via electricity. The shop and the house run off the same meter. A high bill is $120.00. Without doing anything in the shop, a high bill is $70.00.

Now, about the labor issue and being in your seventies and eighties.

All that aside, one car fighting with one pole, or a blown transformer, which happens during heavy loads (cooling/heating) can be a problem. Meanwhile, a rocket stove starts first time every time.

Mike Philips wrote:I think the energy cost depends on the cost of firewood, efficiency of the RMH, cost of electricity, efficiency of the heat pump. I could do a back of the envelope calculation but it might not say much because the factors vary widely. The one with the lower energy cost may have as much to do with the particular installation and use-case.  

If you have cheap electricity and expensive firewood it may favor the heat pump.  If you have expensive electricity and cheap firewood it may favor the RMH.

Similarly, if you compare an inefficient heat pump to an efficient RMH, the RMH has cheaper energy cost. if you compare an efficient heat pump to a somehow inefficient RMH?, the heat pump could have cheaper energy cost.

A standard RMH seems very efficient though.  While an efficient heat pump may require an in-ground installation with water-to-water heat exchanger.

Thanks for the comparison! We hope to use a rocket heater in a permitted LSC house. We have a draftswoman/designer, an energy calculator and a contractor - all sympathetic but with no actual experience building this. Our drawings are almost done but the issues of heating need to be resolved before we can file for a permit. Would you have time for a quick consultation?

Peace,
Steve

Steve where are you? I can help you design and build something permittable. Some states and cities are harder than others.

be inclusive, not so exclusive.......think worldly, not merely The Land. Answers have to address urban populations.

What about mold with conduction heat?  

The Teepee experiment at Wheaton Labs would produce enough heat for human habitation but would it be sufficient to drive away moisture from the canvass structure?

Going further with this thought; what about potential for mold in other kinds of builds whether traditional, cob, etc?

the only time I had a problem with moisture from an in-floor radiant heat/cool system was when I ran cold water through the floor to cool the house. Where there were exposed pipes the pipes would sweat from the condensation. All that was required was to insulate those few pipes. It certainly is nice to close the windows when there is smoke in the air from forest fires, and still have a chilled floor!

We finished the infographic

I thought I would add a plug for the benefits of a RMH efficiency—specifically CO2 emissions—as compared to a heat pump.

If one grows their own wood on a well-managed woodlot, the CO2 emissions can actually be negative.  Take my hedgerow.  Though I don’t have a RMH, I do maintain a hedgerow that gets heavily trimmed every few years.  I turn that wood to woodchips for mushroom fodder, but I could just as easily use that wood in a RMH.  The actual wood that is cut is carbon neutral.  But the hedgerow has a massive root system that keeps growing.  As the hedgerow regrows, carbon gets dumped underground.  The net effect is that the hedgerow (or woodlot) itself is carbon negative.

Now I don’t know exactly how to count the carbon dumped underground, but it does happen and that carbon does not get burned (or in my case, digested by mushrooms).

I thought this was worth adding to the discussion.

Eric

John C Daley wrote:How does a rocket mass heater partially cool your home?

I would like to see temperature logs in homes to show this point.      Perhaps two homes built to the same specs to prove this point one with rocket stove one without.

What is missing from this is size of home that is to be heated / cooled.     Unless one has a defined amount of space it does not reflect accurately the true cost to heat and cool the area, also the amount of insulation on the space would be key as well.


A DIY heat pump can be had for far less than $4,000.

https://hvacdirect.com/brands/mrcool/mrcool-diy-mini-split-systems.html
Right about $2,500  for an 18,000 BTU unit.


I bought my mini split for  $1,000       12000 BTU and installed it myself.


As for the cost of installing a rocket mass heater there is fire brick,   clay, and paying an expert to install it.     If you want to become an expert then there is the cost of going to an event to learn,  or at least the cost of buying the videos...      I guess there is the commercial rocket stoves one could go with if this is the case then the cost of these units should be the factor in how much the rocket stove would compare against other options.

rmh cooling

If this is for an average sized Montana home, what square footage would that be?       Also what is the cost to retrofit the home for the rocket stove, would it not require multiple rocket mass heaters  to heat  or  need  ventilation to move the heat to where it needs to go....?


I was watching this install, they installed the rocket heater in the basement ,   but fans were required to get the heat where they needed it thru the home..

I guess the best way to do this is to design the home  so it works with the rocket stove,    perhaps the rocket stove would feed a radiant floor heating for the thermal mass instead of the cob  thermal mass or bell siphon ....

 

Learning how to build a rocket mass heater ...   I offer the 8-movie set, the free heat movie and there will be another movie with the new kickstarter.

And here is this full length vid on youtube that is very thorough

Mart Hale wrote:If this is for an average sized Montana home, what square footage would that be?

     

I thought we did a pretty good job of plastering that pretty much everywhere.  You did not see it?  2000 square feet.

paul wheaton wrote:

Mart Hale wrote:If this is for an average sized Montana home, what square footage would that be?

     

I thought we did a pretty good job of plastering that pretty much everywhere.  You did not see it?  2000 square feet.

Sorry, I missed it,

So  per this chart would need .....

From ->  https://todayshomeowner.com/hvac/guides/what-size-mini-split-do-i-need/


What Size of Mini-Split Do I Need For 2,000 sq ft?
If you have a large open space (like an office or living room), consider a mini-split with at least 48,000 BTU per hour of heating and cooling capacity. These units will be able to cool larger areas more efficiently and effectively than smaller units.


Thus your $4,000   estimate would be in the ball park.....        

Advantage of Mini Spits is computer control,   and also removes humidity thus needing less power to cool the room.....      

Combining solar with mini splits make them more viable.       They have new mini splits that don't use freon, but they use c02  as the refrigerant.     Tested and work, here in the south, they combine them with a water heater  for hot water for showers.

Interesting to see firewood prices… I am sure there is plenty of variance around the country.

We just built an icf house since the wife wanted something more conventional than a wofati, and I built a Walker style brick bell and bench with a 6” J tube. We have lived in the house for one year now, and source our wood from an Amish sawmill about 8 miles up I-35. We DO have a propane wall heater for the coldest days if desired, but did not use it much last year.

So, last winter, we used about 1 1/2 “short bundles” of wood, trimmings up to about 4”x6” size and almost 4 feet long. Each bundle I would guess at about 1/3 cord of wood, and the cost is $5/$10 per bundle for soft/hard wood. We buy the hardwood bundles… I use an electric chainsaw to cut them in half for about 18-22” length pieces, and a kindling cracker to knock them down in diameter a bit.

Hence, about $15 plus some time and energy handling the bundles, a bit of fuel, etc. Heated our home plus probably a bit ($25-50?) of propane, mostly the pilot light… wife can start it easily with maybe 5 seconds of torch from a little green bottle and we run a fire some days twice to heat the brick mass.

Easy to clean and stays hot for hours after a cycle… we love it in Northern Missouri!