HVAC Using the thermosyphon effect for a floor water heating system

[anorlunda]

load them into the stove feed using a tractor

The auto feeder will be a challenge as well.

The external boilers I've seen allow 24 hours supply of 6' logs to be loaded at once with a tractor. That eliminates the need for an auto feeder. Remember the KISS principle.

The radiant heat is a separate problem. The logical way to proceed is

1. Determine the heat load of your house in all conditions to determine the heating system requirements. As part of that, you may consider different insulation options.
2. Design the radiant heat system, and from that determine the requirements of the external heat source, power, flow rate, temperature, etc. Circulation pumping is part of that.
3. Select the external heat source, boiler/water-heater/furnace that meets the requirements.

 

[jbriggs444]

Heatpump (air - air or air - water) doesn't work well with outside temps below 15C (5F).

You seem to have dropped a minus sign on that 15C. Possibly that is why you had an objection to the apparent claim about COP=1 at 15C.

 

[petterg]

You seem to have dropped a minus sign on that 15C. Possibly that is why you had an objection to the apparent claim about COP=1 at 15C.

You're right. At least I got the 5F right.

The external boilers I've seen allow 24 hours supply of 6' logs to be loaded at once with a tractor. That eliminates the need for an auto feeder. Remember the KISS principle.

The radiant heat is a separate problem. The logical way to proceed is

1. Determine the heat load of your house in all conditions to determine the heating system requirements. As part of that, you may consider different insulation options.
2. Design the radiant heat system, and from that determine the requirements of the external heat source, power, flow rate, temperature, etc. Circulation pumping is part of that.
3. Select the external heat source, boiler/water-heater/furnace that meets the requirements.

I started with 1, Calculated that new 3-glass windows will save 10% on the heating bill. Material cost will make this pay back in 225 years at the current electricity cost, with 0% interest rate and cost=0 for the work. And it won't keep the house warm when power goes out.

To dimension this for electrical heating just to calculate how much is required of the stove seems like a waste of work and money.

I'd rather go the other way around - build a small scale stove for the planned green house (planning to build) and/or for the barn (planning to make a workshop in there). With that experience, I hope to be able to build it right for the house.

 

[anorlunda]

With that experience, I hope to be able to build it right for the house.

How will you know it is right for the house plus greenhouse?

Prudent engineers always work in this order.

1. Requirements
2. Design
3. Construction
4. Operation.

You're free to do it any way you want, but don't expect engineers to advise other than requirements first.

 

[petterg]

Requirement is 7kw continues inside the house. Which means the stove will need to be 7kw + waste
Design is probably rocket mass heater or similar
Construction will follow some plans for rocket mass heater (unless something better turns up). There are lots of plans out there.
When a green house / barn model is build, I will have an idea of what needs to be improved.

Installing the floor tubing is a project that needs to be done step by step. One floor at the time. As thermosyphon effect seems to not do the job, floors can be designed as regular heat pump driven floor heating. That is a topic where there are lots of local knowledge. Whats experimental here is how much energy the pump will need - so I may have to upgrade the pumps battery.

 

[Spinnor]

Would you cut and split the firewood for fun and exercise? My back tells me not to anymore and our firewood piles are shrinking. We could heat our house nicely without supplemental heat with our single wood stove (southeast Pennsylvania, house built in 53, maybe some insulation in walls) but lately I let the oil furnace do more and more of the heating.

If you don't need the exercise a backup generator might be cheaper in the long run?

https://www.google.com/search?q=bac.....69i57j0l5.4639j0j8&sourceid=chrome&ie=UTF-8

Did you tell us what is the main source of heat for your home currently?

 

[jim hardy]

Would you cut and split the firewood for fun and exercise?

I did until a month ago.
Fair Anne has complained for years about the dirt and leaves i tracked in

so i plumbed propane up to the fireplace and set one of those radiant heaters in front of it.
I am far happier with it than i expected - so after this heating season i think i'll change to gas logs inside the fireplace.

 

[Spinnor]

so i plumbed propane up to the fireplace and set one of those radiant heaters in front of it.

A few years back oil was relatively inexpensive, it still is not so bad. I burned a lot less wood then choosing to burn the less valuable fuel, heating oil.

 

[Borek]

The idea of placing the stove 16 m from the house means 32 m of tubes that have to be isolated to minimize heat loses, and need to be somehow protected from freezing and rupturing.

Just another factor to consider.

 

[anorlunda]

We used to have a relatively small and old house with poor insulation. The wood stove seen in this New Years Day picture was our only heat. It was a very comfortable and pleasant form of heat. When the temperature approached -40, it was not enough heat, so I would load it up with coal and that did the job.

But you need to put on extra blankets at night and get up to a cold house in the morning. My part of the marriage was to start the fire in the morning before my honey got up.

 

[anorlunda]

The idea of placing the stove 16 m from the house means 32 m of tubes that have to be isolated to minimize heat loses, and need to be somehow protected from freezing and rupturing.

Yes, the pipes need to be buried beneath the frost line. That's true with all outdoor water piping including your city water or well water service.

 

[Spinnor]

My part of the marriage was to start the fire in the morning before my honey got up.

Hopefully her part was to get in bed at night first and warm it up. I think I sleep better in a cool bedroom.

 

[Spinnor]

Another potential problem with such system would be the resale of the house. In our neck of the woods for someone to get a mortgage for a house that house must meet certain minimum standards, your system may not and potential future buyers of your house might be hampered in getting a loan, just a thought. Also a future owner of your home might not want to deal with such a system, it might be a deal maker or breaker. It sounds like you are interested in doing work on your house, maybe time and thought spent making the home more energy efficient is a good project? If you do use a chainsaw and cut up a bunch of wood get some chainsaw chaps and all the other proper equipment.

 

[Rive]

Another potential problem with such system would be the resale of the house.

As one who actually bought a house built with just a bit more creativity and bit less knowledge than ideal I have to agree.
All the homemade creative kludging feels tempting, but at the end the point when you finally need a handbook for your own house to keep it working is the point when it gets the title of 'need immediate and full restoration' on the market.
Nothing makes people respect simple and regular solutions more than living in a 'kludgehouse'.

 

[anorlunda]

kludgehouse

I like that word.

 

[OmCheeto]

I like that word.

Me too!

I'd never heard of "kludge" before. Did you also look up its history?

Merriam-Webster; "The first recorded use of the word kludge is attributed to Jackson W. Granholm, who defined the word in a 1962 issue of the magazine Datamation as: "an ill-assorted collection of poorly-matching parts, forming a distressing whole." He further explained that it was derived from the German word klug, meaning "smart" or "witty." Why Granholm included a "d" in his spelling is not known. What we do know is that speakers of American English have agreed to keep it silent, making the vowel pronunciation of kludge reflect the pronunciation of German klug (KLOOK). We can also tell you that not everyone agrees with Granholm on the "d" matter: the spelling "kluge" is also popularly used."​

Seems like adding too much "klug" stuff together makes "sludge", = kludge?

Anyways, I'm working on a similar problem, and have solved all the simple stuff, inserting my constants and variables where none were given.

House loses 160 watts/∆°C (common to both houses)
∆T = 44°C (OP, worst case. T_hot: 20°C, T_cold: -24°C)
power required = 7000 watts (OP, worst case)
area of floor = 84 m^2 (my house)
required temperature of floor = 34°C (my house in Norway)​

But I'm afraid my knowledge of thermodynamics & combustion of wood are too weak to be able to handle the details of this problem.

Example:

A $15,000 Woodmaster 6500, fully loaded, would "theoretically" heat my house in Norway for 2 weeks. But it's only rated for 12 hours. So this raises the question: How slowly can you burn wood?

According to this news account; "Trees can retain burning embers throughout the winter, she said, through snow and ice."

Ok then. A novel, well designed wood burning outhouse will work.​

But from here, as before, I'm finding the maths involved to be a bit tricky.
I'm used to thermal conduction solutions where the specific heats of the conductors are ignored.
But once I include them, I get quite confused.
I'm guessing this is a partial differential equations type thing.
Never did like them.

Prudent engineers always work in this order.

1. Requirements
2. Design

...

I'm thinking we may have different definitions of "Design".
My definition would be; "Solve all the maths involved, down to the last partially differentiated detail."
But, I may be overthinking the problem.

Here's another problem I haven't worked out:

What's the optimal way to not have hot spots on the floor?
I'm guessing the initial high differential temperature at entry is going to resolve itself because less heat is going to flow when that section of floor mass warms up.

 

[Rive]

How slowly can you burn wood?

The efficient solution is to burn it very fast at high temperature and add a buffer to store the heat. For an internal stove it can be a big piece of masonry, for external ones a big bucket of water with some heat exchangers. For an efficient combination of stove and floor heating, these kind of buffer tanks are necessary.

Ps.: around the same link some pointers can be found about floor heating limits, control and pipes.

 

[OmCheeto]

The efficient solution is to burn it very fast at high temperature and add a buffer to store the heat. For an internal stove it can be a big piece of masonry, for external ones a big bucket of water with some heat exchangers. For an efficient combination of stove and floor heating, these kind of buffer tanks are necessary.

Ps.: around the same link some pointers can be found about floor heating limits, control and pipes.

I calculate that we need a 2700 liter (700 gallon) tank for a 24 hours period. (7000 watts * 24 hours)
Kind of big, but the https://1q11xr3but6322f0iz1d1vp6-wpengine.netdna-ssl.com/wp-content/uploads/2015/01/24397_15-WM-Classic-BRO-Web.pdf has a built in 1900 liter (500 gallon) tank. So, I guess if the experts do it, it must be necessary.

 

[Svein]

I am surprised that no one has mentioned the problem of the "heating water" freezing whenever the fire goes out (sickness, vacation etc.). My house is relatively new and we have done everything by the book (heavily insulated, using a rock-to-water heat exchanger etc.) and we still use a frost-free liquid (glycol and water) in the system.

BTW: Retrofitting an old house with floor heating is more expensive than building a new house (I know, because I got a quotation in 2007). We built a new house instead.

 

[Borek]

I am surprised that no one has mentioned the problem of the "heating water" freezing whenever the fire goes out

Don't forget antifreeze in the system, otherwise you can't leave for a couple days without draining the whole system.

I believe that's not the only time it was mentioned.