what oven type for 100m2 central heating with hot water underfloor heating

hi guys,

been into rmh for a while and have a little experience with cob ovens. however thats not much good to me with this project as its quite ambitious. i'm looking to heat around 100m2 in this house via a central batchbox mass oven, with warm air ventilation to the rooms plus a small copper tubing hot water system for underfloor heating of a very small, but important space - the kitchen.

the oven will be located where the old roof meets the new flat roof and i'm guessing its gonna require some power. the house has been insulated recently but part of the floor will remain uninsulated, at least for the medium term, so there is heat loss to consider, but not from the roofs or the room where the oven will. still the oven needs to:

- radiant heat the room its in, very high 65m3 room
- warm air vent 3 rooms, ca. 100m3
- hot water underfloor heat a 25m3 room

winter lows are -6 at max. with many sun hours, and the south facing windows collect considerable amount of warmth which is stored in a 2 inch thick cob wall on the northern side. i have a bunch of very capable builders with me, so i'm hoping to save on costs by doing the construction myself.


one general question i have is how do oven builders calculate the required power output for a certain amount of m3 space?


well, the options i'm seeing right now are:

Uzume B14

seem to do great work. obvious advantage is the black baking oven. but i have doubts over the power output being enough? they dont offer a copper tubed hot water system, so the question would be how tricky it would be a add that myself, or have someone add it. also if i chose this, would i be well advised to sacrifice the bench in order to give more power to the other functions?

www.warmteopmaat.be

sent me info on their modell BrulR 1.5 which is a rocket batch box side winder with heatriser, diameter 150mm. 5KW and 93% efficiency. he offers an integrated copper tubed hot water system for underfloor heating in 3 sizes, of which i'm assuming the smallest would suffice. only disadvantages are the lack of a baking oven and a bench.

Hire someone else to make a design:

well, seeing as there will be some level of adaptation / individual design i'm gonna be paying some to help creat plans anyway. what would your advice be? and what options am i missing? what considerations am i missing?



how do you guys, in europe, see the development of governmental regulations on wood stoves? dont really feel like building an amazing 5 grand oven thatll be forced out of order in 10years. or would that be an argument for a prefabricated oven with very good exhaust specifications?


thank you all for your great work - more power to you!

brian

What is the heating at the moment? I would think the quickest way to guess how many kW you need is to see how many you are using at the moment. For example we have a 30kW output pellet boiler that runs around 30% most of the time so our house takes 10kW to heat with the outside at 0 and the inside at 20 using a mix of radiators and underfloor heating. However to heat the house downstairs (175m) from cold takes all 30kW for 4-5 hours.  we have poor insulation and a lot of heat loss from badly insulated pipes between the boiler and house, but also have south facing windows which help a lot, IF the sun shines  Your heating needs to be sized for the worst case so after a week of overcast weather at -6. my fear with a 5kw system is that from cold which it will be every time you let the fire go out for more than a day it's going to take you 6-7 hours to reheat the air and even longer to reheat the mass.

The official way to calculate is very complicated you need to know the R value of all walls roofs and windows which in practice is impossible to find on an old house especially if it's been retrofitted with insulation. I can find estimates online especially if you look at air to air heat pumps they estimate that a new/well insulated house requires 55w per m3 so guessing your house is 2.5m tall inside you would need 13kW to heat it.

Could this give you any ideas?

https://permies.com/t/44806/Cobbling-workshop-heater-cooktop-oven

Skandi Rogers wrote:What is the heating at the moment? I would think the quickest way to guess how many kW you need is to see how many you are using at the moment. For example we have a 30kW output pellet boiler that runs around 30% most of the time so our house takes 10kW to heat with the outside at 0 and the inside at 20 using a mix of radiators and underfloor heating. However to heat the house downstairs (175m) from cold takes all 30kW for 4-5 hours.  we have poor insulation and a lot of heat loss from badly insulated pipes between the boiler and house, but also have south facing windows which help a lot, IF the sun shines  Your heating needs to be sized for the worst case so after a week of overcast weather at -6. my fear with a 5kw system is that from cold which it will be every time you let the fire go out for more than a day it's going to take you 6-7 hours to reheat the air and even longer to reheat the mass.

The official way to calculate is very complicated you need to know the R value of all walls roofs and windows which in practice is impossible to find on an old house especially if it's been retrofitted with insulation. I can find estimates online especially if you look at air to air heat pumps they estimate that a new/well insulated house requires 55w per m3 so guessing your house is 2.5m tall inside you would need 13kW to heat it.

heating has been wood stoves so far, but its been out of use for almost ten years so i've no data to go by. i'm guessing people were simply very cold-tolerant. :-D what i'm getting from you is that 5kw might be marginal and i tend to have the same feeling. especially since the oven is meant to server various purposes. i'm looking at the B28 batchbox from uzume at the moment, which gives more than 8kw from two firings.
what would the disadvantegs of a bigger system be? i'm struggling to see it, other than size and space isnt an issue in our case. bigger simply means i have to fire it less often right?


how does the theory work in terms of how much mass is ideal? is it a simple trade off of direct heat vs stored heat? in other words, what speaks against adding more and more mass, other than the obvious fact it takes longer for the heat to dissipate into the living areas.


and whats the general knowledge on convection heat (hypocaust) vs radiation heat (like hot water panels in the walls)? i'm assuimg in a well insulated house, it makes little difference. what about older badly insulated houses, is it generally a bad idea to convection heat such a house, i.e. wasteful and expensive? in my case the convection heat would go to the top floor, newly built and well insulated rooms. or is it likely more efficient and cheaper to install wall heating via hot water and radiate the heat.


thanks a lot!

brian haitz wrote:

Skandi Rogers wrote:What is the heating at the moment? I would think the quickest way to guess how many kW you need is to see how many you are using at the moment. For example we have a 30kW output pellet boiler that runs around 30% most of the time so our house takes 10kW to heat with the outside at 0 and the inside at 20 using a mix of radiators and underfloor heating. However to heat the house downstairs (175m) from cold takes all 30kW for 4-5 hours.  we have poor insulation and a lot of heat loss from badly insulated pipes between the boiler and house, but also have south facing windows which help a lot, IF the sun shines  Your heating needs to be sized for the worst case so after a week of overcast weather at -6. my fear with a 5kw system is that from cold which it will be every time you let the fire go out for more than a day it's going to take you 6-7 hours to reheat the air and even longer to reheat the mass.

The official way to calculate is very complicated you need to know the R value of all walls roofs and windows which in practice is impossible to find on an old house especially if it's been retrofitted with insulation. I can find estimates online especially if you look at air to air heat pumps they estimate that a new/well insulated house requires 55w per m3 so guessing your house is 2.5m tall inside you would need 13kW to heat it.

heating has been wood stoves so far, but its been out of use for almost ten years so i've no data to go by. i'm guessing people were simply very cold-tolerant. :-D what i'm getting from you is that 5kw might be marginal and i tend to have the same feeling. especially since the oven is meant to server various purposes. i'm looking at the B28 batchbox from uzume at the moment, which gives more than 8kw from two firings.
what would the disadvantegs of a bigger system be? i'm struggling to see it, other than size and space isnt an issue in our case. bigger simply means i have to fire it less often right?


how does the theory work in terms of how much mass is ideal? is it a simple trade off of direct heat vs stored heat? in other words, what speaks against adding more and more mass, other than the obvious fact it takes longer for the heat to dissipate into the living areas.


and whats the general knowledge on convection heat (hypocaust) vs radiation heat (like hot water panels in the walls)? i'm assuimg in a well insulated house, it makes little difference. what about older badly insulated houses, is it generally a bad idea to convection heat such a house, i.e. wasteful and expensive? in my case the convection heat would go to the top floor, newly built and well insulated rooms. or is it likely more efficient and cheaper to install wall heating via hot water and radiate the heat.


thanks a lot!

I was probably living in "your" house the last few years, solid fuel heating to radiators, yes it works yes it takes very little wood but YES it is cold!
We once had a very oversized coal fired heating system and the problem there was it was impossible to keep it burning at a low enough rate for the house it was in, so most of the heat went straight up the chimney.
from what I have read underfloor heating runs on colder water than radiators but runs for longer and is more efficient (up to 40% when using a low heat input) . however upstairs underfloor heating has no mass so it may behave more like a radiator.