Outdoor Wood Boiler with Cast Iron Radiators inside and an auxilary water heater

TheRadiator

I've been reading all posts regarding OWB and radiators but I have still some questions that may be unique to my setup.

I have connected 7 old cast iron water radiators with steel pipe with a two-pipe system in a closed loop, looking to pressurize it at 15psi.

Here's the more fragile part of my plan that I am not so sure about:

I want to put a 60 gal water heater into the radiator system, to act as a thermal storage but also as an auxillary heating source. (I want it to maintain a temp of at least 85F ~ 30C)
This closed system will be connected to the open OWB with heat exchanger plates.

1) Will the water heater be able to keep up or nada? (1500 sqft home, unfinished basement, average insulation, Canadian Prairies)
2) Or, should I put the water heater in the OWB circulation instead? (reciculating and keeping out cold water if OWB is off)
3) What size of a heat exchanger am I looking at to be safe? (With the OWB on, I want the radiators to be at 180-185F ~ 80-85C)

I am so afraid that it might not work out as hoped for... I really want this to work well and get it right the first try, and willing to abandon any bad ideas / pick up new ones (maybe the water heater idea is stupid altogether and I should raterh get an adequate electric water boiler of some sort.. maybe I don't need it at all and I am better off directly connecting the OWB to the radiator circuit with plate heat exchangers etc.)

Hot_water_fan

You need a pro here. There's a whole lot to consider and right now, you're headed down a bad path.
1. Wood boilers need a lot of thermal storage to be efficient. Like 1000 gallons. 60 will not cut it.
2. OWB perform poorly vs. wood gasification boilers.
3. You need to figure out your true heat loss. What was the original heat source? How much fuel did you use before?
4. Wood boilers are best paired with low temperature emitters (back to the storage point). That could be your cast iron if it can meet your heat loss at a lower water temperature. A design temp of 120 would be excellent. An outdoor reset will help.
5. A tank water heater is a bad backup system.

Here's a good resource but also hire help:

https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_10_0.pdf

Jamie Hall

Well, the fundamental problem is heat loss -- which you really need to figure out (Slant/Fin has a very good calculator for that, here: https://www.slantfin.com/slantfin-heat-loss-calculator/).

That said... Canadian Prairies ("Four Strong Winds...") can, if memory serves, get a bit chilly, so a wild guess would be you will find you need somewhere around 40,000 BTUh to stay warm -- maybe more. Most electric water heaters have a maximum output of around 15,000 BTUh -- and aren't really meant for continuous duty and certainly won't reach the circulating water temperature you want (most are set to turn off at 140 F -- 60 C). The water heater you propose will act as thermal storage, which is no bad thing, but it's heat output is likely to be no more than just enough to take the edge off -- it won't come close to keeping up with demand if the wood boiler is off on a cooler, never mind cold, day.

If you are reasonably certain you can keep the wood boiler going, I'd couple it to the pressurized system with a good heat exchanger -- and I would provide a nice big, well insulated atmospheric tank (maybe inside the heated envelope, so the heat it loses isn't wasted) to serve as both a buffer and as a heat dump tank. If you have some concern that there may be occasions when you won't be able to keep the wood boiler going, then I'd look into adding a properly sized electric boiler to the indoor pressurized circuit, and setting its controls so that it only came on when the wood boiler wasn't taking the load.

leonz

What brand of forest eater do you own???

What you want to do will not work well at all as you need much more thermal mass.

You would be money and aggravation ahead investing in a large used insulated bulk
milk tank to do this rather than use a water heater.

You would use a small circulator to provide a balanced temperature between the
forest eater and the bulk milk tank and use the existing circulator for the radiators.


On question one its a big no period.

On question two having and using a used bulk milk tank will save you work as you can use lower temperature water to do your heating this and have a faster recovery time with the forest eater as long as you do not run out of wood.

Refer back to paragraph three and then visit the garn web site to learn more about thermal storage.


www.garn.com

TheRadiator

Great answers.

Other calculators gave me a BTU of 50,000. I'll check today with the Slant/Fin app.

The original heat source was the OWB, it is an old 30yo model and I would guess around 30-40% efficient. We burned through 8 cords last year. The newer part of the house has hydronic in-floor heat (there doesn't seem to be a lot of heat escaping through the wood flooring though, so I think I can get more heat from cast iron radiators). The old part of the building was heated with forced air only.

Right now, I will probably hook up the OWB directly to the readiator circuit with a good heat exchanger, and keep the boiler water flowing through the hydronic floor system.

Leaving out the water heater for now and adding large storage tanks later, if I can find any that can be brought down into the basement.

Hot_water_fan

The newer part of the house has hydronic in-floor heat (there doesn't seem to be a lot of heat escaping through the wood flooring though, so I think I can get more heat from cast iron radiators).

Is the newer part of the house maintaining the thermostat temperature? If so, the radiators won't do anything in this space. Radiant floors shouldn't feel warm in new construction.

TheRadiator

Hot_water_fan said:

The newer part of the house has hydronic in-floor heat (there doesn't seem to be a lot of heat escaping through the wood flooring though, so I think I can get more heat from cast iron radiators).

Is the newer part of the house maintaining the thermostat temperature? If so, the radiators won't do anything in this space. Radiant floors shouldn't feel warm in new construction.

There is no thermostate installed, it is all at the whim of my firing of the boiler. Would you suggest to make improvements on this system and utilise it better? I cannot say much about the goals of the design of the infloor system, as the previous owner probably did it himself and there's a chance that it hasn't been set up quite right.

Hot_water_fan

There is no thermostate installed, it is all at the whim of my firing of the boiler. Would you suggest to make improvements on this system and utilise it better? I cannot say much about the goals of the design of the infloor system, as the previous owner probably did it himself and there's a chance that it hasn't been set up quite right.

If the room with the radiant floor is comfortable (once it runs for a bit), then the radiant floor is fine output wise. I'd talk to a local expert here, there's a lot going on. You could have a good system using wood (less labor) with a better wood boiler or switch to another fuel or live with it.

TheRadiator

The HVAC guy I spoke to said I should add an electric boiler (they've done this for over 30 years). If I want to, I can get a model with more BTU than I need, pull a few elements out to meet the demand and keep the spares.

To make the cast iron radiators more efficient, I will have to heat them at a higher temperature, towards 80C.

We'll have someone take a closer look at our house and whether that demand can be met by the wood boiler and plate heat exchanger.

Feeling a bit more confident now that things are going into a better direction.

Hot_water_fan

An electric boiler will be really expensive to run. You may as well install electric baseboard, they'll be more reliable and redundant.

Heating the radiators to a high temp will not make them more efficient. Who told you that?

TheRadiator

Hot_water_fan said:

Heating the radiators to a high temp will not make them more efficient. Who told you that?

I found it right here: https://inspectapedia.com/plumbing/Electric_Hot_Water_Home_Heating.php

"The thermal conductivity of finned copper baseboard, or of cast iron radiators, is exponentially greater at higher temperatures."

Electric baseboards aren't my favorite, and we already have the cast iron heaters in. The electric boiler is a backup only if the OWB cannot keep up, hence I think it may be fine, no?

Hot_water_fan

That was misleading advice in the link.

Please read the Idronics series.

The amount of heat given off by a radiator increases as temperature increases, but efficiency doesn't increase. Often it will decrease because the boiler has higher losses, and for some boilers efficiency goes down for other reasons (gas ones can't condense, air-to-water heat pumps have higher lifts, etc.) You'll want to use the lowest possible temperature at all times. That temperature changes based on the outdoor temp. Higher outdoor temp = use lower water temp. Newer boilers gain efficiency by using lower water temperatures. It matters for wood boilers because lower temperatures mean you can store more heat with the same size tank and firing a wood boiler at a high heat rate is the most efficient (both for fuel costs and your time).

You'll continue to use the wood boiler? The electric boiler will be expensive to operate and unless you're keeping the option of wood, I'd switch to baseboard or heat pump for the forced air. If just using the electric boiler, you'll have the most expensive heat source with 0 redundancy and high complexity.

All-in-all, this is a poor situation for a hydronic system (low load, using a hydro coil, no zones, no backup) and both electricity and wood (with an outdoor boiler) make poor fuel sources, so I don't want you to sink any more money into this unless you're certain.

TheRadiator

Thanks for pointing that out to me.

Thank you.

Yes, I was going to continue using the wood boiler, and lower temperatures required make things easier still. I will have someone come over and see it all in detail, and let you know.

Jamie Hall

Electric boilers are a very good approach to heating in certain situations. Alberta and Saskatchewan are two places where they make pretty good sense. Also Quebec. Oddly enough, I'm rather a big fan of them -- where the electricity rates are reasonable in comparison with other fuels, particularly if there already is a hot water or steam system in place, or if they are used in conjunction with another source of heat, such as a wood boiler might be.

Which is the situation here on both counts.

They can modulate to fit the load very precisely, with no or minimal loss in efficiency, using inexpensive but robust solid state controls. They can fit in tight spaces. They require no venting. They do benefit from some maintenance, but with decent water treatment it's minimal. They can be adapted to high temperature hot water, low temperature hot water, in floor radiant, snow melt radiant, even steam.

What's not to like? Only the price per unit energy -- and if the electricity rates are competitive...

As I've said before, in the heating trade the very first essential is to keep an open mind as to what approaches are available and what approach will fit the specific situation in the best way for the client.

TheRadiator

Jamie, I would love to hear more about the solid state controls, do you have any examples?

Jamie Hall

TheRadiator said:

Jamie, I would love to hear more about the solid state controls, do you have any examples?

No, honestly, I don't have any commercial examples for you. And considering my recent rant on custom controls I should probably shut up! But, being me... you are looking at a resistance load on AC, and there couldn't be anything easier to control. The control would be exactly the same as a solid state light dimmer -- an SCR device -- but instead of being controlled by a hand turned rheostat, the control signal would be from a thermistor and set point selection circuit.

Jamie Hall

A bit of further research on that. Theatrical lighting dimmers have ample capacity, and a number of them now are electronically controlled, usually a 0 to 10 volt signal. Matter of rigging up a thermostat circuit...

TheRadiator

Hot_water_fan said:

You'll continue to use the wood boiler? The electric boiler will be expensive to operate and unless you're keeping the option of wood, I'd switch to baseboard or heat pump for the forced air. If just using the electric boiler, you'll have the most expensive heat source with 0 redundancy and high complexity.

All-in-all, this is a poor situation for a hydronic system (low load, using a hydro coil, no zones, no backup) and both electricity and wood (with an outdoor boiler) make poor fuel sources, so I don't want you to sink any more money into this unless you're certain.

Do you have any other ideas for alternative heating sources that would also work well with cast iron radiators?

As much as possible, I'd like to make use of the cast iron radiators and eliminate forced air altogether (the main complaint I have with them is that they are loud, and make allergies worse. When they are on, the air in our house becomes significantly more polluted).

Because the basement is unfinished, I could install hydronic heat in the old part of the house as well.

Furthermore, I'm not sure how well air-to-air heat pumps will work in our climate, as we regularly have winters of -38C, so geothermal heat pumps would be the only option.

Jamie Hall

What are your electric rates? Am I mistaken that they are quite reasonable?

My recollection is that they are, and you should compare the cost of various ways of firing up your system. I will stick with my thought, though -- electric boiler. No, it isn't a really great option in most of the US -- but most of the US doesn't get down to -38C and stay there, nor does most of the US have relatively low electric rates.

The only heat pump options you can use, despite the enthusiasm of some, would be ground source.

TheRadiator

We are currently at CAD 8.983¢/kWh from Manitoba Hydro.

Jamie Hall

An electric boiler and an oil boiler just about break even for you. LP would be considerably more expensive.

Hot_water_fan

Electricity seems like the way to go! My hesitation with the electric boiler would be that you gain little by having an electric boiler vs. electric radiators/baseboard. It's the same efficiency (minus some distribution losses with the boiler) and if any part of the system breaks, you have no heat. If one electric baseboard breaks, probably have 6 others that work. Electric baseboards can be controlled individually as well. You can do that with a boiler, but it'll be extra.
So you'd:
1. increase complexity
2. decrease redundancy
3. decrease controllability

using the electric boiler vs. electric baseboard/radiators.

TheRadiator

I see what you're saying.

I can wire up one large baseboard heater I have lying around. They are cheap, and installing one won't take much effort, maybe I can install a few more.. (I will have to talk that one over with the wife).

Regarding the cast iron heaters, I will hook them up to the wood boiler through plate heat exchangers, and I will also look out for large storage tanks. Maybe I don't need an electric boiler, and even if I put one in later, having added two or three baseboard heaters can't hurt.

Thank you very much.