Hot Water Radiator System Design

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Old 01-08-12, 09:15 PM
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Hot Water Radiator System Design

Hello Everyone!

I need some help, please.

I'm installing four antique hot water radiators into an attic I'm renovating, but I can't find any reliable information on pipe layout and design. I need four 38" high x 18" wide x 8" deep rads which I'm sourcing now. What I don't know is how best to hook them into a system. I have 170F water coming into the basement two floors down from an outdoor wood boiler. I need to bring supply and return pipes up through two floors to the attic, run these pipes through the floor joists of the attic, and connect them to the rads. Two rads are side by side (6 feet apart) at one end of the attic, and two more at the other end of the attic 40 feet away. Here are my questions:

1. Should I connect these rads to a system plumbed in parallel?

2. Is 1" diameter pipe large enough for the supply and return branches of the system?

3. What size pipe should I feed the rads with?

4. Is thermostatic control just a matter of turning the circulating pump ON and OFF via a wall thermostat?

5. Does this system need to run under pressure as a closed system, or can it work under zero pressure via a direct connection to the outdoor boiler (which is vented to the air)?

6. Since I'm working in an existing structure, I'll need to be snaking pipes in and around floor joists and such. Flexibility is an advantage. Can I use PEX? Do I need to go with PEX-AL-PEX? Will I need to use steel pipes for connecting to the actual rads, for physical strength and anchoring power?

7. Will I need to install valves to allow me to balance the flow o hot water through the rads after installation?

8. What's the best hardware to install for bleeding air from these rads?

I have lots of experience plumbing, and some experience installing radiant infloor heating systems. but none installing hot water rads from scratch. There isn't any clear info on the web I can find, either. Is there an expert who can point me in the right direction?

Thanks very much for your help,

Steve
 
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Old 01-09-12, 05:34 PM
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If the attic is all one room, then I wouldn't worry about balancing or parallel piping. A series loop would be fine.

You should do a heat loss calculation of the attic. That will answer a lot of questions for you. A 1" pipe is probably way more than you need, but the heat loss will tell the story.

You need to use oxygen barrier pex. The stuff they sell for domestic hot and cold water is not sufficient. They do make oxygen barrier pex that does not have the AL layer.

Get these for bleeding: How to Bleed a Hot Water Radiator and Clear a Steam Radiator Vent | The Family Handyman My local Home Depot sell them. They have a small hydronic section with zone valves and pumps. Don't forget to get a a radiator key with them.
 
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Old 01-09-12, 06:14 PM
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What else is running on this wood boiler? heating the downstairs also?

Droo makes a good point about the heat loss. I'm thinking that a 40' attic space sounds HUGE.

How much heat loss will the attic have?

Are the four rads that you are sourcing going to have enough BTU output to heat the space?

1. Should I connect these rads to a system plumbed in parallel?
It depends... piping in parallel, as long as it's done so that you have equal flow through all radiators would maximize the heat output because they will all get hot water... as opposed to in series, the water will progressively cool as it passes through each rad... the last rad on the loop will have less output because it's getting cooler water.

You need to do a heat loss and know the BTU output of your rads.

2. Is 1" diameter pipe large enough for the supply and return branches of the system?
3. What size pipe should I feed the rads with?
Depending on the heat loss, and how much heat you need up there, you may be able to get by with 3/4" tubing... that will get you a bit less than 40K BTU available... going to 1" will let you flow more water, and will be able to supply 80K BTU. I highly doubt that the heat loss of that attic space will exceed 40K.

You need to do a heat loss and know the BTU output of your rads.

4. Is thermostatic control just a matter of turning the circulating pump ON and OFF via a wall thermostat?
No. It depends on how the rest of the home is set up as far as 'zoning' goes. Are the existing areas controlled by electric zone valves? or does each zone have it's own pump?

The thermostat in the attic area would have to be able to a) fire the boiler, and b) circulate water.

You need to know how the existing setup is designed and follow suit.

5. Does this system need to run under pressure as a closed system, or can it work under zero pressure via a direct connection to the outdoor boiler (which is vented to the air)?
Since the wood boiler is an 'open' system, meaning not pressurized, it will be open to air. Water plus oxygen quickly rust ferrous metals. If those are C.I. rads you're installing, I don't think it's a good idea to run it open.

You will need to pressurize the system and take the heat from the wood stove through a 'heat exchanger'. One side of the HX would accept the open system from the wood stove, the other side would be pressurized.

Pumping water UP into an open system requires a HOG of a pump! Think 'electric $$$'
 
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Old 01-10-12, 05:55 AM
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Thanks very much for your help. Your answers have been useful indeed. I'll answer a few issues that were raised.

A 40' long attic is large (total size is 800 sq. ft.), but it's well insulated. I've had a heat loss calculation done and it comes in at 17K btu/hr. This seems right since the space is currently heated easily with three 2000W electric heaters. Hot water rads have been professionally spec'd at four 38" x 17 1/2" x 7 1/2" two-column rads (28 EDR each).

The rest of the house will be done with radiant infloor heating, plumbed from a separate line coming off the insulated pipes from the outdoor boiler. As far as the attic rads go, I think we can consider the whole thing a self-contained proposition.

The outdoor boiler is hot all the time, with 180F water constantly available in the basement. That's why it seems to me that all the attic thermostat needs to do is turn the circulating pump for the rads ON and OFF. Am I missing something?

When I said that the outdoor boiler is open, it's not wide open. There's no pressure in the system, but it's not open to the air. At least not much. There's a small vent whole in the top of the boiler filler pipe. The whole boiler and fire-tube heat exchanger is mild steel, and it's protected against corrosion by the chemistry of the fluid in it. This same chemistry would protect cast iron rads, too.

That said, pumping energy is not something I considered, though. If I transfered heat from the boiler through an exchanger, allowing me to pressurize the radiator side of the system, would this reduce strain on the circulating pump for the rads?

I had figured I'd need to go with 1" pipe, but I've got a bunch of 3/4" PEX-AL-PEX pipe and fittings. It seems to me that this might be large enough to properly feed the rads in the attic. Am I right on this?

Thanks a million again! What a resource you guys are.

Bye for now,

Steve
 
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Old 01-10-12, 04:39 PM
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it seems to me that all the attic thermostat needs to do is turn the circulating pump for the rads ON and OFF. Am I missing something?
You may be right about that... I know you said it was an OD Woody, but I forgot that part when I posted about the relay. If your sole source of heat during the winter is going to be your woody, then you can just run the circ from the thermostat. It's still advisable to use a relay box though. You wouldn't want to run the circulator directly from the thermostat.

it's not wide open. There's no pressure in the system, but it's not open to the air. At least not much.
Understood... but what constitutes an 'open' system is one that is operating at atmospheric pressure, one that will allow air to come in contact with the boiler water. The water that circulates in the system will be oxygen rich because it will absorb the o2 from the air. Your OD boiler is considered an OPEN system because of this.

A CLOSED system is one that is pressurized and there is NO communication with the atmosphere and no chance for the water to absorb oxygen.

If I transfered heat from the boiler through an exchanger, allowing me to pressurize the radiator side of the system, would this reduce strain on the circulating pump for the rads?
I don't know about 'strain' so much as ENERGY (electricity) consumption. I would just take a simple wild a55 guess that the pump you would need would probably use about TEN TIMES as much electricity, and cost at least FIVE TIMES as much to purchase. In a closed heating system, one that is completely full of water, the only energy that the pump has to put into the water is that which is required to move the water past the pump. These pumps typically consume about as much energy as a 60 W light bulb. The pump does not have to 'lift' any water to altitude. The pump both pushes AND pulls. Gravity pulls the water back down also. Think of it as a big Ferris Wheel of Water.

In an open system, the pump would be required to be able to 'lift' or 'push' the water all the way to the attic, because there is no help from gravity. Yes, the water would 'fall' in the return pipe, but it would fall back to the same atmospheric pressure ... NOT to a 'suction' created by the pump.

Because the system is not pressurized, as soon as the pump turned off, all the water in those pipes would fall back down to the level in the boiler. The water level in the system would rise and fall as the pump cycled on and off.

You CAN run the system open as long as the proper anti-corrosion chemicals are used, but you will have to run that HOG of a pump to do so... and ... you will probably not have a 'quiet' system. You will likely hear air at times gurgling through the pipes.

I've got a bunch of 3/4" PEX-AL-PEX pipe and fittings. It seems to me that this might be large enough to properly feed the rads in the attic. Am I right on this?
Yes, I believe so. In order to supply say 20K BTU to those rads, you only need to pump around 2-3 gallons per minute through them. 3/4" tubing is good for up to about 4+ GPM, or 40K BTU. You would only need to go to 1" in order to move up to 8 GPM, or 80K BTU up there.

One more thing to consider though... if you DO go with an open system, it might be to your advantage to run the 1" tubing... because in ADDITION to the vertical lift that the pump has to produce, it will also have to overcome the FRICTION in the piping, which effectively ADDS to the size of pump you will need. The 1" tubing will have less resistance to flow than the 3/4", so consider this when choosing your HOG of a pump!
 
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Old 01-10-12, 04:43 PM
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One more quick thought...

If you do run an open system, I would probably opt for PARALLEL piping, and piped as REVERSE RETURN so that all rads get the same flow. It will be easier for a hog pump to move the water through the rads in parallel than in series. You want to design the piping so that you introduce the absolute MINIMUM of resistance to flow. As few sharp bends, and fittings as possible.
 
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Old 01-10-12, 04:47 PM
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The rest of the house will be done with radiant infloor heating, plumbed from a separate line coming off the insulated pipes from the outdoor boiler.
I just noticed this...

If you are planning on pumping that 180 water through the radiant tubing, you might want to rethink that plan. It's not advisable, or good design practice, to use more than say 120 water through floor tubing. You would need to temper that water somehow... with a thermostatic mixing valve... and this would mean yet another pump.

In the long run, you will be MUCH better off by building a standard pressurized distribution system in the home, and transfer the heat from the boiler into the system via a heat exchanger.

Will there be, or is there, a standard boiler in the home?
 
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Old 01-10-12, 05:43 PM
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Wow! You've really rolled out the red carpet for me. Thanks very much. Your information is very valuable.

You've convinced me to go with a close, pressurized system. That makes much more sense.

As far as temperature goes, my plan was to use untempered 180 water for the rads, and tempered 120 water for the infloor heating. My thinking is that it's easier to install rads in the 3rd floor attic than it is to create an infloor hydronic installation on top of the wooden subfloor up there (there's no finished floor at the moment). Do you agree? I also really like the old cast iron rads with floral patterns and would like to work them into the house somehow.

One more thing . . . Does the ideal parallel piping layout involve nothing more than running a main supply and return line, then tee off from the main supply to each rad, with another line leaving each rad and going back to the main return line? I imagine that the last rad could simply connect to the ends of the main supply/return lines. Does the reverse return design vary from this?

Thank you very much again, NJ Trooper!

Bye for now,

Steve
 
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Old 01-10-12, 06:16 PM
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Steve, I do that for everyone! (well, there are exceptions... but very few) But if it makes ya feel good, then that's fine!

Do you agree?
Yes, it makes perfect sense... You will enjoy the heat that comes from those rads to be sure!

Does the reverse return design vary from this?
Yes, but if you are going with a pressurized system, you could probably pipe them in series... it might be easier.

Or, you could pipe the two pairs that are next to each other, and then pipe the series pairs in parallel.

Reverse return... I think a picture is worth a thousand words... basically it's a method of piping such that the water to and from each parallel element travels the same distance. The LAST rad to get water on the supply side is the FIRST rad to return the water.

I think this graphic pretty much sums it up:

 
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Old 01-10-12, 07:29 PM
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Another great answer! Thanks very much. Now I'm all set to start making good things happen. That diagram was very helpful.

Bye for now,

Steve
 
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Old 01-10-12, 08:43 PM
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Four rads at 28 EDR each is about 27k BTU/hr output with 180F water. For room with heat loss at 17k (and if a Manual J calc, likely an overestimate by about 10-30%), that is quite over-radiated. You don't need 180F water. Probably 120 like the rest of the house would do it. Find some output curves and see. Would make the system a very simple, one temperature throughout. And if you used a mixing valve that has built-in outdoor reset, like the Taco I-series, you could give yourself even better comfort than simple tempering.

http://www.taco-hvac.com/uploads/FileLibrary/100-19.pdf

Concur on parallel or reverse return as a nice way to do this, particularly to heat a long room evenly. Isolate the whole system with a flat-plate exchanger. Use a manifold, some electric low-draw zone valves, and pump the whole house with a high-efficiency ECM pump like Grundfos Alpha or Wilo ECO.
 
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Old 01-11-12, 05:41 AM
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Every time I get another answer here, important light bulbs go off for me. Thank so much for your help.

Given how easily the attic space heats with electric baseboard heaters right now, I also figured more rads were spec'd than necessary. But if this lets me run one water temperature instead of two, that's a good thing indeed. The Taco valves look terrific. Thanks for directing me to them.

Here's something else you might be able to help me with: My plan had been to use the incoming 180F water to heat domestic water in a tank, then continue the water stream to the radiant infloor system and the rads. Ideally I'd like this tank to be a propane water heater with an internal heat exchange coil. This way when I'm running the outdoor wood boiler, it would heat the DHW and house space, but when I'm not around to stoke firewood, the propane would kick in and keep the entire system hot (boiler tank included). I was planning to run the circ pump to the boiler continuously, thereby keeping the outdoor boiler tank hot for heating my workshop (which has infloor heating connected to the outdoor boiler that is now operational). Here are some questions:

1. Are there any drawbacks to using a big propane water heater to keep the entire system hot like this? The underground pipes (Logstor) and the outdoor boiler itself are all well insulated, so I don't think there would be much heat loss.

2. I'd like to go with a propane heater that has an internal heat exchange coil, but the only one I can find is the Phoenix Solar. It's designed to allow supplemental heating of the tank from an outside source (i.e. solar panels), but I'm concerned about its ability to transfer heat efficiently back from the tank to the heat exchange coils, for those times when I'm in propane mode. The coil sits on the bottom of the tank (where the water is coolest), and only appears to be two or three revolutions not a lot of surface area. Do you think this model will work? Are there better propane heaters with internal heat exchangers? Should I just go with a regular, commercial duty heater of about 200,000 btu/hr and use an external heat exchanger? Should I just forget about the propane back-up altogether, plumb in a simple, indirect hot water tank with coil, then use the electric baseboard heaters I have now as back-up heat?

Thanks very much again! For the last 20 years I've earned my living writing about the trades I know: home improvements, construction, woodworking and tools. If I can answer any questions along these lines for you guys, send me an email: [removed personal info, sorry, not allowed]

Bye for now, with much appreciation.

Steve
 

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Old 01-11-12, 03:34 PM
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Given how easily the attic space heats with electric baseboard heaters right now,
6 KW of electric heat is about 20,500 BTU just for a reference point.

the propane would kick in and keep the entire system hot (boiler tank included)
Maybe not... what is the BTU output of said water heater?

1. Are there any drawbacks to using a big propane water heater to keep the entire system hot like this?
Yes, I'm sure there are... number one being, 'will it work?'

I'm concerned about its ability to transfer heat efficiently back from the tank to the heat exchange coils,
I would be too.

commercial duty heater of about 200,000 btu/hr
Why on earth are you thinking you need such a big boiler?
 
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Old 01-11-12, 05:02 PM
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Thanks very much again! For the last 20 years I've earned my living writing about the trades I know: home improvements, construction, woodworking and tools. If I can answer any questions along these lines for you guys, send me an email: [removed personal info, sorry, not allowed]
Go to the other forums and participate. That would be the best way.
 
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Old 01-11-12, 05:11 PM
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Good point Droo! In fact that goes for everyone... PAY IT FORWARD!

Steve, how many square feet is your home? I wanna get a 'rule of thumb' idea of your heat loss...
 
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Old 01-11-12, 05:59 PM
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My house is 3000 sq. ft. in total (2x6 construction, with fiberglass batts and 1 1/2" extruded polystyrene foam). My shop is 1500 sq ft. (SIPs construction), with plans to expand it to about 5000 sq.ft. There's also the domestic hot water that my wife and our five kids use.

As for my water heater idea, I don't have the heater yet. I'm tracking down models with an output that matches the boiler and an internal heat exchange coil, if possible. In practice, if I plumb a water heater into the system, it would essentially be in series with the boiler, with water circulated back and forth between the boiler and water heater. Energy for heating would simply be siphoned off from the continuously flowing loop as needed. The only question is whether the boiler would be providing the heat or the water heater.

I've been looking at a 200K btu/hr water heater since I've seen these available as commercial duty water heaters and it approaches the output of my wood boiler (300K btu/hr). I live in Canada, and houses are routinely fitted with 90K to 100K btu/hr furnaces up here. What with my workshop and the DHW as part of the deal, I figure a 200K water heater is the right size.

I'll certainly visit and help out in the other forums.

Bye for now,

Steve
 
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Old 01-11-12, 06:24 PM
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Don't figure your loss, calculate it. You don't want to be over sized.
 
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