I will be replacing my old steam heat system with a high efficiency boiler with a combination of radiant underfloor and baseboard heaters. I was told that running 180 degree water to baseboard heaters was old technology and was inefficient. The supposed more efficient method would be to run longer runs of fin tube at a lower temperature. This doesn't make sense to me for a number of reasons, so I have been searching for an answer to this for a while now, but haven't found anything on the subject. Anybody have any input on this?

The key here is boiler return water temperature. If you are considering a condensing boiler the return should be kept lower than 130F to take advantage of the boiler and its maximum operating efficiency.

Ah, I see. Any good links on condensing boilers? The best I have found so far is the bellgosset site which only mentions 180 degree temperatures.

all major manufacturers have condensing boilers.

even if you don't do a condensing boiler (and if you're on gas, you should) you should run lower temperature heating systems. At the very least it allows more sophisticated boiler firing and purging cycles which can enhance efficiency and boiler lifespan.

whatever you do, do not do a standing cast iron boiler with a domestic coil in it.

Ideal would be to run baseboard and radiant at the same temps or skip the baseboard entirely and use something lower temp. Then you save mixing and get your operating temperatures down as far as they can go.

Is there an optimum return water temp?

My original thought was to run the return from my first floor baseboards to underfloor radiant. From what it sounds, this would be doable.

I would run all underfloor but I don't believe it would be enough heat in my 1870's built house.

A heat load calculation would answer that and (cue the chorus) would be the best way to figure out what will and will not work for you.

The general rule of thumb is with non-condensing boilers, that you reap ~3% in fuel savings for ever 10F you can drop the distribution temp. If you have a very tight and well insulated house with the boiler and all distribution plumbing within the fully conditioned space, or all of your near-boiler and distribution plumbing is insulated to R6 or better your savings will be somewhat less, but it's often cheaper to add more baseboard to allow lower temp operation than to insulate all of the plumbing. About half of the gains are in raw combustion efficiency, which is only ~83% (best-case) with 160F return water, but can be ~86% with 130F return water (about the lower-limit for cast iron to protect it from condensation damage.) The rest of the savings is related to lower distribution loss, and lower boiler-standby losses.

AFUE testing on high-mass boilers yields numbers significantly higher than you'd actually achieve at 180F output, since it's done at 140F out, 120F return (which would violate the warranty on said boilers)

If you use a heat-purging boiler control such as the Intellicon 3250 or it's cousin the Beckett Heat manager, the savings are similar to using "outdoor reset" type controls on a high-mass cast iron boiler, in part because the average temp of the water is lower, and the boiler is "parked" at a lower temp during standby, for lower standby losses.

Fin tube baseboard output is often specified with only it's 180F rating at distributor sites, but most manufacturers have charts showing different ratings at different temps & flows at least down to 140F average temp, but they still have reliable output down to ~120F that can be calculated or inferred, eg: http://www.slantfin.com/documents/675.pdf In general, the 140F output is about half the 180F number, and the 120F output about a third.

The fun begins when the return water to the boiler is 120F or less, since that's about where you begin to get a real efficiency boost form the heat-of-vaporization return by condensing water from the combustion products on the heat exchangers in the boiler. Condensation on the heat exchangers is destructive to cast iron, but this is the GOAL with condensing boilers. If you're running 140F water out with a 25F delta-T, the return water is ~115F and you're into the mid-90s for raw combustion efficiency, roughly 8-10% better than you're getting with 130F return water with only a 15F drop in temp.

Below ~120F the output curve of fin tube is non-linear and changes easily with variables like dust buildup, dinged fins, etc. High mass radiators or cast-iron baseboard are much more predictable low-temp emitters, as are convecting panel radiators (ubiquitous in Europe, not so much in N. Amer.) Getting it up into high-90s efficiency with fin tube isn't in the cards, but it can be with other types of emitters (inlcuding different types of radiant floor.)

What Rob said, about getting a very good room-by-room heat loss analysis to be able to get the balance right and the running lengths of fin-tube, etc. The room that has the smallest length of wall-length suitable for fin-tube could be what determines your design-day water temp, putting a constraint on what approaches can be taken on the rest.

Depending on the severity of your local climate,you should be careful not to set the lower temperature too low! The gradient between the temperature outside and the lower temperature inside could cause pipe freezing and other problems if the thermostat is set too low for too long!


_________
SIPs

Posted By Dana1 on 30 Mar 2011 03:03 PM
The general rule of thumb is with non-condensing boilers, that you reap ~3% in fuel savings for ever 10F you can drop the distribution temp. If you have a very tight and well insulated house with the boiler and all distribution plumbing within the fully conditioned space, or all of your near-boiler and distribution plumbing is insulated to R6 or better your savings will be somewhat less, but it's often cheaper to add more baseboard to allow lower temp operation than to insulate all of the plumbing. About half of the gains are in raw combustion efficiency, which is only ~83% (best-case) with 160F return water, but can be ~86% with 130F return water (about the lower-limit for cast iron to protect it from condensation damage.) The rest of the savings is related to lower distribution loss, and lower boiler-standby losses.

AFUE testing on high-mass boilers yields numbers significantly higher than you'd actually achieve at 180F output, since it's done at 140F out, 120F return (which would violate the warranty on said boilers)

If you use a heat-purging boiler control such as the Intellicon 3250 or it's cousin the Beckett Heat manager, the savings are similar to using "outdoor reset" type controls on a high-mass cast iron boiler, in part because the average temp of the water is lower, and the boiler is "parked" at a lower temp during standby, for lower standby losses.

Fin tube baseboard output is often specified with only it's 180F rating at distributor sites, but most manufacturers have charts showing different ratings at different temps & flows at least down to 140F average temp, but they still have reliable output down to ~120F that can be calculated or inferred, eg: http://www.slantfin.com/documents/675.pdf In general, the 140F output is about half the 180F number, and the 120F output about a third.

The fun begins when the return water to the boiler is 120F or less, since that's about where you begin to get a real efficiency boost form the heat-of-vaporization return by condensing water from the combustion products on the heat exchangers in the boiler. Condensation on the heat exchangers is destructive to cast iron, but this is the GOAL with condensing boilers. If you're running 140F water out with a 25F delta-T, the return water is ~115F and you're into the mid-90s for raw combustion efficiency, roughly 8-10% better than you're getting with 130F return water with only a 15F drop in temp.

Below ~120F the output curve of fin tube is non-linear and changes easily with variables like dust buildup, dinged fins, etc. High mass radiators or cast-iron baseboard are much more predictable low-temp emitters, as are convecting panel radiators (ubiquitous in Europe, not so much in N. Amer.) Getting it up into high-90s efficiency with fin tube isn't in the cards, but it can be with other types of emitters (inlcuding different types of radiant floor.)

What Rob said, about getting a very good room-by-room heat loss analysis to be able to get the balance right and the running lengths of fin-tube, etc. The room that has the smallest length of wall-length suitable for fin-tube could be what determines your design-day water temp, putting a constraint on what approaches can be taken on the rest.

Thanks a bunch for your input here. Very helpful everyone that has responded. The boiler I am planning on using is a Baxi Luna HT 330 condensing combination boiler/DHW unit. I did a heat loss calculation using the calculator on this website: http://www.warmlyyours.com/hlc/room/index_main I'm assuming it should work well enough. I came up with 42,000 btu/h for the entire house. Adding the sf of the interior space I have 1350sf. I know for evaluation purposes square footage is measured from the outside, but that is a lot smaller than the 1850sf on the tax record. Why do you say "Getting it up into high-90s efficiency with fin tube isn't in the cards"? So basically find the room that will limit the water temperature due to lack of space for fin tube and go from there? I was hoping to run the least amount of baseboard possible, but I guess I just learned that isn't the efficient way to do it! And that is what I am going after here; The lower the gas bill, the bigger the smile. So is there a point where the return water temp can be so low that it will impede efficiency? If I ran 140F out to the baseboards with a 25F delta-T which than supplied my underfloor with a return temp to the boiler of 100ishF (guessing here), would that be doable/desirable? Thanks again for sharing. I'm very happy I found this forum!

That warmly-yours heat calc isn't really a substitute for a real design tool, and running the radiant in-series with the fin-tube as a single zone is a whole-nuther design problem. Also note, 140F out with 100F return is near or at the operating limit for delta-T of many boilers. (Don't know about the Baxi Luna without looking it up.)

Getting the return water temp down to 100F or less is a desirable design goal, but you can end up spending a ridiculous amount of money getting there just to squeak the last 2-3% out of it. Without a REAL heat load calc and a REAL hydronic system design the danger of screwing it up is high. This is not something easily amenable to design-by-web-forum for the DIY set (not impossible, but you'd have to be an obsessive nerd like me to cook it all up yourself. ;-) )

You might try this cheapie heat loss calculation package out: http://www.hvaccomputer.com/ I haven't used it, but have read favorable reviews.

Designing the radiation/emitters to run at a single water temp is a HUGE simplifying factor increasing your odds of success. I highly recommend starting there, once you have more accurate room by room heat loss numbers.

I will aim for running 120F to both the baseboards and the radiant and size the baseboards lengths accordingly. I have an experienced installer who will help me with the installation, but I always like to arm myself with the most accurate, up to date information, which is usually obtained from people such as yourself on boards like this one! Thanks again for your help. Very helpful.

After performing a room-by-room heat load analysis on dedicated radiant floor software I usually choose a condensing boiler based on the load, the local support and the design water temperature. Note that most condensing boiler manufacturers will not support DIY boiler installations and many will not warrant a DIY boiler unless a licensed or "qualified" professional has claimed it.

http://www.badgerboilerservice.com/boilerchoice.html

I design and install a lot of radiant systems for old houses. Mixing radiation is the most challenging of these designs. If you do sub-floor radiant floors you may find the water temperature needed may be higher than for fin-tube (not really a radiator) in the same space. If you want to be warm without the work or advanced design, steel panel radiators are second only to radiant floors for comfort and efficiency.

Here again, careful room-by-room design is essential, especially if you want your boiler to condense.

http://www.badgerboilerservice.com/radiators.html

What is it about flat panel radiators that make them more efficient than fin tube?

Posted By ryry on 01 Apr 2011 02:19 PM
What is it about flat panel radiators that make them more efficient than fin tube?

Flat panels are taller, which allow them to induce reasonable convection rates at much lower temperatures than fin-tube can, and continue to work predictably down to 100F or lower (where the output of fin tube is unreliable.) The lower the temp at which the heat is delivered, the greater the condensation heat is returned by a condensing boiler, and the lower the distribution & standby losses from the system as a whole.

This is mainly because the "stack effect" of 2" fin tube in an 8" tall enclosure is pretty pathetic at 100F, where the delta-T to the room air is only ~30F (whereas with 180F water and a 110F delta-T it works great)  while with a 24" tall flat panel the induced convection flow is still rock-solid at 100F, and unlike fin tube, it's delivering a significant fraction of the heat via direct radiation.  Fin tube delivers only a tiny fraction of the heat to the room via radiation- it's mostly convection. Flat panels use both (as do cast-iron radiators and baseboards.)

You know your stuff Dana1. You don't live in Central NY, do you?

European style steel panel radiators are more comfortable because that have a radiant surface, which fin-tube lacks.

At higher temperatures the stack effect is quite effective, but properly sized will not be a major factor, but for the coldest days. Naturally, placement is critical. Ask your designer, supplier and installer how many steel panel radiators they have installed.

Posted By ryry on 01 Apr 2011 08:49 PM
You know your stuff Dana1. You don't live in Central NY, do you?

I don't know anything- I make it up as I go along!

(I live in central MA- a somewhat similar climate.)

If it wasn't clear - while you may need x degrees at full load, most of the time you aren't at full load and can run cooler (with the right boiler). In other words, a 180F system is more efficient than you might think - because it usually isn't at 180F.

Hello,

I found this thread on a google search, and while it may not be on the same topic as the original post, it does pertain some similar information.

Here is my question:

I purchased a home that was built in 1979, it has an older traditional boiler system (is that a condensing or non-condensing?). It has baseboard radiant heat that is plumbed with copper lines throughout, the house is divided up into 3 zones.

As we slowly remodel rooms, I have debated running some underfloor heat with some pex tubing. 3/4" with some transfer plates fastened to the bottom of the subfloor, and tying that into the existing copper lines that are plumbed to that room.

Now, the issue here is the water temp. I've read/been told that your baseboard heat is around 180F+, and your underfloor heat should be around 140F.

Can I make this work? Can I say turn down my boiler to maybe 160 to make the whole system work, with a mix of underfloor and baseboard heating systems? Can I run everything at 180F and be safe? Will that get to hot and buckle the subfloor? If I turn the boiler temp down, do I risk the colder water return cracking the boiler?

I've only had one person tell me that I can't easily replace 1 baseboard piece with some underflood pex tubing, I just want to get a few more opinions before I try this.

Thanks

cutting base board loop to make in floor staple up,
first 3/4 staple up will not work so well, it will cause you to cuss excessively
Think 1/2 pex and heat transfer plates.
next. you will interrupt the base board loop with #2 3/4 copper T's (assuming base board is all run in 3/4) next, a 3 way mix valve to lower staple up to about 135, next a pump and then manifolds to you your staple up area. pump can be wired to come on when base loop goes on or with temp sensor on base loop.

Dana, Nice definition on Base board rad VS Flat panel,

Dan

Posted By ctech99 on 22 Oct 2011 12:23 PM
Hello,

I found this thread on a google search, and while it may not be on the same topic as the original post, it does pertain some similar information.

Here is my question:

I purchased a home that was built in 1979, it has an older traditional boiler system (is that a condensing or non-condensing?). It has baseboard radiant heat that is plumbed with copper lines throughout, the house is divided up into 3 zones.

As we slowly remodel rooms, I have debated running some underfloor heat with some pex tubing. 3/4" with some transfer plates fastened to the bottom of the subfloor, and tying that into the existing copper lines that are plumbed to that room.

Now, the issue here is the water temp. I've read/been told that your baseboard heat is around 180F+, and your underfloor heat should be around 140F.

Can I make this work? Can I say turn down my boiler to maybe 160 to make the whole system work, with a mix of underfloor and baseboard heating systems? Can I run everything at 180F and be safe? Will that get to hot and buckle the subfloor? If I turn the boiler temp down, do I risk the colder water return cracking the boiler?

I've only had one person tell me that I can't easily replace 1 baseboard piece with some underflood pex tubing, I just want to get a few more opinions before I try this.

Thanks

If the boiler is circa 1979 it is not a condensing boiler and is not tolerant of temps lower than ~130F (if gas) or ~140F (if oil) coming back from radiation entering the boiler.  If you run the radiant floors as indivitual zones with a thermostatic mixing valve (as Blueridge recommended) that isn't much of an issue.  But this really needs to be designed, not hacked to get best results.  If you're doing it room-by-room you'll end up with a very micro-zoned heating system with micro-loads, and keeping the boiler from short-cycling itself into low efficiency and an early grave may become in issue.

But if the boiler really is 30 years old it's probably time to bite the bullet and buy a condensing boiler anyway. This is even more true if the boiler is more than 2x oversized for the load at outside design temperature, which is common.   Gas-fired oilers of the late-'70s ran steady-state efficiencies  of  ~80%, and 75-78% AFUE when brand new, and 30+ years hence with normal corrosion, liming & wear & tear on the heat exchangers it's probably not doing better than 75% combustion efficiency and 65-70% AFUE.  If it's 3x oversized for the load, knock that down to ~60% AFUE.  With three zones it has to be more than 3x oversized for the smaller zones, and it may in fact be short-cyling on zone calls, cutting further into efficiency.  Replacing it with a "right-sized" modulating condensing boiler it wouldn't be surprising to see a 35-50% reduction in fuel use.

180F is an arbitrary number- fin-tube's output is predictable down to ~120F.  Most older systems were designed with both the boiler and the fin-tube lengths oversized for the actual heat loads.  If the house has been tightened up since it was built and it has storm-windows or insulated-glass windows in decent shape the probability that the baseboards can deliver sufficient heat at outdoor design temps is quite high, but running it there with the existing boiler without some modification to the near-boiler plumbing to feed in some direct boiler output water to keep the return temps up would wreck the boiler in short-years from condensation. But running it at 140F or lower is ideal for getting the condensing-efficiency out of the boiler, since the return temps would almost always be in the condensing zone.