View Full Version : Heating basement baseboard zone + domestic hot water

09-18-2011, 10:14 AM
I own a 2-story colonial with a finished walkout basement, located about 10 miles SE of Philadelphia, PA. The 1st and 2nd floor is forced-hot-air heated by a condensing, gas furnace that was installed in September 2010 (replacing an old oil furnace), while the basement zone remains heated by a Thermo-Dynamics HT-110 oil boiler (114,000 BTU/h output), which also supplies the domestic hot water (a typical summer/winter hookup).

The finished basement and laundry room (approx 700 SF total) are heated with 45 feet of finned-tube baseboard, while the adjacent utility room (approx 375 SF) is warmed by the furnace and boiler located there.

I just used an online heat load application that calculated ~ 60,000 BTUH heat loss for the 700 SF finished basement and laundry room Ė but I donít know how accurate that is. Iíve read that baseboard radiates ~ 580 BTU/h per linear foot, which would mean the 45 feet of baseboard puts out ~ 26,100 BTU/hÖ and the basement remains comfortable on the coldest days.

Weíre now planning to replace the Thermo-Dynamics HT-110 in order to move from oil to gas as the means of heating the basement and DHW Ė and while there are only 2 of us living in the house (showering at separate times), we want enough hot water that 2 showers could be taken simultaneously with water to spare and a reasonable hot water recovery rate (for when guests stay over).

Iíve considered several options including: 1) converting the Thermo-Dynamics HT-110 from oil to gas + indirect hot water heater or heat pump hot water heater; 2) condensing boiler + indirect hot water heater; 3) condensing combination boiler (for both heat and DHW); 4) smaller condensing or atmospheric boiler + heat pump hot water heater; and 5) atmospheric boiler + indirect hot water heater.

Iíve talked with two reputable heating companies in my area and have received estimates ranging from X to 4X depending on equipment.

For instance, one company will charge X to convert the Thermo-Dynamics HT-110 to gas and install a 50 gallon indirect hot water heater Ė and while the other company has suggested more options, its most expensive is about 4X for a condensing boiler (like a Triangle Tube Prestige Excellence or Prestige Solo) + a 50 gallon indirect hot water heater.

However, I recently read that finned-tube baseboard operates best with 180 degree water and condensing boilers are far more efficient with 130 degree water (i.e., finned-tube baseboard radiates only about 250 BTU/h using 130 water and condensing furnaces operate at ~ 85% efficiency with 180 degree water).

Consequently, I think Iíve ruled out a condensing boiler owing to their high initial cost and apparent incompatibility with finned-tube baseboard.

Instead, Iím now thinking about configurations like (but not limited to) the equipment listed below:

1) Burnham P202 gas atmospheric boiler, 27,000 BTU, Electronic Ignition, 81% AFUE + 60 gallon A.O Smith Voltex heat pump hot water heater, 2.4 EF (yipes!!!). Obviously, with this type setup the boiler only heats the basement zone. (Equipment cost about the same as X above.)

2) Buderus CG124 gas atmospheric boiler, 110,000 BTU/h (DOE), Electronic Ignition, 84% AFUE + 40 gallon Triangle Tube Smart indirect water heater, 112,000 BTU/h input. Obviously, with this type setup the boiler heats both the DHW and the basement zone. (Equipment cost about the same as .8 X above.)

Iím looking for the most practical and cost effective approaches to achieving the goals of switching from oil to gas, heating (with finned-tube baseboard) a 700 SF finished basement and laundry room, and having an adequate supply of DHW.

Your ideas and suggestions are welcome and appreciated.


I've just read a recommendation to a similar question regarding the heating of a small baseboard zone where DHW is supplied by an indirect hot water tank.

The person making the recommendation says (in part):

"... low-mass boilers reduce the standby loss factor that can dominate in cycling losses a less than full load (not just short-cycling which is even worse.) The 2-stage Lochinvar Solution series of mid-80s AFUE type low mass copper tube boilers are probably more appropriate to [the required] retrofit than the Lochinvar mod-con. Buffered with an indirect HW heater it'll probably meet/beat the performance of the mod-con + whatever hot water heater he's using... Getting the burner size matched to the actual load is the first critical step on the efficiency path though. You can only reduce cycling losses so far with a buffering strategy. Sizing it correctly to start with pays huge dividends: A 93-95% AFUE mod-con boiler may run at 88-90% when right-sized in the higher-temp radiation situation found in most retrofits, but will only hit 83-84% if it's 3x oversized. (Which is VERY typical.) Similar scaling factors occur with mid & standard-efficiency boilers too."

The Lochnivar Solution is a 2-stage boiler with a copper finned-tube heat exchanger, available in 7 output sizes between 36,900 and 213,200 BTU/h.

Does this sound like a possible solution... and given that my zone's 45 linear feet of baseboard can radiate no more than 26,100 BTU/h, what size unit would be best when used in conjunction with an indirect hot water heater to supply DHW as well as serving as a "buffer," if needed..?

Also, can someone explain how an indirect hot water heater serves as both a buffer and the source of DHW..?


09-18-2011, 02:31 PM
A couple of observations...don't go by BTU/ft to calculate your boiler size. The odds are highly against the boiler running 100% of the time to produce that max output you predict. it would be unusual to need even the 26K BTU for that area, more likely less than half that. You need someone to actually do a manual-j calculation or find a better heat loss calulator.

09-18-2011, 08:11 PM
Thanks, Jim.

My basic dilemma seems to be:

1) Small heating requirement -- let's say 12,000 - 16,000 BTU/h for the basement baseboard.
2) 40 or 50 gallon indirect hot water heaters are typically rated ~110,000-140,000 BTU/h input.
3) Condensing gas furnaces that can modulate down to 16,000 BTU/h output cost (installed) ~ 3X more than an atmospheric gas boiler.

Any thoughts on the Lochnivar Solution 2-stage (100%/50%) atmospheric boiler, copper finned-tube heat exchanger, available in (high-fire/low-fire) 36,900/18,450, 61,500/30,750, 73,800/36,900, and 110,700/55,350 BTU/h outputs.

Could the 36,900/18,450 BTU/h model work with a 40 or 50 gallon indirect hot water heater, knowing that they are rated at ~ 110,000 and 140,000 BTU/h input..? Would fewer BTU/h input simply mean that it would have a slower recovery rate..?

09-19-2011, 08:24 AM
First, your heat load in the basement is nowhere near 60KBTU unless you open the window and install a fan to blow cold air through it all winter. (60K would be almost 2x the my whole house in central MA at 0F!) I'd be surprised of the true heat load of just that basement was over 10KBUT/hr, but it's probably north of 5K if it has a lot of windows or the foundation is uninsulated or leaking air at the foundation sill & band joist.

Second, fin-tube puts out ~600BTU/foot at 180F sure, but it also put out 300BTU/ft @ 135F or 200BTU/ft @ 120F. There's nothing magic about 180F. See"


Download a real heat loss calculator and re-run your numbers:


A gas-fired tank HW heater with an isolating heat exchanger can probably handle it, and ANY of the boiler solutions will likely be oversized for just that zone. If it's a fairly open basement with few doored-off areas a cheap/better solution than the Lochinver might be a 1-ton or even 3/4 ton mini-split heat pump with an HSPF of 9 or better. (eg. Fujitsu AOU12RLFW/ASU12RLFW or Mitsubishi MUZFE12NA/MSZFE12NA.) With a mini-split you'd get some AC and dehumidification function out of it as well, and at 15 cents/kwh in your location the operating cost in heating mode would be comparable to "right-sized" mid-efficiency gas fired goods. Since a mini-split is also a modulating heater, it's comfort and efficiency are quite high- much higher at part load than at full blast. Since there are no gas-burners right sized for this low a load, a mini-split will almost surely be cheaper to operate. A 1-ton mini-split would typically run ~$3.5-4.5K installed.

You can then stick with a decent but inexpensive 40gallon gas HW heater for the DHW without complicating factors such as a Rube Goldberg exterior heat exchanger heating system growing off of it's side.

But if you insist on using the baseboard to heat the basement a Combi-Core (http://www.bradfordwhite.com/images/shared/pdfs/specsheets/553.pdf) would be more appropriate. The Lochinvar Solution would be extreme overkill for heating a 700 foot basement, and more than 3x oversized for the space heating load even at the lowest fire.

In fact any full-on space heating boiler options mentioned would be like using a sledgehammer as a fly-swatter- far too big to be efficient for the space-heating load, and expensive to boot.

By contrast a mini-split heat can be right-sized for the load, and in your climate would deliver an average coefficient of performance (COP) of about 2.5. The average cost of electricity in PA is about 10cents/kwh, so it would deliver 2.5 x 3412= 8500BTU of space to the house for 10 cents, or ~85,000 BTU/$.

At $1/therm for gas a Combi-Cor would be only giving you ~ 80,000BTU/$, and at a more realistic $1.25/therm even less. Also note, at $1.25/therm a 95% AFUE gas furnace is delivering only 76,000 BTU/$ or ~10% less than a mini-split. Even if the COP averaged only 2.3 (as it would in cooler climes such as southern ME or southern NH) you'd be at rough parity with $1.25 gas if you have 10cent electricity.

If your heat load numbers come in at 9K or less (possible, even likely) you'd be able to use a 3/4 ton mini-split, but there isn't a significant cost savings for doing so. The sweet-spot is still likely to be a 1-ton, which wouldn't be oversized to the point of lower efficiency unless your under 5K for a heat load. (They're modulating systems, typically with a ~3/1 turn-down ratio.)

09-19-2011, 07:18 PM
Thanks Dana, for your reply.

PECO is my supplier for both gas and electric. Looking at a few bills over the past year, I see my gas costs ~ $1.05 Ccf and electricity costs ~ $.177 kWh (which includes Transmission and Distribution Charges).

If my math is OK... $.177 kWh (ILO $.10) would decrease your mini-split calculation from ~85,000 BTU/$ to ~48,000 NTU/$.

Given these numbers... would the Combi-Cor be a better option..?


This Combi-Cor is new to me -- but as it apparently supplies hot water heat to a small zone as well as DHW, it seems like the optimal solution.

Can you explain more about it... Thanks.

09-19-2011, 08:40 PM
Heat pumps deliver more BTU than they consume to make them. Depending on the unit you select and the outside temp, dictates what the multiplier is.

09-20-2011, 09:07 AM
Thanks Dana, for your reply.

PECO is my supplier for both gas and electric. Looking at a few bills over the past year, I see my gas costs ~ $1.05 Ccf and electricity costs ~ $.177 kWh (which includes Transmission and Distribution Charges).

If my math is OK... $.177 kWh (ILO $.10) would decrease your mini-split calculation from ~85,000 BTU/$ to ~48,000 NTU/$.

Given these numbers... would the Combi-Cor be a better option..?


This Combi-Cor is new to me -- but as it apparently supplies hot water heat to a small zone as well as DHW, it seems like the optimal solution.

Can you explain more about it... Thanks.

At 18 cents/kwh & $1.05/therm the Combi-Cor would be cheaper to operate. (8530BTU/$0.18= 47.4KBTU/$, to the Combi-Cor's (0.8 x 102,000)/$1.05= 77.7KBTU/$

In the warmer weather of the shoulder seasons the COP of a mini-split rises considerably- most are at 4 or better @ 50F outdoor temps, but most of the operating energy will be likely used at outdoor temps below 30F in your location where the COP will be at or under 3.0. Most are rated ~2.5-2.7 at 17F at mid compressor speed, but unless you oversize it it'll be running a bit below 2.5 when it's that cold. While a COP average of 2.5 is intentionally conservative, odds are you won't beat a seasonal average COP of 3 under any circumstances. At 18cents/ kwh that works out to 3412 x 3.0/$0.18= 56.9KBTU/$

The mini-split can also dehumidify and air condition, and only 20% or less of your heating would be supplied at this higher-cost method (still FAR cheaper than using an oil boiler to heat that space) but won't heat your hot water.

A Combi-Cor or similar is probably the "right" solution here. It's essentially a tank HW heater with in internal heat exchanger for the heating loop. If you set it up to 125-130F for the hot water, that's probably going to be hot enough to cover your heat load, since with an average water temp of 125-ish you'd be getting ~250BTU/foot out of the baseboard. Times 45' that's 11,250BTU. If somehow it's not keeping up (did you leave the windows open? :-) )cranking it to 140F you'd be north of 300BTU/foot and 13.5K+. At 140F storage temp you'd have higher standby losses, but the losses are to the room, which is inside the zone, so it's heating the zone slightly out the sidewall of the tank- not a big deal 6-8 months of the year, but you may want to crank it back in summer. In some locations 140F storage temps are required by code anyway.

Installing a Combi-Cor for a 1-zone heating system with 45' of baseboard is simple enough that it's practically a DIY project, but that also means it won't break the bank to let a pro do it. It won't take much of a pump to drive a head of 45' of baseboard + heat exchanger either- it's probably worth doing the math and down-sizing it rather than blindly dropping in a Taco-007 and calling it "done". (But if you have to pay $100/hr for a hydronics designer to do it, fuggedaboudit- do the latter!)

If you want to go condensing high-efficiency, there are condensing versions of combi-heaters out there, but like a full-on boiler they're all extreme overkill for your miniscule space heating load, and it would take several decades to make up the difference in installed cost with fuel savings in buck-a-ccf-land. (They might make sense if you were heating your whole house with one though.)

09-20-2011, 08:26 PM
Thanks, again, Dana,

I couldn't find any online pricing for the Brandford White Combi2 (which I think is their "latest/best" release with a double-walled heat exchanger that, from what I've read, overcomes some problems with the original Combi model).

But I did come across links for a 45 gallon Laars "Combi Core" for $2,232.88 + shipping as well as a Lochinvar "Double Duty" - both of which appear to be a rebranded "Combi2." Do you know anything about these units..?


Do you know the prices of the Combi2 and where they can be had (or are they available only to the wholesale/contractor market)..?

Since there are only 2 of us in the home, I'll likely get the 45 gallon (not the 72 gallon model):


Or would the larger, 72 gallon model lend itself to supplying adequate heat and DHW on cold winter days... or, instead, simply be wasting energy via standby losses..?

I'll talk with the plumber (from the company who installed our Lennox SLP98UH110V60C gas furnace last year) regarding installation-configuration and circulator pump.

I agree that any condensing gas boiler would be overkill and require decades for payback... and if the Combi2 (or clone) fails in 10 years, it can easily be replaced.

It's also interesting that a moderator at another major HVAC site - in response to my initial lengthy post - simply suggested going with a Weil-Mclain Ultra 80 which he said can mod down to 16,000 BTUs. Nothing more... no discussion of any other issues or questions raised in my post.

By way of contrast, your interest and thorough discussion of this topic has been educational and enlightening - leading me to what seems a most practical solution. It's funny that your mention of the "combi-cor" was the first I had ever heard of a unit of this type, designed for just this sort of application - yet, this type of unit was exactly what I've been asking myself for the previous week or so, "Why isn't there something that will both heat my basement and supply DHW.

Dana, I thank you... and I'll keep you posted at this thread on the progress of this matter.

09-21-2011, 10:35 AM
Dana, I've done some more reading and now have a few more questions:

1) Would the power vent model of the BW Combi be advisable -- as I've read that a lot fewer BTUs go out the chimney with a power vent..?
2) I've read that it this area it's typical for 40 degree F water to enter the home rather than the "rated" 55 degree water. What impact would 40 F water have on the recovery time or overall ability of the Combi to heat both the DHW and the baseboard zone... and does the size of the unit (50 versus 72 gallons) play into ..?
3) Would I need an expansion tank (I assume so)..?
4) Where would the circulator pump be located and what type of pump do you suggest..?
5) I've read that the Combi 1 (single-walled heat exchanger) puts 40% more heat into the zone system that the Combi 2 (double-walled heat exchanger). If code is not an issue, which do you recommend..?
5) I've also read an old (1998) but interesting article "Using Water Heaters for Radiant Heat" that recommends using a separate heat exchanger ILO BW Combi when code permits... what are you thoughts..?


Thanks, Keith