Is 100k BTU the right size of boiler (or HTP heater) for both baseboard heating and indirect DHW

Tom C._3

We are thinking to install a new 91% gas boiler and a 45 Gal indirect DHW to replace the old 62kBTU/44kBTU boiler and 50Gal water heater. Two contractors suggest about 100 BTU (input) for the new boiler, does it sound right size to you?



Here are details:

* two zones of hydronic baseboard heating, the total room area is 1300 sq ft (the rest of house is heated with forced air by furnace)

* I did a preliminary heat loss cal, by slant/Fin heat loss calculator, which is about 31,000 BTU for those two zones. We are in Boston area, 70F / 5F was used when calculating heat loss.

* the existing baseboard: 77 ft. The baseboard Fin is about 3 inch by 3 inch

* 45 Gal of Indirect DHW



I hope I can hear your opinion about whether 100K BTU (input) sound right size for new gas boiler with 91% AFUE to provide both baseboard heating and DHW ?

If we use HTP Versa Hydro (or Pioneer) as a combination unit to provide both DHW and 160F water for baseboard heating, instead of a gas boiler plus an Indirect, how much BTU a Versa Hydro needs to have for such a load, giving the fact that Versa Hydro is using lower temperature water (160F install of 180F) to heat the baseboard?

Will 77ft Fin baseboard sufficient long when reducing water temp to 160F?

Thanks

SWEI

Math

Your heat loss of 31k is smaller than any modulating condensing boiler currently available in North America.  Time to look at minimum instead of maximum firing rates.  My current rule of thumb says you should look for a boiler whose minimum firing rate is about one third of your design day heat loss.  Here are the options I know of with minimum net outputs [in square brackets] under 12k (maximum rates in parens):



Viessmann 200-W B2HA 19   [11,580]    (64,655)

Lochinvar WHN055                [10,450]    (53,250)

Dunkirk DKVLT-050                [10,000]    (50,000)

Lochinvar Cadet CDN040         [8,545]    (37,600)



A higher maximum rate will provide faster recovery for your indirect.  I would upsize the indirect long before I specified an oversized boiler.  Keep in mind that most gas water heaters have net outputs of 30k or less, so any of the above should outperform the old tank.

Rich_49

Heat Loss

Has anybody done one yet to determine if your baseboard is sufficient at 70' using 160* fluid ? This would need to be a room by room heat loss .

  How did you come to find out about Versa Hydro ?  Swei is correct in all he has stated but the Versa (PHE130-55) would be a good fit for your project if in fact 160* is sufficient . The Versa is a water heater that uses the stored mass of water in the DHW tank for space heating . When there is a call for heat the Variable speed pump pulls water from the tank and through a flat plate heat exchanger , it will not short cycle when programmed correctly . Your installer should know how to do the proper math to accomplish this , that is the main concern .  This unit quite simply enables a system to operate as if there were a mod / con boiler , indirect water heater and a buffer tank which is what most would specify , the only sticking point would be , do you have enough installed radiation for the 160* temp .  Along with whatever equipment you use a Delta T circ will help you insure that you are matching the load at any given time .

  Like Swei said , the low end is what you need to be concerned with , so does everyone now . Properly designed systems should be able to match the load at a given point in time without overshooting fuel usage . As far as equipment and space / labor saving go your mentioned choice is a sound one .

In the future if you decide to or need to replace that furnace you can just use an air handler with a hot water coil and that too can enjoy the outdoor reset feature by changing the water temps to the coil . remember a house doesn't change size , the load does

ced48

100,000 Overzied-

Go with something like the Loch WH55-

Ironman

Base Board Water Temp

At 160* supply water temp, your B.B.s will have their output reduced to about 75% of their rated capacity. Right now, you're getting about 38.5k btu's with 170* AVERAGE water temp (180* supply, 160* return). 75% of that would be 28.8k btu's at 150* AVERAGE water temp (160* in, 140* out).



That may not be quite enough, based upon your load calc, on very cold nights. Particularly in some rooms if the BB are looped in series. The end rooms may suffer.

Tom C._3

boiler BTU cal. = room heat loss + DHW load? and chimney concern

Thank everyone for your valuable replies. As you pointed out, our limited length of baseboard could be an issue if we use 160 degree HTP Versa Hydro. Do you think it may be better to use HTP Versa Flame instead, whose max temp is 180F ?



I am confused about total BTU calculation for boiler plus Indirect DHW. It seems to me you do not add the load of Indirect DHW into equation. Doesn't it make sense to add the load of the Indirect DHW on top of spacing heating load? With this thought, one contractor proposed WM GV90+4 (105 MBH input) plus an Indirect. If I assume an Indirect DHW equ. to 40 MBH, adding room heat loss of 31 MBH, I will get a total load of 71 MBH, which would let us choose a larger boiler, or larger HTP Versa. Am I right or wrong?



Chimney is a concern: our existing gas furnace (very large MBH), gas boiler, and water heater all vent into same un-lined chimney flue. If we replace the gas boiler and water heater with a side-wall-vent boiler (or Versa Hydro), the chimney will have only one vent from the old furnace, with reduced BTU. Will it wok if no change is made to chimney? (We may remove that furnace vent from chimney in a few years when replacing furnace)

RobG

Why?

If you have a 32K heat loss why is everyone quoting you on a 100k BTU boiler? Does your 50 gallon water heater provide sufficient hot water now? If the answer is yes, a 50 gallon indirect will give you PLENTY enough hot water. If your heat-loss is correct, as was stated above, even the smallest mod-con will be slightly over-sized. Go with the smallest unit that is well represented in your area. It is funny how over the years as boilers are replaced every replacement seems to get bigger than the last, no contractor wants to under-size a boiler so they keep on putting in the next larger size. Don't buy into it!



Rob

SWEI

And don't forget

that your 77 feet of baseboard will top out around 40,000 BTUs per hour no matter what boiler you install.



Do not add the DHW load to the heating load -- they rarely coincide with each other and the newer boilers handle the switchover and switchback seamlessly.  Any of the boilers I listed above will outperform your existing 50 gallon gas water heater using a 40 gallon indirect -- most would using a 30 gallon.

Tom C._3

issues with small boiler and small sub-40 MBH load

Thanks for letting me understand there is no need to add Indirect DHW load on top of Baseboard load when sizing the boiler, and I should choose a small boilder (50 MBH) for our sub-40MBH baseboard to reduce short cycling.



A small boiler seems lead to other concerns and issues:



(1) I don't understand why many Indirect tank manufacturers require large 100+ MBH boiler for Indirect.



(2) How bad it is with short cycling in term of operation cost? A few previous posts suggests that if the load is small (less than 60 MBH) they recommend standard cast-iron boiler, since the Mod-Con boiler won't save enough annual operation cost to justify the extra upfront equipment/installation investment compared to standard boiler since the gas consumption is small anyway. Other reasons include the cast-iron boiler lasts longer, requires less maintenance, less complicated. If so, it seems we'd better to install a standard 80% 50 MBH cast iron boiler. Sure, there is no Mod, and will short cycle a lot. But we can add Out-door Reset, and Del-T circulator with the cast-iron (can we do that？). With ODR and Delta T, how much more costly it would be to run a standard 50 MBH boiler than a Mod-Con (10%, 20%, 50%, 100% more costly if you can estimate ? I try to get an idea of magnitude to understand why for small load people recommend cast-iron)



(3) The standard cast-iron boiler is usually not designed for Indirect, can we still add an Indirect DHW tank to it ? The water will draw from the same/only water tank for both DHW and space heating. Will this cause problem ?



(4) In summary, among the following three, which one will you recommend for our sub-40 MBH load?



(i) standard cost-iron (e.g. WM CGi-3 as in one proposal): GOOD: longevity and low maintenance, low upfront cost. (With Indirect or Direct DWH ?)

(ii) condensing but not Mod (e.g. WM GV90+3 as in another proposal). GOOD: good fit with Indirect (?), higher efficiency. BAD: upfront, short life and more maintenance, short cycle

(iii) Con-Mod: GOOD, highest efficiency, less short cycle. BAD: upfront, short life more maintenance.

Rich_49

Oversized

Alright , I gotta do it .  Tom C , The HTP products that you mention all have one thing in common , that thing is something nobody that has commented can deny , MASS .  It is a new world as of late with all these mod cons and tighter houses and more zones , that's why HTP has developed some of the units they have . Truth is that it's hard to oversize a mod con , it has a variable range and oversizing is bad because it promotes short cycling , when we install mod cons and the smallest zone / zones are below the low end modulation we add buffer tanks , a buffer tank allows everything to be set up so that short cycling does not happen . The HTP versa line offers combi convenience , buffer mass , and lower BTUs on average than all other combi type units , the other thing it offers is a different type of heat exchanger which does not require such a rigorous maintenance schedule because you'd be hard pressed to foul up a very large center tube and several 1 1/4 tubes spiraling back toward the flue .  The only thing you may maintain is the FPHX for space heating on the Versa Hydro or the FPHX for water heating on the Versa Flame which is less likely if you have city water for obvious reasons . If local support is an issue you in particular won't have one because they are manufactured in East Freetown and are represented by Emerson Swan who recently dumped the AI Smith line of same old trash to rep HTP .  So , to be truthful you cannot oversize a mod con if you know anything about designing and installing Comfort systems , the fact is because these mod cons have a range you are able to choose one that fits and just because it is capable of reaching 100K but only ever operates at 20% - 50% for space heating it cannot be oversized but it sure is nice to have those BTUs available if you need them like this past winter .  Keeping a 55 gallon mass at 180* during non heating season . Enough fear mongering please , at 55 gallons using the default 7* differential to fire on standby the Versa Flame needs 3,207 BTU about twice a day to keep temp or about 3.60 a month in fuel in an unheated area , it will be much less in conditioned space .  Tom,  The Versa Flame is probably what you need and depending on your hot water needs the size could vary , I would probably use a small storage tank , maybe 20 gallons ,  This unit when all summed up costs the same as all the others mentioned after installation . Remember this the versa series can provide heat before the burner fires , is ODR enabled modulates and saves space . The wall hungs must be firing to provide heat unless they have a buffer tank , the mass units will enable you to heat your home even below the low end modulation by engaging a circ , possibly for an hour at very low loads in the shoulder seasons . We have all spent a long time being the best we can and the HTP offers Tom a real good option , he asked about it why do we throw so many more confusing things at him . Besides that load there is the other hot air zone that can be added to the load later on and he can enjoy odr and modulation for an Air handler also with a coil .

  How many days was it below 5* this past winter Tom ? Are you willing to be cold in your home when these things happen or would you like the extra capability to not be when it does no harm ?

SWEI

Oversized mod/cons

short cycle just like an oversized conventional boiler.  Even though there is a bit more headroom thanks to modulation, a low minimum firing rate will lead to longer run times and a happier boiler.



ROI will always be longer with a small load.  The added comfort and safety a mod/con provides may be worth a few extra bucks even if the hard dollar payback is less than optimal.



The HTP designs add mass, which is great -- just wish they made one with a smaller burner.  I could sell a 60-80k model all day long around here.

Rich_49

Smaller HTP

would be nice but the design allows for the larger burner not to hurt Swei , that was my point . With a lot of the earlier suggestions sized for the baseboard load only though the opportunity for a later conversion of the furnace to AHU with a coil was lost .

 Did I neglect to say mod cons with mass ? If so I apologize and I am sure you know what I meant .

Tim Potter

recap

your 61k/44k boiler seemed to be sized correctly on paper, but in real life was it cycling even on a design night? Some in the know here have reported boilers correctly sized to the load with a correctly figured heat loss still cycled up to 50% on a design night. Point being there is a bunch of fudge factor in the heat loss.



SWEI makes 2 good points, 1 being look at the minimum firing rate over max rate, & 2) up size the indirect rather then up-sizing the boiler.



any chance that fur-nasty might go away & an air handler might be installed? you might pre-plan for a 2nd boiler to be added in the future. 2 CI boilers can be staged to modulate & 2 mod-cons would have a 10 to 1 turn-down



If you decide to go with a correctly sized CI boiler, you might try a reverse indirect like a "turbo max". It acts like a buffer & makes hot water. I have one installed in my rental house, works like a charm. I have long boiler run times, long boiler off times, & plenty of hot water for showers after skiing. kinda kills 2 birds with 1 stone, gives you happy boilers (no short cycling) & give you lots of hot water



you have lots of work to do to get it right.



Tim

Rich_49

Wow

I just don't know what to say .

Tom C._3

too many choices and options

Thanks gentlemen for all your professional suggestions and opinions. It is a very good learning experience. However I feel like it is pretty hard to make a choice given so many choices and options. Too many choices isn't an easy thing to us as a homeowner.



To answer some questions you asked in your reply:



Rich: I checked accuweather.com, last winter is pretty cold, and there are about 20 days when the min temperature went below 5F at night in our region.

Besides, when you said “ (Versa Flame) I would probably use a small storage tank , maybe 20 gallons “. Are you suggesting add a separate 20 Gallon storage tank besides the Versa Flame? Add the 20 Gal tank on the DHW side or space heating side?



SWEI: when you said upsize the indirect, how large an indirect you are suggesting



Tim: yes, there is possibility to replace the existing old furnaces (300k BTU input, obviously oversized, will be even more so after we add insulation into empty wall ) with new one or AH,



Rob: the existing 50 Gal tank is fine with us now, since my kids/wife/myself seldom take shower in the same time.



Besides, I just checked the baseboard fin element in our house today, they are Slant/Fin Multi-Pak 80, which has slightly larger BTU output than standard Slant/Fin baseboard.

Rich_49

Multi Pak 80

Derating that for the imaginary 15% heating effect factor you will be able to disburse 52,360 BTUh through the installed radiation . This is good . You should also be able to lower the supply water temps also and really benefit from the outdoor reset . Storage tank would go on the DHW side . I would also plumb in a zone to be used at a later date for the Hydro fan coil and possibly use an outdoor reset Taco I Series there in the case that the heating coil used in your future air handler requires a different reset curve than the baseboard in the house .

  This past winters folly . Depending on where you are in Mass be very cautious of who you employ for this work . Also watch the something for nothing programs that the state and utilities offer . Everyone here can verify what kind of disastrous results and the frustration that many Mass residents felt this past winter during record cold days .  I would stay away from anyone who does a majority of those program jobs and that tells you they do most of the work for them .

I would contact Emerson Swan and try to ask Bruce Marshall for a recommendation to a qualified designer / installer in your area .

  In case you have trouble deciphering all you have heard here . If you choose one of the earlier recommendations from Swei , go with the Dunkirk , it is local also from NW NY and has a generously sized Heat exchanger that will be less likely to foul than the others . The Versa Flame or the Versa Hydro would be a fine fit for your home and the strategy that each uses will allow you to begin heating the home without firing the burner , this is achieved through MASS . They will not shortcycle because when the burner does fire the minimum rate of fire will be required .  As stated early in the discussion by a pretty bright guy , look at the low end numbers , some of the boilers mentioned will require a good maintenance schedule . The HTP's will require less attention due to their construction and in your house they will NEVER fire at more than 50% in your current configuration but if you change that furnace you will have adequate room to do so and really upgrade your heating system further with minimal investment .  

  In short , my suggestion is the  HTP  PHE130-55 . Versa Hydro , 55 gallon tank .  Call Bruce Marshall .

SWEI

Indirect sizing

should you elect to take that route, can be calculated based on fixtures and use patterns in the house.



Assuming the existing TWH fires at 40,000 BTU/hr, it will deliver at most 32,000 BTU/hr into the water.  Continuous output will be higher with the new setup even with a 30 gallon indirect.  First hour requirements may dictate a larger tank (or not.)  No soaker tub there?