Advice needed please…

Skyline

My house is a small one, three bedrooms, semi-open concept living/dining room, kitchen, bathroom, laundry room, about 1,300sqf, occupied by two adults.

Currently, the heat/hot water is provided by Triangle Tube Phase III boiler, with 30G HW tank in the boiler. The majority of heat is via seven cast iron radiators in zone 1, while zone 2 is single room with baseboard. The boiler is still in working condition, does not have major issues, nor did it have during the last 20 years. Minor issues here and there, but they remediated rather quickly.

The radiator in the kitchen need to go, due to the remodeling plans that brought up the question about the boiler. Should we update the boiler as well? We answered yes and started to have contractors giving us estimates. My question is more about sizing the modulating condensing boilers.

The Phase III is 120,000 BTU/hr and seems oversized to the size of the house. The first estimate received includes the IBC DC 33-160 combi-boiler natural gas boiler. That is a 160,000 BTU/hr unit, about 33% increase over the current one. I do not know much about sizing the boiler, but the estimate seems to move to the wrong direction.

If anything, shouldn’t the size of the new boiler be around 80-100K BTU/hr?

This is actually the first time I heard about IBC boilers. I do know that they made in Canada, but that is about all. Where does the quality of this boiler, where compared to others?

TIA…

Jamie Hall

When you are dealing with hot water heat, and especially with the possibility of mod/con boilers, it's all about building heat loss.

The back of the envelope question is -- does the existing boiler keep the house warm on the coldest nights and days? if it does, it's at least big enough, if not bigger than need be, and it's simply ridiculous to suggest going to a bigger boiler.

Now to size a mod/con boiler properly, it needs to be sized so that it is matched to the heat loss of the house; no bigger and no smaller, within a few percent. So your contractor should do -- or have done -- a Manual J heat loss for the house and provide you the result, and select the correct boiler based on that. For that matter, you can do it pretty well yourself. There are several on-line heat loss calculators; I happen to like the one Slant-Fin provides, but there are others.

Now there is a joker in the pack. The domestic hot water. Combis are nice, but... your instantaneous domestic hot water requirement, in terms of burner power, may be considerably greater than the heating requirement, and this may cause you to have a bigger boiler than needed for heating, just to keep your showers from being chilly. This will hurt your efficiency on warmer days, which are most of them. I've never really been keen on combis for that reason, preferring a regular mod/con boiler with an indirect tank for the domestic hot water.

Zman

I am not familiar with that boiler. What type is it? Do you have a picture?

Brewbeer

You can use gas consumption and weather data to estimate boiler size, I forget how but someone here can post it. Your location makes a big difference in heat requirement, but 120k BTU seems excessive for your house. For example, I'm heating 2,000 sq.ft of 1965 split ranch with a 0F design day requirement of 35k BTU.

Steamhead

I've encountered a lot of cold kitchens caused by remodelers insisting on removing radiators.

You can still get cast-iron radiators new, and there are plenty of used ones. Look into changing to a radiator of a different shape, to fit the available space.

HVACNUT

I'm not a huge fan of TT, but if it ain't broke...
Do all of the above as far as heat loss for a future upgrade.
And don't plan on replacing one cast iron radiator in a loop to fin tube and expect it to work. Hope you're just relocating it.

Skyline

Thank you all for your help...

@Jamie Hall

I had two contractors visits to update my boiler, both of them suggesting the IBC mod-con boilers. Neither of them did any heat loss calculation, one provided an estimated cost so far.

The Triangle boiler did keep the house cozy during the last 20 years. The last eight years or so, we keep the house a 70-72 degree Fahrenheit in the winter, we babysit the grand-kids during the days.

I'd love to have an indirect tank, but the "boiler room" is really small and it also has the exit to the backyard.

I did a rough estimate for sizing the boiler for the radiators, based on this link. The raw heating capacity of all of my radiators is 55,000 BTU/HR, then multiplied by 1.4 for estimating the boiler size. That gave me 80,000 BTU/HR, but it does not take in to account the heat loss. I just don't know how accurate this estimate is.

Thanks for the Slant-Fin calculator, that's going to be my next item to tackle.

@Zman

I've never heard of IBC either, until the two contractors suggested their boiler. Here's a link to the quoted IBC boiler:

https://usa.ibcboiler.com/consumer/products/dc-series/dc-33-160/

@Brewbeer

In CT along the shoreline, it seldom gets to 0F, if ever. The average winter temperature is around 25F.

@Steamhead

It's a galley kitchen that has an open doorway to the laundry room that has a larger radiator. It also has an open doorway to the living room. The radiator removed has the smallest raw heating capacity of the radiators the house has.

In the place of the radiator, there will be a cabinet, effectively eliminating the last place where heating could be added.

Steamhead

They could probably find someplace to put a radiator if they tried. But most remuddling contractors are lazy.

hot_rod

You need to size the boiler to the largest load, in some, many, cases nowadays that is the DHW. So with a combi you may run into an oversized boiler to supply adequate DHW. For a family of two a 120,000 boiler can supply 2 gpm or more, so plenty for a single faucet or shower load.

For the heating side, the boilers can be locked at a low firing rate to better match the load. A 120K boiler could lock down to 10,000 firing rate for example.

The combis, like most mod cons have outdoor reset and many other features built into the control.

About 40% of mod cons sold ware now combis, so they are being accepted. Same with tankless water heaters taking a big share of that market.
If you do not have space for a conventional boiler and indirect, combis are worth a look. IBC is one of the top brands, the installer is the key to any project.

Skyline

@Steamhead (in transit)

Removing the radiator is our design to make room for more kitchen cabinets. There will no replacement and as stated earlier, there's really no room to put even a baseboard, much less a radiator anywhere. Neither of the plumbers so far seen any issue with the heat in the kitchen with the radiator removed.

@hot_rod

In case of the IBC, both of the D series has a max flow rate DHW heating of 4gpm. For two of us, that is plenty in my view. I'll talk to the plumber to have the DC 20-125, instead of the DC 33-160.

I maybe able to fit in a small indirect water heater, like the BTI-30 or the BTI-40 from IBC. In which case the quote needs to be changed to the HC 20-125 or the SL 14-115G3 from IBC. Wouldn't that be the same that I have now? Meaning that the boiler would kick in periodically to keep the DHW in in the indirect heater at operating temperature?

It's all about cost, like anything else. I don't know much about boiler costs, but it seems that the combi boiler will lighten up my checkbook less. I could be wrong...

mattmia2

The max flow rate is dependent on the heat input and delta T. You might be able to move 4 gpm through it, but if it doesn't have the heat output to heat that volume of water from the incoming temp to the desired outgoing temp, it will be 4 gpm of cold water.

Indirect capacity doesn't map directly from that of a direct fired water heater. They are slower to respond to a call for heat, the boiler takes more time to come up to temp and start heating the water but once it does it usually has a lot more output than the burner in a direct fired water heater. You may need a bigger tank than a direct fired water heater to accommodate that start up time depending on your usage pattern.

It isn't entirely clear, but I assume that the system is forced hot water. If that is the case, a kick space heater may be an option in the kitchen although balancing it with the radiators will be a challenge.

Steamhead

@Skyline , I'm just going by what I see in the field. I see a lot of chilly kitchens and bathrooms that used to have radiators but someone removed them.

The Steam Whisperer

If skylkline is using a low mass boiler, The warm up time is negligable. Even on the higher water content Triangle Tubes, it only about 25 gallons of water and that may already be partially up to temp, especially during the heating season which is also when DHW incoming temps will be the lowest.

HVACNUT

> @Steamhead said:
> They could probably find someplace to put a radiator if they tried. But most remuddling contractors are lazy.

Or they suggest a toe kick heater.

Don't go without heat. Can you add a dedicated kitchen zone? You know you always wanted radiant in the kitchen. If the boiler is using outdoor reset, a small zone shouldn't be a huge issue. Even the radiator on an interior wall if cabinets are taking up the perimeter. Better to have than have not.

PerryHolzman

I'm a homeowner, and know enough about hot water and steam heat to end up talking to a lot of other home owners about issues; and removing radiators almost always creates lack of heat issues.

I also happen to live in a house where the prior homeowner removed the cast iron baseboard on a wall and corner to install cabinets on that wall - the removed sections are in the garage attic, and the slant fin calculator says that those sections are needed). Fortunately the 8 ft under the window is still installed, and the house allows free circulation with rest of the 1st floor so that the kitchen is only slightly colder on cold days.

I'm with Steamhead. You are very likely to end up with a chilly kitchen if you remove that radiator and do not replace it with something.

May I suggest that you study the options for modern wall panel radiators at Runtal (there are other companies as well).

https://runtalnorthamerica.com/residential-radiators/

I suggest that you figure a heat loss for the kitchen (do the slant fin calculator for the house), and then figure out how to include modern wall panel radiators in the kitchen. They could be on the wall above a counter and below the upper cabinets (leave some space for electrical receptacles at the ends). They could be mounted on the ceiling for "radiant" heat.

Depending on your kitchen, there may be other options.

I wish you well with this,

Perry

Skyline

Thanks everyone for your advise...

The radiator will be moved, instead of removed. We've made minor changes to the kitchen redesign and will fit it in. So many people cannot be wrong and I do appreciate all of your concerns about a cold kitchen. She'll appreciate keeping the kitchen at the same temperature in the winter. Thanks guys and gals...

My current Triangle Tube has a 42 gallons reserve capacity. I don't believe that it is for the DHW only, but we didn't have issues with DHW until last year. The reason for running out of hot water was that DHW mixing valve got stuck and didn't mix as it should have. Couple of turns back-and-force did fix the issue, didn't have to change it.

Please help to understand couple things with modcon DHW...

The IBC DC series DHW delivery @70°F/ 39°C temperature rise is 3.2 gpm. In my neck of the wood, the tap water temperature in the winter is around 56F. If I add the two together, the DC series delivers 126F DHW at the rate of 3.2gpm in the winter. Yes, they are nominal values, but as long as it's not lumber yard deviation from the nominal, wouldn't the DHW be a OK for two people with a single bathroom?

My understanding of the indirect water heater is that the 30 or 40 gallons of the water is kept at the desired DHW temperature. The chances are that there's a pump to circulate the water, if it gets under a certain, predefined temperature for bring the temperature back to the desired level.

If that's the case... On the surface it does not seem much different from what I have now with the Triangle Tube, except for the pump.

I could be wrong in both case, in which case, please explain. TIA...

@mattmia2

Yes, it is "forced hot water", there are number of pumps that circulate the water around, if the system calls for heat. I think that was your question?

Jamie Hall

The thing to remember is that it's all about how long can you flow hot water at a given rate, and then how fast can you recover.

The idea of the IBC series -- and, for that matter, all the "tankless" or combi systems and so on and on -- is that they have a big enough burner in there to need no storage. They will raise the temperature of the incoming flow a certain amount; if the flow is greater than a certain amount, they won't be able to give you the temperature you want. The formula is the good old hydronics formula, which you can rearrange (how's your algebra?) any way you want: BTUh equals Flow in gpm times delta T in degrees F times 500. They have a flow switch, so that when there is no flow, there's no heat added. This can sometimes trip you up -- the switch is often around half a gallon per minute. They also have a high limit, so that if there isn't enough flow they will either modulate the burner down -- or turn it off completely.

So long as you don't ask too much flow from them, they're fine.

The storage idea (including indirect) is that you have a certain amount of hot water stored all the time -- typically 30 gallons as a minimum, but sometimes a lot more -- and you can run the flow as fast as you want, provided you don't actually run more than that volume (for a 30 gallon tank, for instance, 6 minutes at 5 gom). As soon as the temperature in the tank starts dropping, though -- due to the cold water coming in to replace the hot -- the burner or electric elements (or heat pump, for a hybrid) for a self contained heater, or the circulator, for an indirect, kicks on. At which point your recovery rate is determined as in the tankless above -- and is usually much less (not always! The oil fired hot water heater in Cedric's home recovers at a delta T of 70 at 2 gallons per minute -- not bad for a tank type!). Of the various direct types, hybrids are usually slowest, followed by electrids; gas and oil stand alones can be pretty quick. Indirects are partly determined by the boiler feeding them, but often by the capacity of the heat exchanger involved.

So long as you don't use too much hot water all at once, they're fine, too.

The trick is to figure out how much hot water you need for how long, and then how best to meet it. There's no one right answer.

Zman

I meant, what type is your existing boiler? I believe the Triangle Tube Phase 3 is an indirect water heater.

hot_rod

mattmia2 said:

The max flow rate is dependent on the heat input and delta T. You might be able to move 4 gpm through it, but if it doesn't have the heat output to heat that volume of water from the incoming temp to the desired outgoing temp, it will be 4 gpm of cold water.

Indirect capacity doesn't map directly from that of a direct fired water heater. They are slower to respond to a call for heat, the boiler takes more time to come up to temp and start heating the water but once it does it usually has a lot more output than the burner in a direct fired water heater. You may need a bigger tank than a direct fired water heater to accommodate that start up time depending on your usage pattern.

It isn't entirely clear, but I assume that the system is forced hot water. If that is the case, a kick space heater may be an option in the kitchen although balancing it with the radiators will be a challenge.

Certainly you want to read the DHW output data and numbers on the manufacturers spec.
Most brand name tankless and combis rate at a 77° delta. So working backwards from that, a 33° inlet would heat to 110F.
33° incoming should be a rare occasion
103- 105F should be plenty warm for a shower bath and laundry load. Even if they use a 70∆ that would just give a smaller gpm rate. I use the temperature boost on the dishwasher just to assure a high temperature cycle for any bacteria concerns.

Most shower heads are 2 gpm so IF the unit provides 4 gpm at a 70- 77° rise, that is usually plenty for even two DHW loads running.

BTU formula is 500 X flow rate X ∆T
So a 110,000 output boiler should supply around 2.5 GPM at 77 rise
A 140,000 output around 3.5 GPM

My wife and I get along fine with a 120,000 combi, endless DHW at a 2.5 flow.

A 120,000 should be able to derate on the heat side close to your heat load. I have mine locked at 30% for heat, 100% for DHW

Most well water runs 50- 55F year around so that would up the output numbers considerably. Public water stored above ground in some areas could drop into the 40's or high 30's I suppose.

I have also lived with indirects for 30 years, I feel the combi use less propane compared to a boiler/ indirect package. Just the tanks heatloss, especially if you maintain 140F for adequate drawdown.

Skyline

Thank you all for your help, much appreciated...

@Jamie Hall

Like most people, I tend to believe that my algebra is pretty good. Maybe not, let's see with your formula:

System Delivered Btu = 500 x GPM x DHW Temperature Rise

112,000 = 500 x 3.2 x 70*

*-The GPM and DHW values are based on the IBC DC 20-125 technical data

Setting aside the efficiency rating and adding the tap water temperature of 56F in the winter, a 112,000 Btu unit would deliver 126F DHW at 3.2 gpm. The actual Btu rating for the DC 20-125 is 20-125 MBH; the IBC provided technical data checks out, even if efficiency rating is taken in to account.

The rain shower head in my only bathroom has flow rating of 2 gpm, includes both the cold and hot water. While my shower is about 6-7 minutes on the cool side, my wife's is around 30 minutes and on the hot side. In either case, there seems to be more than enough DHW output from the DC 20-125, if the given technical data is correct. Of course, if the wash machine/dishwasher running and someone doing dishes as well (not going to be me ) at the same time, most combi-boiler system would run out hot water.

Provided that the DC 20-125 gives us sufficient DHW output, there's really no need for the indirect water tank. The benefits of having one in our case does not seem to justify the additional cost.

@Zman

I'd call the Phase 3 a "combi-boiler". The 42 gallon reserve has coils for DHW, both the heat and DHW thermostats can kick in the boiler. The only time we may have issue with the DHW, if the heat just kicked in, the return water temperature low and there's high demand for DHW. It does not last for long, but there's a small window for deviation from the set DHW temperature.

@hot_rod

The 33° incoming water would allow the fridge to make ice cubes faster...

My tap water temperature is 56F in the winter. It seems to me that the IBC DC 20-125 would provide plenty of DHW for the two of us.

While the estimate cites the IBC DC 33-160 to replace the Triangle Tube, its DHW delivery is the same as the DC 20-125. I am still waiting for the plumber to revise the estimate.

Setting up the 30/70% split is done by the installer, that could be modified later, if needed.

I still need to check, IBC requires certified installer, should have started with that...

Zman

Thanks for the clarification. I have seen that boiler. Probably not one of Triangle Tube's finest moments.
I would suggest the following:

• Pay close attention to your load calcs, oversizing is the Devil's work.
  
• Design for efficiency on the typical day not the coldest day. Boilers with high turn-downs help you do this and still cover the coldest day.
  
• It is all about the installer and local manufacture support. A brand that is popular and well supported in one part of the country, may be a bad idea in another part of the country. Local training and support is everything
  

EdTheHeaterMan

Sometimes you might want to check on other projects the installer has completed. Also, check with the Supply house where the contractor is purchasing the boiler. How long have they offered the product? What is the track record of the product as far as warranty claims, what's the track record of the contractor?

I was hired by a supply house from being a contractor/installer because they knew that I was not calling them for tech support all the time. If your guy is the "problem contractor" then you might not want to use them. Remember the supply house may not want to bad mouth a customer of theirs so you may need to read between the lines. How long has he been a customer? does he attend many training seminars?

Skyline

@Zman

What does it meant the Phase III "not one of Triangle Tube's finest moments". It's still working after 20 years, makes me wonder just how long the Triangle Tube finest moment would last...

Anyway...

The first contractor has got back to me about the IBC DC 33-160 boiler, yes, I questioned the size. He explained that it is a modulating unit and better have more, just in case. Ok...

The second contractor showed up today, who actually did a load calculation by measuring the radiators and the the one baseboard to get the BTU requirements. He was concerned about short cycling the boiler, if it is oversized. His number was 87 MBH , a bit higher than I've calculated earlier. He had stated, that the Phase III had been sized correctly. I am pretty certain, that his estimate will list the IBC DC 20-125 model as the replacement.

All in all, he's the first guy, who was actually concerned about not to oversize the boiler and the operation once it is installed.

He also explained the origin and meaning of the EDR, that I've got somewhere on the web.

EDR = Every Darn Radiator

He actually guessed two websites, from where I've got it. That was impressive...

Come to find out, in the two houses across from mine, he actually installed Navien boilers 3-4 years ego. My neighbors like him, he had quick response time, when they need him. I am still evaluating companies, who did or will give me estimates; also have one more company coming next week.

I might be pushing my luck, but...

I'd love to have the IBC DC 15-96 combi-boiler, the annual cost is close 400 bucks less, than the DC 20-125.

Would the DC 15-96 actually be suitable for my house?

mattmia2

The edr may be useful in a hot water system to figure out if it is properly balanced, but it should not be used to size the boiler. the heat loss of the structure should be used for sizing.(and dhw needs if the boiler is also providing dhw).

Jamie Hall

Good analysis of your need for hot water vs. the boiler, @Skyline . Makes me happy to see that!

Brewbeer

Measuring the radiators isn't the same thing as a load calculation. Measuring radiators tells you the rate heat can be supplied to the structure, not how fast the structure is losing heat. Radiator output is useful in determining maximum boiler size; It's likely your structure's actual design day requirement is well below 87MBTU.

The Steam Whisperer

AT 1300 sq ft and a 0F design temp, your heat load is probably somewhere between 32,000 and 45,000 btu/hr boiler output, depending on insulation, storm windows and air tightness. I've got about 1600 sq ft plus full basement and my 105,000 btu/hr Cast iron boiler runs at about 50% capacity or less when temps are well below zero. We have r-13 walls, r-25 attic and 1903 windows well weatherstripped and mostly new basement windows.

Skyline

I have been struggling to make sense of all of the information that you guys/gals kindly provided and very much appreciated. Some of this information I pick up easy, while others researched, but it is getting easier as I go along.

Question…

Isn't there a correlation between the maximum output of a boiler and the DHW GPM? Looking at number of different combi-boilers seem to show, that the lower maximum boiler output lowers the DHW GPM. While I am comfortable with the 3.2 GPM DHW, less than 2 GPM would not be enough in my case.

It might be possible sizing the new boiler under 100K Btu/hr; on the other hand, doing so may require an indirect hot water heater. The saving from the lower boiler size could be quickly offset by the cost of the indirect hot water heater, both installation and operational cost.

Is this correct, or I don’t know enough yet about combi-boilers?

My house built in the early 50s. About eight years ego, Okna Enviro-Star 800, double pane, double-hung windows, vinyl siding and installation under the vinyl added. No clue what the R-values might be.

Looking at my last 12-month gas usage, including average temperatures, it shows this:



If I look at the IBC DC 20-125 estimated yearly MMBtu at EnergyStar website, it estimates 116.3. That is 24.2 MMBtu more, than my last 12-month of natural gas utilization.

This is confusing, mainly since the EnergyStar website does not indicate how they got the quoted number. I hope my gas bill will not go up, if I replace the boiler…

Jamie Hall

You are quite correct that there is a connection between the combi burner's maximum output and the domestic hot water. You had it, a couple of posts ago when we were talking about algebra. If you are going to depend on a combi -- or, for that matter, a tankless -- to supply your hot water, you need to figure your maximum flow and the temperature rise at that flow -- 70 F is a good number to use to allow a bit of a margin, even on city water -- and then the BTUh output of the burner just comes out of that equation.

Personally I'd never size a system for less than 2 showers going at the same time. But that's me. Cedric's home has 5 showers, and has been known to have all 5 going -- but it also has a stand-alone oil fired hot water heater, so it's never been a problem!

That EnergyStar website is vaguely useful -- for comparing different boilers. It's utterly useless for figuring you own gas use. How they get their numbers is never explained, but it is quite clear that however they are doing it has some relationship to the maximum capacity of the boiler -- which has no relationship at all to how much gas you will use in a given building in a given climate. Nor is it useful for comparing different boilers of different net outputs. So don't let it worry you.

The Steam Whisperer

Looking at your average daily usage in the 3 coldest months (average 36f) and subtracted you summer usage ( water heating, etc), you are using only 3.58 cu/ft per day or about 15 334 btu/hr for heating on a typical 36F day. If you start needing heat at 60F, then running the numbers you only need 51,000 btu/hr input when it is -10F. Your boiler is grossly oversized for the space heating load.

hrlevy

We put radiant in our kitchen. Love the warm feet.

Skyline

@Jamie Hall
I rather have the DHW GPM oversized for my needs. It seems that some of the combi-boilers do have relatively high DHW flow, without the oversized maximum output for the heat.

@The Steam Whisperer
We like the house warm in the winter for two reasons. One, we work from home, while the other is grandkids. As such, we keep the temperature 70-72F and drop it at night to 66F.

Last winter has been one of the warmest in my area, maybe due to global warming. We do get couple of days of close to 0F and the average winter temperature in the mid-20s most of the times. How did you calculate the 51,000 Btu needs, can you share it with me?

Based on your 51,000 Btu calculation at -10F and my DHW GPM needs, the Navien NCB-180E would be more than sufficient for my needs:

• Space heating: 14,000 - 80,000 BTU/H
• Domestic hot water: 14,000 - 150,000 BTU/H
• Flow rate (DHW): 3.4 GPM (77°F temp rise)

PS: I have nothing against the IBC DC20-120 combi-boiler, other than its output, that "grossly oversized" for my needs it seems.

Zman

There is a correlation between the boilers ability to heat water on demand and the amount of storage you have.

Comparing an electric tank water heater to a gas instantaneous is an extreme example.

The electric heater likely has 5,000 watt coils.5,000 x 3.412 = 17,060 BTU/hours. 17,060 / 70 (delta T) / 500 = ) O.5 GPM demand capacity. A typical american family gets by with a unit like this because they have 50- 60 gallons of storage.

A typical 200,000 BTU gas instantaneous water heater works out like this. 200,000 X .9 (efficiency derate) = 180,000 / 70 / 500 = 5.14 GPM.

There are numerous formula's and online calculators to assist with the decision. I have used one that AO Smith puts out as well as Lochinvar's.

In my opinion, sizing the boiler to meet the heating load and then up sizing the indirect if needed to cover the DHW is the most efficient way to go for most projects. In your case, an 80 K/btu boiler with 10-1 turndown and a 40 gallon indirect with a large coil would work great.

The Steam Whisperer

I took the 3 coldest months and calculated the average temperature. Then I assumed that internal heat gains such as breathing, baths, cooking, sunlight can keep the house warm down to 60F ..... this is being quite optimistic. Must old homes need heating below 65F average daily temp., so there will result in a higher heat load number. I looked at the average daily usage for those 3 months, about 4.3 cu ft and then subtracted the summer use ( .72 cu.ft./day for hot water, cooking etc) and ened up with 3.58 cuft per day. Converting that to btus/ day.... thats 3.58 x 102,800 btu/cuft = 368,024 btu/day
368,024 btu/day / 24 hours= 15, 334 btu/hr when its 24 degrees colder out than in ( 60F - 36F). 15, 334 / 24 degress = 638 btu/hr for each degree warmer inside than outside. For -10F outdoor 638btu/hr/ F x (60- -10) = 44724 btu/hr. However, as it gets colder outside air leakage accelerates, so I usually add about 15%.
44, 724 x 115% = 51, 432 btu/hr boiler input. This doesn't take into account boiler efficiency changes, but gives a good ballpark number.

hot_rod

A combi example, the 150K with your incoming water temperature should get you close to 5 gpm continuously. You can lock down the heating output as low 13,900.
Personally I think a 110 might be adequate, but if you want to error safe on DHW performance the 150 would work well, not a huge if any price difference.

Depending on the brand snd size indirect, I suspect a combi would come in $1000 or more less, compared to boiler and indirect, if that is part of the decision.

15 years ago combis were barely even on the radar, now manufacturers tell me they account for 40% and growing of the mod con market. I'd guess 80% or more of Europe and the rest of the world generate DHW with combis or tankless.
Really no need to store a tank full of potential bacteria in you home anymore

I've lived with combis, 3 at one point! for 15 years now, just removed one of the first Laars/ Baxi that were brought into the market.

Skyline

Thank you all, this is great information, filed them away...

As long as the DHW GPM is 3.x, it'll be just fine with a single bathroom in the house. I was more worried about the heating capacity, but @The Steam Whisperer put me at ease with the explanation of his calculation.

While I don't know the heat-loss is in my house, it's probably average for "This old house". Even if it is worse, it's not but say it is by 3-5 times, 44,724 btu/hr x 150% = 66,990 btu/hr boiler output that's all I would need. The 80,000 btu/hr rated combi-boiler, with 150,000 btu/hr DHW should handle this load with ease and provide 3.5 GPM DHW at the same time. It may even cover the loss in efficiency.

At least that's how it looks for me. I am at the point with my knowledge on this subject of being dangerous; please set me straight if that's the case...

hot_rod

I think you have it.

One comment, if an 80,000 boiler goes up to 150,000 for DHW, it’s really a 150,000 boiler😉

Skyline

@hot_rod

Makes me think, does the Navien NBC-E really have Dual stainless steel heat exchangers?

mattmia2

Skyline said:

@hot_rod

Makes me think, does the Navien NBC-E really have Dual stainless steel heat exchangers?

Probably. One to transfer heat from combustion to the system water, one to transfer the heat from the system water to the dhw.

hot_rod

> @mattmia2 said:
> (Quote)
> Probably. One to transfer heat from combustion to the system water, one to transfer the heat from the system water to the dhw.

Best I can see they are calling the boiler one heat exchanger, the flat plate DHW the second. How does that differ from other combis?

Need to see a cut away to know for sure

Skyline

mattmia2 said:

Skyline said:

@hot_rod

Makes me think, does the Navien NBC-E really have Dual stainless steel heat exchangers?

Probably. One to transfer heat from combustion to the system water, one to transfer the heat from the system water to the dhw.

So, what happens, when both the system water and DHW is calling for heat?

If there's a priority for the heat, would I loose the DHW? Say it ain't so...