Mod Con boiler sizing on new install in a brick building

pexhead

I am looking to heat a 2900 square foot (total) 2 story brick 2 unit apartment building with 10 foot ceilings in Albany NY built in 1860s. This is a new install of a radiant floor with plate with a natural gas mod con boiler heating the entire structure. I plan on using an indirect tank (40 gal. @ 140) to provide DHW for 2 showers( 4-5 GPM )and 2 laundry rooms I have done a heat loss calculation for heating only using low insulation values as I am not sure I can insulate the outer walls well. Heat loss calculation is 123,000 at 70 degree differential and 104,000 at 60 deg.
I am planning on maxing out 1/2 inch radiant loops at 280 feet to get the best delta t possible on return temps. With 1450 feet of full accessible floor space I think I can do this pretty well.

What size Mod con should I be looking at to achieve the best efficiency. I don't want to short cycle. It seems that oversizing is a problem with condensing units and don't want to make that mistake. You guys have been a great help over the years with my current system. I would appreciate any thoughts you have.

Gordy

Hmm. Two unit apartment building. One boiler?
Any zoning with in each unit?

What is your calculated AWT in the loops?

pexhead

One boiler if I can manage it. I have a great spot for a mechanical room to centralize the heating system on first floor. One zone for each floor because demand on second floor apartment will be less. AWT I calculate at 125. Are you thinking that 2 units makes more sense?

Gordy

No, trying to get a handle on smallest loads. You want to keep the smallest load with in the lowest modulation of the selected mod/con.

Example: a uft 120 will have a 12k low end modulation. So keeping the zone at, or slightly above helps with cycling. Of course on less than design day temps it become an issue with cycling.

Will there be an indirect supplying DHW?

Gordy

43 btus a sf seems pretty high for the heat loss.

pexhead

That is the plan. Not sure if 40 gal is quite enough. Average winter temps in this are are -1deg. so I don't want to have DHW suffer.

Zman

Most radiant systems will require primary secondary piping. In that case the tubing length is irrelevant. Keeping your tubing more like 200' will increase circ efficiency and improve performance.

I suspect your heat loss numbers are high. Old brick houses are difficult to figure. Do you have historic fuel use to double check?

You will have trouble getting 40+ btu/ft out of any radiant assembly. You definitely won't with 125 deg water. It is also not advisable to get the floor that hot.

pexhead

Shouldn't have any problem keeping zones above 12k.

Zman

You need to get an accurate heat loss number.
Out door reset is advisable,it will however reduce the output on a typical day.
What is your existing heat system.

pexhead

z man You think radiant on this is not possible? I have 24" joist spacing so I can fit 3 runs of pex per bay. With good aluminum plates i might get to 40 but /ft. I used Watts flex plates on my last system but that would be too pricy on this size system. My current system runs at 118 AWT but I have better insulation.

pexhead

I don't have historic fuel use numbers. This had steam heat and system was stripped during foreclosure. I'm sure my HL is figured high. I have double pane windows and figured single pane because my experience with brick is nil.

Zman

pexhead said:

That is the plan. Not sure if 40 gal is quite enough. Average winter temps in this are are -1deg. so I don't want to have DHW suffer.

The domestic can be setup as a priority. The indirect size just pertains to the storage. The boiler will determine the continuous output.
Do you have large tubs to fill or high flow shower heads?

pexhead

no big tubs zman figuring 2 showers at 2 gpm each plus 2 washers and dishwashers. This is a totally new system. Old system was steam and rads were stripped out before I bought it out of foreclosure. In the process of gutting it out but really can't insulated the exterior walls well because window trim is directly on the brick floors and ceiling will be well insulated.

pexhead

Gordy, The heat loss calc is making me crazy because brick without insulation on the outside walls make heat loss calculations go off the charts. Radiant floor with Aluminum plates (three runs per joist bay full plated) should be enough to deliver the BTUs needed don't you think? windows are double pane. The floors are plank and will be well insulated as will the ceilings. It is a total gut job.

Bob Bona_4

I would be doing a radiant heat loss design here.

Zman

I think radiant is very possible. I also think your heat loss is inaccurate and you are oversizing the boiler to both the heat loss and the available emmiters
Don't randomly change the windows to compensate for the walls.
How is the brick wall constructed? Is there a vertical air space?
How about attic insulation?

Paul48

I think this was tested by Hatterasguy, and found to be quite accurate. https://www.ag.ndsu.edu/pubs/ageng/structu/ae1373.pdf

pexhead

Zman R34 at least in attic. and double pane insulated windows. I can come up with HL of 104,000 on the low side. The brick walls are 8 inch at minimum but no insulation.

EBEBRATT-Ed

The walls are going to be cold. Ever here of cold 70? Put some baseboard around the exterior to hit the walls and radiant the rest if you can't get enough baseboard in

NY_Rob

I would look into using a larger than 40gal indirect for a two family dewlling.

Murphy tells us that there will be occasions (7am workday mornings) where both showers will be running at the same time... and someone will run a load of laundry and/or use the dishwasher at that time too

Make sure the tank you choose has 1" or larger boiler connections- some only have 3/4" boiler connections and take longer to recover. The longer it takes to recover DHW means the longer your spaceheating loops are not working.

Gordy

pexhead said:

Gordy, The heat loss calc is making me crazy because brick without insulation on the outside walls make heat loss calculations go off the charts. Radiant floor with Aluminum plates (three runs per joist bay full plated) should be enough to deliver the BTUs needed don't you think? windows are double pane. The floors are plank and will be well insulated as will the ceilings. It is a total gut job.

The most you will ever get from a radiant floor assembly is 30-35btus a sf. The limiting factor is exceptable floor temps. 85 max Radiant ceilings, and walls offer higher outputs due to the ability of having higher surface temps.

I think you need to get a better heat loss number with a radiant program.

Gordy

Some very good advice from some heavy hitters also. I don't agree with your long loop length approach. Keep head loss down in ecm circ range. The return temps to boiler are a function of awt.

pexhead

Gordy said:

Some very good advice from some heavy hitters also. I don't agree with your long loop length approach. Keep head loss down in ecm circ range. The return temps to boiler are a function of awt.

What would you recommend for max loop length. I'm sure I can hit 30 BTU/sqft. Maybe a hybrid with baseboard on exterior walls and radiant in the center would be a better alternative. Primary/secondary would make that possible bringing higher temps to the outside and leaving radiant floor to keep the floors warm?

Zman

I think Ed makes an excellent point. The last thing you want is a super hot floor and cold wall, it makes the body uncomfortable. You will have cold walls. Maybe your house would be a good candidate for a hybrid system using in floor and/or in wall radiant in critical areas like bathrooms. You could then do a low temp low profile panel radiator along the wall and under windows. Runtal makes a nice one.

Rob also makes a good argument for a larger tank. The last thing you want is have one neighbor pissed at the other about water usage. You will also get better efficiency out of a small mod con with an indirect with a big heat exchanger, due to the lower return water temps to the boiler.

Tinman

The building you describe is similar to what we see in Chicago. 2 flat, brick apartment buildings with zero insulation. 2 zones, oversized CI boiler, over-radiated with CI radiators, and a high load calc.

Here's what we've been doing successfully. We advise the customer to button up the envelope. R-45 ceilings, weatherstripping, whatever can be done. Most agree and we move forward based on that.

We've been installing 5:1, 110K mod cons with a 50 gallon indirect knowing that, on paper, the new boiler is a bit "undersized". The new small boiler with the more than extra EDR from the radiators has made for some sweet systems. And surprisingly, when we've had temps near design, the boiler is more than keeping up w/o any additional insulation. 15-20% buffer on load calcs may be an understatement?

Gordy

The mass of a stone, or brick structure carries the load through design conditions. Slow to warm , and slow to cool.

Can't beat those ci rads either.

pexhead

Thank you all for your input. You have given me a lot to think about. The constructive advice on this forum is intelligent and adult and much appreciated. Will keep you updated.

kcopp

How think ae the floors? Old buildings like that usually have 6" thick dense flooring. Especially where you say 24" spacing. I would opt for oversized panel rads or Refurbished CI rads w a ModCon. Another option for a boiler would be 2 wall hung combi boilers. This would allow metering usage and have some redundancy if there were an issue w a breakdown in the future.

Gordy

@kcopp hit another variable on floor thickness. As he said with joists 24" on center the sub floor has to be at least 1 1/2", and then what is finished floor? Looking at a high r value to push the heat through to heat the space.

EBEBRATT-Ed

@Gordy &@Zman are correct. The thermal mass of the brick must be considered.

saw a commercial building in Hartford, Ct they converted it to warm air ripped out the steam...an office building all exposed brick no insulation

You couldn't make the people comfortable the complaints were horrific. Touching the brick wall was like touching an ice cube

Rich_49

pexhead said:

One boiler if I can manage it. I have a great spot for a mechanical room to centralize the heating system on first floor. One zone for each floor because demand on second floor apartment will be less. AWT I calculate at 125. Are you thinking that 2 units makes more sense?

Why will demand for second floor be lower ?

pexhead

Rich said:

pexhead said:

One boiler if I can manage it. I have a great spot for a mechanical room to centralize the heating system on first floor. One zone for each floor because demand on second floor apartment will be less. AWT I calculate at 125. Are you thinking that 2 units makes more sense?

Why will demand for second floor be lower ?

I would have thought that heat rising through the floor from downstairs (10 foot ceilings) would tend to lessen the demand for heat in the upstair apartment, Maybe that is optimistic.

kcopp

HEAT doesn't rise... it goes to cold. Plus the floor will need to be insulated very well to drive the heat UP through the floor.
How thick is the flooring?

pexhead

kcopp said:

HEAT doesn't rise... it goes to cold. Plus the floor will need to be insulated very well to drive the heat UP through the floor.
How thick is the flooring?

The joists are 16 on center and floor is 1" -1 1/4 " plank

Bob Bona_4

You've got to do the radiant design. So much will be revealed. Forget anything you've heard about 200/250/whatever length loops. You have to know what the floor loading is, if supplemental heat is required, flow rates, head loss etc etc. You could have an area that needs one 225 foot loop or three 110 foot loops. Or you need 10k btu supplement to make design temps. I cannot over stress the importance of knowing what it will take before even committing to a quote.

Gordy

Agree with the above comments. Radiant heats objects. The air is only a secondary effect from the heated objects. Everything in the envelope has mass. Walls, floors, ceiling, plaster, furniture etc.

We are not trying to be difficult, only trying to lead you down a path that insures success with your project. If you don't have time, and money to do it right the first time you certainly don't the second. We see a lot of folks having to do the second time around after coming here.

The first step of your success is getting the heatloss dialed in room by room. This is the beginning step that dictates what happens in the next steps. Loop centers, flow rates, water temps, boiler size, the possible need for supplemental emitters.

CMadatMe

Heat loss is wrong.. If you did a convention loss and came up with 123,000, radiantly its more like 80,000... and in radiant we aren't heating air, we are controlling the heat loss of all objects in the room so you can take the hot air rises, cold air falls to the garbage can.

Jamie Hall

Chris said:

Heat loss is wrong.. If you did a convention loss and came up with 123,000, radiantly its more like 80,000... and in radiant we aren't heating air, we are controlling the heat loss of all objects in the room so you can take the hot air rises, cold air falls to the garbage can.

Um. Well... Maybe. Wearing another one of my assorted hats, however, I might mention that I am a concert pianist -- and I can assure you that if the air isn't warm, then my hands aren't warm and I am not going to play your venue! I suppose you might aim a heat lamp at the keyboard (and my hands) -- but that would wreck the piano. I like a warm floor as well as the next fellow -- but I'll take warm air over a warm floor, if I have to choose.

CMadatMe

Jamie Hall said:

Chris said:

Heat loss is wrong.. If you did a convention loss and came up with 123,000, radiantly its more like 80,000... and in radiant we aren't heating air, we are controlling the heat loss of all objects in the room so you can take the hot air rises, cold air falls to the garbage can.

Um. Well... Maybe. Wearing another one of my assorted hats, however, I might mention that I am a concert pianist -- and I can assure you that if the air isn't warm, then my hands aren't warm and I am not going to play your venue! I suppose you might aim a heat lamp at the keyboard (and my hands) -- but that would wreck the piano. I like a warm floor as well as the next fellow -- but I'll take warm air over a warm floor, if I have to choose.

If I can control the heat loss of that keyboard and those hands what does it matter if the air is warm? The misconception is that radiant is a warm floor. The floor is just the emitter.. How I control the heat loss of everything in the space is all that matters.

Zman

@Chris
First off, what does a fixed speed circ have to do with the need for a buffer tank? The need for a buffer tank is related to the minimum firing rate of the boiler, the minimum output of the emitters and the system's thermal mass. I agree that the fixed speed circs provided with many boiler are not appropriate, what does it have to do with buffer tanks?

I am not familiar with a "radiant heat loss calculation". Could you please elaborate? How is it different to a regular heat loss calc?

Are you suggesting that Jamie's piano keys should be included in the heat calculation? Are you suggesting that the piano keys need to be heated? I would agree that the contents of home with radiant heating will be slightly warmer than a conventionally heated home. I think you are suggesting that Jamie would be happy playing the piano in a 50 degree room as long as his feet are warm and the piano is warmer than the the air. If so,I respectfully disagree...

To the OP, I think that the thermal mass of your brick home has a tendency to make heat loss calcs a challenge. I own a home very similar to yours and can tell you that the calculators overstate the heat loss when compared to appliance cycling and actual energy usage.

I would also pay attention to the mean radiant temps of the floor compared to the exterior walls. If your floors end up needing to be above 80 degrees on the design day and the walls, because of the lack of insulation, are in the 50's, this difference in mean radiant temps will likely make you feel uncomfortable. The human body does not like to be exposed extreme differences in radiant temps.

Your project would be a great candidate for a hybrid system. I would think that radiant floors and/or walls in the kitchen and baths with flat panel radiators on the exterior walls would be the best of both worlds.

CMadatMe

Fixed speed circ has everything to do with it. Boiler pump is moving a fixed rate gpm on the primary side no matter high fire or low fire. If you cannot exhaust that flow of nice hot water (btu/hr) into the secondary side it just b-lines right back in to the boiler return. Buffer tank just allows you exhaust it. If the boiler pump varied its speed to the modulation rate and could also control a secondary side VFD pump (like the do across the pond) there wouldn't be a need for a buffer tank.

Infiltration rate is less in a radiant heat loss then a conventional heat loss. Why? ie, lack of convection...

The piano becomes an emitter in a sense, just as the floor is, the couch, the table and every other object in the room. I'm not stacking hot air into a box to get it nice and warm, I'm controlling the boxes rate of heat loss.