Weak heat from tankless HW in city apt.

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Old 01-21-15, 07:45 AM
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Weak heat from tankless HW in city apt.

My fiancÚ and I are looking to improve the performance of the heating system in our 750 sq ft 2BR co-op apartment in NYC. It has its own gas-fueled water heater (unusual for NYC) that is supposed to provide both DHW and baseboard HW heat. It's a Baxi Luna 310Fi.

There are 32ft of aluminum fin baseboard radiators in a single loop around the perimeter of the apartment. There's no thermostat - we just use the switch on the water heater to select either heat+HW or just HW.

We're OK with the simplicity of that. What we're not OK with is that it produces very little heat! It's especially weak in our living room, which is where we most need heat. The bedrooms are very sunny, and we aren't in them during the day anyway. Yet the BRs have 21ft of baseboard vs only 8-1/2ft in the LR.

It was suggested to us that we switch out the slant fin baseboards in the living room for cast iron baseboards. But after a little googling, I see that you're not supposed to mix fin and cast iron baseboards in a zone. What I don't find is an explanation of exactly what happens if you do. If the outcome was that the aluminum fin baseboards in the bedrooms produced less heat, that would be just what we want. Is that what we'd get?

Being in an apartment building, we don't have basement access. And we don't want to tear up the floor just now. So putting the LR on its own zone isn't an option.

Thanks for your input!
 
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Old 01-21-15, 09:46 AM
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If it is a rented contact the landlord,you can't mess with it,one thing you could check is to be sure you have adequate air circulation up through the baseboard ,make sure the flap on the top of the heater is opened fully and carpet is not stuffed up underneath it.
Just a thought
Geo
 
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Old 01-21-15, 02:57 PM
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What Geo said about a rental is true, but I suspect that you OWN this property and are responsible for maintenance. Is that true?

co-op apartment in NYC.
Still, check your local codes and regulations. NYC is pretty strict about only LICENSED contractors working on heating and plumbing...

Let's use a 'ballpark' figure for heat loss of say 25 BTUH / SQ FT of space. If you've got heated space above and below you, and a minimum of wall to exterior, this is probably a reasonable GUESS at your heat loss.

So, 750 x 25 = 18750 BTUH of heat emitters requires.

At 180F water (which I sorta doubt you will get from the Baxi), this would require approximately 34 feet total of baseboard (figuring 550 BTUH / LF @ 180F water)

Already I see a shortfall.

It's even greater if your heater can not output 180F water because the BTU output of the baseboard goes down as the water temp does.

How many square feet is the living room? Multiply that by 25 and divide that by 550 and it should tell you how many feet of baseboard you need in the living room... IF you can get 180F water through them.

It sounds to me as if somebody didn't do their homework.
 
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Old 01-21-15, 03:26 PM
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Sounds like take some baseboard from the BR and move it to the LR.
Or add a pile more in the LR and turn the CHW temp way down
 
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Old 01-21-15, 03:34 PM
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There's no thermostat - we just use the switch on the water heater to select either heat+HW or just HW.
WTH no thermostat? Did they reno your place on THAT much of the cheap? Wow...
 
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Old 01-21-15, 03:47 PM
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In NYC, an apartment is often what the rest of the country calls a condo. Similarly, "regular coffee" in NYC means with cream. Strange.
 
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Old 01-21-15, 05:09 PM
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Thanks to all, especially NJ Trooper for that analysis.

Yes, we own the apartment (a co-op is not exactly the same as a condo, but it's close). The co-op board is very easy-going, and is likely to go along with whatever we decide to do. The board president is a close friend. I don't intend to do the work myself, but I need to understand the options in order to know if the plumber we hire knows what he's doing. Clearly the one who installed the system didn't, and the guy sent over by the company that sold the heater to the previous owner seemed pretty clueless about radiators.

I don't know what the water temp actually is, but the readout on the heater says 78 (degrees C). NJ Trooper's other rough assumptions sound about right. The living room is about 305sq ft.

So we need more dissipation in the LR, and we don't have much more wall space. The LR is also the kitchen, and the cabinetry takes up a lot of wall. Also, as I said, we'd like to leave the under-floor pipes as is. And that's what lead me to the idea of putting cast iron baseboards in the LR. Would that help?

I see that there are also higher output tube/fin baseboards available. Would those Smith Environmental baseboards be a better idea?

Also, the water flows to the BRs first. Would having it reversed make a big difference?
 
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Old 01-21-15, 06:25 PM
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If the LR/KIT is 300 sq ft, that means the BRs and BATH make up the other 450.

That 21 ft that you said was in the BR... was that split between BOTH BRs?

How much in the bath?

It's clear that you've got too little heat emitter in the LR/KIT, but I'm wondering how much extra is in the BRs... how many SF in the other rooms, and how much baseboard?

Too much baseboard is relatively easy to fix with a roll of heavy duty aluminum foil. You remove the covers and wrap the part you don't need tightly... but you need to have an idea how much you don't need...

The problem with mixing cast iron and fin-tube is that the cast iron is slow to heat, and slow to cool. By the time the BRs are warm, the cast iron might just be starting to heat up. Then the flow shuts off... and you could end up in a colder room.

It's also slower to cool... but it has to get hot in the first place, right?

I'm guessing that you need a bit more than 1.5 X the footage you have now. Say 13-14' of standard output baseboard.

Is there room for that?

If not, certainly the high output stuff is worth looking into.

You would need something rated at 800-900 BTUH / LF to be installed in the same space. I think the Smith can do that. I just looked at the specs... at 180F and 4 GPM flow (which you should have no trouble achieving) you're looking at 1100 per foot when piped with both the upper and lower elements in parallel (there are other options for piping, but less output)

Slant-Fin also makes high output models... and I'm sure others do too.

the water flows to the BRs first. Would having it reversed make a big difference?
It might... most ppl like the BR a bit cooler anyway... and it seems you may have excess in there anyway... it's a good thought worth thinking about.

Think about the bath too... nobody likes a cold bathroom!

Oh... the water temp looks fine.

Consider having a thermostat installed.
 
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Old 01-21-15, 06:45 PM
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Just spent a little time perusing the Slant-Fin high output...

You won't get what the Smith offers unless you go to their 'commercial' offerings with 2 tier elements (like the Smith).

I personally don't care too much for the Smith look... if it turns you off too, consider the S/F commercial units.

Commercial Baseboard, Compact, High Output, Heavy-Duty Construction
 
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Old 01-21-15, 08:29 PM
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I guess this is where a picture is worth 1000 words:
Name:  baseboards.jpg
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The radiators are the red lines; the blue rectangle is the Baxi water heater which is mounted high on the wall. The 750 sq ft number includes the long entrance hallway, which we try to shut off with a heavy curtain.

What I don't understand about the Smith baseboards is how, in a simple loop like ours, you use the two tubes. Would you split the line to flow through both pipes in parallel? With a simple T?

Another thought I had is a kickspace radiator, installed right under the water heater, pointed into the LR. We're planing a bathroom renovation, so we'll be tearing out the little vanity behind that door anyway. Would a kickspace unit want its own zone?
 
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Old 01-22-15, 06:54 AM
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I've got the HeaTrim baseboards and I wanted to add some extra BB in one room, so where the copper came out of the floor, I cut it close, ran some copper right on the floor, made a tight U connection and added about 8' of BB in that room that way. It was tight, but it fit in the HeaTrim cover. It may not be completely correct but I didn't have to dig into the floor to add BB. Don't know if you can do that or not with yours. But it looks like all your space is taken up so maybe you'll have to make up a double section and fabricate a cover somehow.
 
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Old 01-22-15, 09:16 AM
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Would you split the line to flow through both pipes in parallel? With a simple T?
Yes, exactly................
 
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