Designing a simple two-zone system

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Old 01-02-12, 12:23 PM
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Designing a simple two-zone system

Happy New Year, everyone! This site is absolutely great, and I've greatly enjoyed reading through numerous threads and learning a lot of the ins and outs of home heating systems.

However, it seems that the more I learn, the more dangerous I become.. to myself. I had started off pretty sure that when we retire our aging oil boiler this spring, we'd install a propane tank and a hi-tech modcon boiler.

Now I'm thinking that I'll just stick to oil and install the smallest Buderus G115WS boiler (74K BTUs) with a Logimatic ODR. I'm not convinced that oil will become cheaper than propane per btu in our market (NH). Propane is currently 50% more expensive per btu than oil... and even with the added efficiency of a modcon, it's at least 25% more expensive compared to oil, even when the latter is burned in a good cast iron boiler with ODR.

Nobody knows what the future holds when it comes to energy pricing, but oil would have to take a big jump (with propane prices holding relatively steady) for it to become comparatively expensive compared to LP gas. Current pricing in our part of New England is $3.49 for oil and $2.95 for LP.

Anyway, the proven longevity and relative maintenance simplicity of CI compared to modcons is appealing, and we already have an indoor oil tank and a decent chimney, so those are further reasons for staying with oil. Still, the environmental benefits of burning LP are points in its favor.

Anyway, our 1986 2100sf cape has two zones-- downstairs with 66 feet of copper fintube and upstairs with just 30 feet. Ooops, make that 3 zones, counting an indirect water tank.

Since our upstairs is such a small zone, I was wondering if I should insist that a buffer tank like Boiler Buddy be included as part of all our bids. My understanding is that this buffer would prevent the short cycling that we get from our current boiler situation (the tag on it says 124,000 BTUs).

Is a buffer necessary or beneficial in our situation or can short cycling be adequately addressed by how the boiler is plumbed?

Is there anything else I should be aware of when this goes out to bid? I've done my own heat loss calcs using an online calculator, and came up in the 47,000-56,000 range. Obviously, the 96 total feet of baseboard in the house yields 56,600 at 180 degrees water temp... but when it's colder than, say, 10 degrees outside it takes forever to warm up the house if I've set back the thermostat to 62 or 63. I can't figure out why, except perhaps when the baseboard is so exactly matched to the heat loss, you have to expect a very slow recovery process.

We've insulated our attic to R40 and replaced some exterior doors and windows, so the house is tighter than before. This week I'm having foam sprayed into the joist boxes in our basement to seal any air infiltration down there-- maybe that will help baseboard performance a bit.

Anyway, I'm hoping that a smaller boiler with ODR and (perhaps) a buffer tank will prove more miserly with fuel burning than presently at 830-840 gal/ calendar year.

Any thoughts?
 
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Old 01-02-12, 12:56 PM
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Obviously, the 96 total feet of baseboard in the house yields 56,600 at 180 degrees water temp... but when it's colder than, say, 10 degrees outside it takes forever to warm up the house if I've set back the thermostat to 62 or 63. I can't figure out why, except perhaps when the baseboard is so exactly matched to the heat loss, you have to expect a very slow recovery process.
That's exactly the situation. To recover quickly from a set back, you need to put many more BTU's in the the building than are being lost. The smaller the spread between the two, the longer it will take. You should probably give up your set backs or at least reduce them to no more than 3 degrees. You will find the house to feel far more comfortable.
 
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Old 01-02-12, 03:44 PM
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Righto-- I've settled for setting back only to 65 and having the warm-up to 69 degrees start 90 minutes before we rise. When we get up it's at 69... but then again it's been a very mild winter so far. Temps have settled below 20 just a few times this year. Can't deny there global warming going on... or perhaps it's just one of those natural cycles that happen every 15,000 years...

Not a speck of snow to be seen!
 
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Old 01-02-12, 03:46 PM
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Buderus is nice... no doubt... but don't throw out the idea of an MPO... there's one HUGE advantage to the MPO IMO... it's EASY TO CLEAN! That means that it will be kept cleaner in the long run.

The IQ control is very competitive to the Logamatic.

Cleaning is the job I absolutely hate the most in the whole world. I'd rather bathe in septic water. So that makes me biased... but think about it. It really is a great boiler!

You might still have to line the old chimney, btw...

Is a buffer necessary or beneficial in our situation or can short cycling be adequately addressed by how the boiler is plumbed?
No, not necessary. You could get by without one. Beneficial? Maybe. If you downsized the boiler to something more appropriate to your heat loss, I don't think there would be much short cycling going on even if the upstairs zone called all by itself... I guess it comes down to whether or not the benefit would offset the higher cost... and actually finding an installer that would know what to do with it.
 
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Old 01-02-12, 05:23 PM
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Thanks for the insight, NJT! I gather the MPO is easier to clean than the Buderus... hadn't realized that. Hopefully the newer lo-Sulfur fuels will make that process simpler all around.

Is the MPO as quiet as the Buderus is supposed to be? The boiler is right below the uninsulated floor of our family room where we sit in the evenings. Our current boiler with a Carlin makes quite a racket, so I was hoping a new boiler might be quieter; maybe that's a delusion, tho!

I did check out the MPO online and it looks pretty slick. Then again, it's not like the boiler is going next to our fireplace!

Glad to hear that the buffer tank won't be necessary to prevent short cycling-- can I quote you on that?
 
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Old 01-02-12, 06:19 PM
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can I quote you on that?
No, of course not! I'll delete the post and my secretary will disavow any knowledge!

Does your current system ever start and fire for less than say five minutes or so?

The thing is, even if the buffer does add significant water volume to the system and causes longer burn cycles, what would be the benefit if you left all them BTUs stranded in the tank at the end of the heat call?

Unless you add a lot of fancy control to the system, you won't be able to use the extra heat you generated anyway.

A buffer tank by itself is often not a panacea for short cycling!
 
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Old 01-02-12, 07:16 PM
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Hah! Didn't realize you were Tom Cruise's controller!

Yes, my current system often fires for like two or three minutes....sometimes for 45-60 seconds. I didn't realize that buffers required "fancy controls"-- I assume you're not referring to gold plated valve handles. I thought buffers held the hot water until a zone required it, thus sparing the boiler from having to fire so often. I guess that means an extra water pump or circulator... But thats not such a big deal, is it?

I guess I'm looking to have a system that runs efficiently and have read here that short cycling is bad karma for the boiler.
 
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Old 01-02-12, 07:20 PM
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We'll talk more about the buffer tanks tomorrow... my brain is fried...

Yeah, basically you would want to set the boiler up to 'recharge' the buffer tank, and the temp in the buffer tank would control the boiler firing. Then, the heat demand from the home would be drawn from the buffer, via another pump(s) ...
 
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Old 01-02-12, 07:35 PM
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We'll talk more about the buffer tanks tomorrow... my brain is fried...
No worries-- I appreciate the help you've already given.. Enjoy the evening!
 
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Old 01-03-12, 04:28 PM
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Not only is the MPO easier to clean it is more efficient and the DOE is 64,000 which is closer to your heat loss. It has built in pre-purge on the boiler so on a demand you can just turn on the pump if the water temp is above 140f. This could be a big advantage when that small zone calls. Before the boiler cools down and the boiler fires many times the larger zone will be calling again so the boiler does not shut off and come right back on again.
 
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Old 01-03-12, 06:02 PM
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That's very interesting about the MPO... I know both the heating contractors I've met with carry Burnham, so I'll ask them about it. Thanks!
 
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