PM Magazine

Sonny B

in PM Mag. He is showing the virtues of a high mass condensing boiler with a solar coil installed and a low pressure drop. He says that these units are already available but doesnt say by who. Can anyone here help with some of the manufacturers he is eluding too.

Mark Eatherton

The rAy boiler

is a high mass mod con.

Steibel makes a dual coil tank, along with VIESSMANN and others.

ME

Singh_9

I believe

it looks like the phoenix solar with burner combi tank by HTP.

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Mark Eatherton

Ah, could be...

I'm living in the mountains, and my PM magazines are living in Denver.

ME

Theealchemist

I too Believe

I too Believe That Siggy is refering to the HTP Solar Phoenix. http://www.htproducts.com/ has a you tube video with Dan Holohan and Dave Davis talking about the Phoenix....

Josh_10

I'm not so sure, the only thing I can't see on the Phoenix is a microbubble air separator, or an air separator of any kind. It would be ideal with that huge low velocity cylinder.

Unknown

frankly I think the whole thing would be better with the tank being domestic and a coil for heating, or internal DHW coils in the top a la the Turbomax.

I hate the idea of adding a pump to do DHW, or heating, as long as we're dreaming of the perfect heat source.

I also hate the idea of heating a solar receptacle with fossil fuel.. and for this to be a good solar receptacle, it would have to be pretty darn big I think..

In the meantime, if we could just get 10-20 gallons of water content in a mod/con heat exchanger, it sure would allow for one flexible, efficient heat source. Like the Prestige but with serious water, or the Vitola with a mod/con burner sensibility..

actually with enough water content I'm not sure what the mod does for you really. the vitola does great just with water.. though I wish you could bypass it for DHW demands.

siggy

ultimate gas-fired heat source

Glad to see this column is stirring some discussion.

The device shown is not meant to be an exact copy of any existing product. The HTP Phoenix comes close, but there are some distinct differences:

1. The Phoenix holds domestic water in a stainless tank. This device is a simple carbon steel tank holding system water (less expensive).

2. The external heat exchanger for DHW allows easy replacement if ever "cruded up", or due to some other failure. Can't do that when the coil is in the tank. The circulator is VERY small (think 10 watts or less), and runs only when there's a call for DHW, very little energy consumption. Also very little DHW being stored reduces potential for Legionella. This concept is used for DHW production in many district energy "substations" in Europe.

As far as solar input - yes, the tank would need to be more than perhaps 30 gallons, but the point is that this concept can be transitioned to "solar ready" with mostly a tank volume increase.

The main features are:
1. superinsulated shell for minimal standby loss
2. sufficient mass to stabilize system with many small zones
3. condensing burner efficiency (modulation turndown could be small - perhaps 50-100%)
4. very low flow resistance - no separate heat source circulator, no P/S, no separate hydraulic separator.
5. Consolidate some customary "external" devices into the unit (air separation)

Hope this provides some clarification.

siggy

CC.Rob_16

a bit more, please?

John, thanks for the article, and for checking in here. Could you please explain a bit further about a couple things.

"3. condensing burner efficiency (modulation turndown could be small - perhaps 50-100%)"

This is part of the equation I'm not getting. What kind of modulation range and bottom/top end will be needed in typical residential application?

Your description of domestic control function sounds sort of like standard DHW priority, but not quite. Is that right? If so, what is different?

Thanks much. (Hope these make sense. Been a long couple days....)

Josh_10

Let's say you have a microzone of about 5,000 BTU/hr. And the input to the burner is 110,000 BTUH. Give it 90% efficiency for argument sake and a 2:1 turndown. Let's say the high mass boiler has a 60 gallon capacity.

By using the following equation you can solve for cycle time:

Gallons= (cycle time)(Lowest Firing Rate - smallest load requirement)/(500*DeltaT)

So take the firing rate of 49,500 minus 5000 and you get 44,500. Divide that by 10,000 and you get 44.5. Take the gallon capacity and divide it by 4.45 and you get a burner on time of 13.5 minutes.

That to me is pretty a pretty compelling reason of why you only need a 2:1 turndown.

Unknown

just remember your smallest zone load is not at design day, it's when it's 64 degrees. I usually presume a load of zero but if you need to be more precise, you can figure for the BTU content and temperature rise of your emitter... that is, for example, concrete heating up 2 degrees on a call for heat provides some buffer capacity. A wood subfloor, not as much.

Unknown

Thanks for the response Siggy! I can't really argue against any of these points. I will muse, on a slightly different note, that rather than these "heat the top of the tank" tanks, I would prefer to see a two chamber tank model with a small heated top tank and a cold, much larger bottom tank for solar apps.

For heating systems we really don't need more than 10 or 20 gallons of buffer capacity max in the residential market, especially if you're going to use a modulating burner. That could be a small heated top chamber, leaving lots of room for a nice, spacious cold coil tank in the bottom. and the top chamber could be a modular unit, say in gas or electric versions, I would think. edit: goal being to absolutely prevent mixing, and absolutely minimize the stored volume. I would like on demand best ideally, but a little mass does eliminate a lot of issues there.

You are raising the bar of discussion Siggy... thanks, as always.

Josh_10

Yes very true, I tried to use east coast numbers at my best guess.

I use zero too. And generally our loads are below 50KBTUH here in Seattle. So we actually need either more turndown or more storage.

scott markle_2

I was intrigued by this proposal but wonder about some aspects.

The Phenix, Vertex,and Polaris all claim efficiencies of around 96%. I'm not clear on what the conditions for this efficiency are though. If 96% is possible while maintaining a 130 deg tank temperature this is impressive. I think most mod cons are lucky to be doing 90% efficiency when they are making DHW through an indirect. I'v watched a solo60 connected to a smart 80 produce almost no condensate during a DHW call.

Since DHW in a well constructed modestly sized northeastern residence can amount to a third of the total load we should be almost as concerned about achieving high (condensing) efficiency from DHW as we are from the heating load.

An often ignored problem with boilers and indirects is heat left in the boiler during summer operation. If we are looking at the twenty year savings of ECM Vs conventional circulators shouldn't we also be considering the thousands of summertime heating cycles that leave usfull energy behind in the bioler and pipe work, even in low mass mod cons?

Assuming a condensing tank hot water heater can achieve 96% efficiency while maintaining 130deg. with none of the losses associated with a boiler indirect paring I think a strong case can be made for wider use of these appliances in DHW/low temperature heating applications.

What I'm a bit fuzzy on in the PM article is the reversing of boiler and dhw volume. In my experience with earlier instantaneous water heaters flow rate has a huge effect on temperature, wide modulation and fast responding electronic sensors have largely solved this issue on the newer units, but it was a real problem with the earlier units,even ones that could modulate.

In the design proposed wouldn't (typical) variable flow rates on the domestic side wreak havoc with stable DHW water temperatures. Also wouldn't the plate heat exchanger need to be quite large and expensive to supply higher volumes of DHW and still maintain tank temperatures in the 120 range?

Great food for thought, thanks Siggy for challenging conventions with this interesting idea, this is the kind of stuff that makes PM interesting to read.

Unknown

I can answer the efficiencies question.. with water at ground water temperature at the heat exchanger, efficiency is very high. and with water heaters, that's most typical. But the Phoenix gives a chart showing the efficiency dropoff with higher temps. It's real, but you can still do 90%+ in heating it looks like.