Small Cottage Development

bruce pirger

Thinking of building a number of small "cottages" as an investment in this college town for graduate and professional students. Small buildings, 16x20, loft. Thinking of radiant heat and building on a slab. I imagine a small cluster of these buildings, perhaps 3-5. Heat load estimated to be less than 20K. Slab would be properly insulated.

What would you suggest to heat this small development?

Initially I was thinking about building a "central building" to house a boiler, water, etc. and distributing hot water to the units, each with their own indirect for DHW and also a mixing valve for floor heat.

I then thought about using a CombiCore to heat the slab and provide DHW in each place, with no central boiler located in the "extra" building.

All comments appreciated. Thanks.

Brad White_2

Interesting Project!

The small heat loss is a challenge. Are you planning on metering the energy? That small btu need may not seem worth it because BTU meters can be expensive for small amounts.

Heresy Alert!: Purely from a development view and tenant responsibility you might do an economic payback of electric resistance heat. For 20K BTU's that is about 6 kW. Expensive sure, but the tenant pays and a lower first cost. Due diligence says to suggest it, then market hot water telling the tenants how much they will save in utilities. A base of comparison at least.

The central HW system approach seems intelligent; the space required to support all say five cottages would rival the space needed for one. Transporting heat via the outdoors or in a common chaseway is a challenge. For boilers, a Monitor MZ, Munchkin, others, all have application here. But think antifreeze, like Noble No-Burst.

The heat loss per works out to 62.5 BTUH per SF which seems reasonable for small area stand-alone buildings with a 0F to single digit design outdoor temperature. (It would seem high for larger buildings even if little insulation.)
Infiltration is your biggest component even and especially if you have good floor wall and roof insulation.

But the high BTU/SF load may allow only about half of that required heat to be provided via the floor. If you figure maybe 35 BTUH per SF as the most for a comfortable floor, then subtract any unusable area, you may get to 10,000 of the 20,000 required.

You could use Climate Panels in the floor and then up the walls to a point to make up the deficit. Too high up and tenants want to hang pictures? Well, that gives me the willies too. I'd say no to that.

I would put in a good radiant panel radiator such as by Runtal, Burnham, Buderus. High density, radiant effect and can take advantage of low water temperatures (thinking condensing gas boilers here). In the alternate, a blower coil such as by Burnham (wall or kickspace) can be sized for lower water temperatures and not take up much space.

Not sure about the CombiCore. Even though it is of my namesake company (Bradford White by coincidence only) I have heard mixed reviews. Other I am sure would fill you in.

Just my two-cents. Looking forward to learning more.

bruce pirger

Hi Brad:


yeah, electric would be the cheapest to install... But, I don't like the idea of the cost...even if it is pushed onto the student. Plus, with concrete, the plan would be to stain the concrete somehow somewhat inexpesively and that would be the finsihed floor. No rugs to clean, replace, etc. They can lie down an area rug...although again, that will affect the BTU's/sq ft. You are absolutley correct that 20K in 320 sq ft might be tough! Even a bare concrete floor might be rather warm, eh? Maybe they won't need a frying pan for the stove!

The losses involved with the underground plumbing are a concern...plus if there was a central boiler, it seems harder to meter the energy use. I just assume have the renter responsible for that, it would most likely encourage a wiser use of energy.

One thing about the concrete of course is without the radiant, it would be cold. I figure the cost of building the sub-floor, finished floor, etc. would exceed that of tubing...especially in time. So, the thought was to warm the floors...and maybe use supplemental electric baseboard. The floor would probably be enough for most times. I'm up here in NY State, so 0 degree outside would be my design temp.

A radiant wall or panel would be neat...but I would like to minimize the investment. I don't know if students would appreciate the radiant for comfort, or as a selection feature, so I don't really wish to maximize comfort for this application.

I could use a stand alone water heater for the floor...it would be very simple to do...and for this heat load, I suspect it would be almost ideal. I would like to minimze the usage of floor space, as these are small places already, and hence the centralized boiler idea. Don't know the price on a Combi-core, I am certain they won't last as long as a boiler.

This is a fun project, indeed. I find myself thinking of ways I can pour concrete to heat!!

Dale

Elect.

With such a low load and such a well insulated structure I would go all elect and separately meter each structure. Then the renter pays the meter charge and is more motivated to save on fuel. Since you want to limit fuel use why not have elect. heated slabs. If nicely insulated the mass holds alot of heat and is very comfortable, the 12" thick ones are used in Northern Wisc. as a backup heat source if the power is out for a few days, you could have a slab sensor to limit the temp to 85 or so and some decent cal rod type strip heat to run of a normal stat for quick pickup. A well insulated 50 gal. elect water heater and the cost for one or 2 people should be minimal. If you need AC a mini split and you may want to live in the place yourself.

DaveGateway

That's the trade off

with the Combi-core and AO SMITH's ProMax SL: they're still just glass lined WH's and probably won't last to 10 years, but you do get the cheapest radiant /domestic HW solution. I would think that all the tubing and labor would still cost more then 2 or 3 pannel rads. I would always want access to all the piping for any repairs on a rental property.

I can't see justifing the cost of a shared heating plant with the complexity and cost of running super insulated piping to each unit unless they were in a common building.

While a munchkin for each unit would be overkill, what about a combo boiler wall hung like Baxi Luna? With just one bathroom and the space savings of no tank, I would think it worth the added cost.

Brent_2

ac?

Are you going to be installing AC? I think you are correct that if you give the students bare concrete you will need in floor heat. I would think that the first thing most students would do is buy a big carpet remnant to cover most of the floor. The perimeter of the floor would still be exposed so that is good. I think a central plant would be cost prohibitive. Is gas available or will the buildings be all electric?

brent

hr

What is the load?

of the space. I come up with 6400 BTU/ sq ft at 20 BTU? foot?

If you do want radiant slabs, and you are considering an electric fired system, you might consider electric radiant cable or mat.

Simple to install, easy to record energy use, etc.

While not my first choice as a "wet" contractor, there are some good applications for electric radiant. This may be one. Watching energy cost soar it could be cheaper to operate than a 70% efficient water heater of combi unit!

hot rod

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Brad White_2

20 BTU's per SF

in my experience (and only as a check figure as we both know, Hot Rod) would apply only to larger buildings in such a climate. Being detached, the infiltration rate is higher and the external surface envelope is amortized over relatively few square feet of floor area. Four exposures, roof and slab is really a box in space radiating in all directions.

The 60-some-odd BTUH per SF may seem high at first. My experience with my parents cottages on Cape Cod have come out similarly when finally heated.

Jack_21

Rinnai Energysavers

www.rinnai.us Ductless heating. Simple install, high effieciency, excellent reliability. From an investment standpoint in your type project, nothing can touch it. go with a small Rinnai Continuum on demand water heater and you are in great shape for both heat and hw.

bruce pirger

Thanks all for your replies.

I will have to look more closely at electric and investigate the dual rate usage here with NYSEG. I guess with some 20,000 pounds of concrete or so, one could easily heat the slab at night only I suspect. Provide a little electic baseboard as supplemental...and boom...all set. Easy, inexpensive for install, and probably not to bad to pay for the renter either.

I will also look at the space heating stuff more closely as well...but I'd prefer to warm the slab somehow.

Thanks again folks!