Low Temperature Hyronic systems?

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ecobuild

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Hello everyone. Although a new poster I've been reading this forum for a couple years and I've heard a lot of great advice here. I am designing a new home with energy efficiency as the top priority and I am in a quandary about the heating system. Here's the specs.

1500 sf. 2 br 2 bth. R40 walls, R65 attic, 3X glazed windows. The house is in Western MT. (zone 5 I believe)

I would like to use a hydronic system in this house, but the heat loss calcs don't require much heat in the house at all with all this insulation. High mass floor radiant is too expensive, and will likely overheat the house, even if well designed based on experience. My idea is low temp (<120) radiators or fin tube with a condensing boiler. Theoretically if I oversize the units I can run lower water temps through them and thus actually make use of the extra efficiency afforded by the condensing boiler. My concern is that there will be a serious lag in warming the house with low temp water. Should the system be run like a high mass system and kept at a constant temp instead of turning down the thermostat, or will the outdoor reset be sufficient to overcome the lag. Will this even work? Plan B would be to install a basic fin tube system with an old technology boiler and just learn how to work with the thermostat. Any thoughts would be appreciated. Thanks
 

Bill Arden

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>R65
Is that fiberglass? Just keep in mind that if it is fiberglass you will still need some cellulose to prevent air "piping" from lowering the R value

>High mass floor radiant is too expensive, and will likely overheat the house

What you need is "low mass" floor radiant heat and/or fan driven radiators.

Ideally you would use a outdoor temperature compensating system where the water temperature increases as it gets colder. Just in this case you are targeting a lower temperature to improve efficiency .
 

NHmaster

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Forced hot air is by nature more dfficient than forced hot water because you only have one exchange. A properly sized and installed forced hot air system will give you comfortable heat with the option of adding things like electronic air filtering and humidification. People have a misconception that warm air is uncomfortable and dry but that's just not true. If the furnace and ductwork are properly sized and installed you will never know the system is running. Besides you can also ad A\C if you want. Take a Look at Ruud's 90++ furnaces.

New construction I would go with that and a Rinnai tankless for my hot water. Should run for pennies a day.
 

NHmaster

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It occurred to me that I never answered the low temp issue. Baseboard works by convection. Cooler room air enters the bottom of the enclosure, is heated and then passes out the top of the enclosure. Unlike Radiatiors that put heat into the room though "radiation" which is a different process. When you put low temperature water into baseboard, two things happen. First off, the temperature difference betweent the room air and the wter temperature is failry low (delta T) so the baseboard will not convect the heat as well as it would with higher temperatures. Second is that if you put low temperature water in the inlet, the drop through the baseboard (delta T again) can be such that the end pof the loop does not have enough temperature to heat that area. We do attmpt to design now for lower water temperatures than we did in the past. Partly because baseboard has higher thermal efficiency then it did. However water temps below 140 don't work out well for a convection system. www.taco.com has piles of technical design information that is free on their site.
 

Jadnashua

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If you may want or need a/c, then forced air may be a good choice. But, either a track system on top of the subfloor, or staple up radiant underneath should not overheat the house. zone valves and separate thermostats for areas that are functionally separate should keep things from becoming overheated, and a condensing boiler with an outdoor reset should compensate to keep the place comfortable and ecconomical. Keep in mind, though, that with a radiant system, it may not pay to try to do a setback because of the slower response. But, I find that if the house is really cold-soaked, neither radiant nor forced air heats the walls, or furniture that fast...you can get the air temp up, but it takes awhile to warm all the rest of the things in the house.

Living in Montana where the incoming supply water in the winter is likely near freezing; I'd think twice about a tankless system. A good quality indirect with an efficient mod-con boiler can be had that only has 1/4-degree/hour standby loss.

If you do decide to go radiant and want a/c, split systems are pretty good and don't require ducts. Or, you can install ducts in the ideal location for a/c, rather than a compromise for heating and a/c.
 

BigLou

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How about radiant baseboards or wall panel radiators ? Even the old cast iron radiators would work well in your situation.

Like NH master said at low water temps you don't get a whole lot of convection out of fin tube so don't count on it for heat. I have lots of steel fin tube(heavy) baseboards in my home and they do keep the place warm even at low water temps.However I am not so sure I would be able to do the same thing with the much lighter and smaller copper fin tube.

Lou
 

ecobuild

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Thanks for the thoughts folks. Regarding the project... The Attic R65 is a combination of sprayed polyurethane foam and cellulose. I've found fiberglass to severely under perform in attics.

Re the heating system.

Staple up or track systems have a number of drawbacks in my mind, You have to be careful what you put in for finish floors, they aren't as efficient as High Mass, and I haven't seen one yet that runs below 120 when the house is really calling for heat. Plus it's significantly more expensive than fintube ,around here at least.

Hence the idea to use over sized fin tube. I do understand the principle of convection with traditional high temp hw systems and the point about its effectiveness at low temps is a good one. I guess I didn't think that all this insulation will hinder convection that much since there is still a pretty significant temperature difference between the 120 degree water and the 70+/- room air temp. I would oversize the fin tube, and maybe even use a High Efficiency version.

Plan B. Use steel panel Radiators instead. Any Thoughts? Thanks
 

NHmaster

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There is a big misconception that radiant floor heat is more efficient than convective baseboard. It can, under the right installation conditions, be as efficient but not more. What it is is more comfortable because it tends to heat the body more than the surroundings.
 
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