Cycling Losses for Gas Furnaces

[AC_Hacker]

Originally Posted by Servicetech

Hydronics is practically non-existent in the south due to lack of AC capability.

I know that Germany isn't actually part of the south, but they have learned how to do hydronic (radiant) cooling, by circulating cooled water through ceiling panels. There was an issue with the possibility of condensation, but it was resolved by automatically monitoring the temperature and humidity of the room, calculating dew point, and adjusting water temp to keep it from crossing into the dew point.

Their energy costs are 2X what ours runs, so they have to be much more intelligent about how they use it.

-AC

[NiHaoMike]

So they use a mini split to do the dehumidification? Actually, that's a great idea. Install a very undersized unit to do the dehumidification and use hydronics to pick up the rest.

[AC_Hacker]

Originally Posted by NiHaoMike

So they use a mini split to do the dehumimore humid and dification? Actually, that's a great idea. Install a very undersized unit to do the dehumidification and use hydronics to pick up the rest.

I don't know if it was exactly a mini-split, but that would work in a home... but I'm sure that there was some de-humidification involved.

randed tried to run water straight from his ground loop directly through a HX that was in a ceiling mounted blower unit to cool his shop. He reported that the setup was able to lower the air temperature, and that there was considerably more condensation on the fan HX that he had expected. But the overall experience that he reported was that his shop became more humid, and not much more comfortable.

So yeah, some de-humidification would be required, but not as much power would be required to make the whole thing work as a straight on air conditioner.

-AC

[jeff5may]

Originally Posted by Servicetech

Back to the original topic, it does appear that duct losses are significant in the cycling loss calculations. Not sure hot much the 45 second burn time to heat up the exchanger costs in efficiency, and how much is recovered during the 180 second blower off delay after the burner shuts off. 90%+, 80% and old school pilot light furnaces have different cycling losses.

The real problem as it relates to this thread is that when using old-school, forced air "blast" furnaces, the duct losses are calculated separately in the design. The AFUE or thermal efficiency measurements are done for the heating unit only, under a tightly controlled set of conditions. They basically consider what heat is available to distribute, and tell nothing about the ductwork outside the box. Once it leaves the unit, there's no telling where the heat may go, only how much and how hot it is, at what pressure.

In the real world, the professionals in the industry are left to solve the problem of delivery. Any mechanical contractor or engineer will tell you that it's all about trade-offs. Finding the balance of airflow vs temperature vs raw btu's delivered here or there is an art in itself. It would be great to have a wide open, straight path to follow, but bottlenecks are unavoidable due to the nature of the installation. Working around or through the real world situations is what these pros are supposed to be devoted to. As always, your mileage may vary.

There are lots of ways to try to approximate the "transient" losses of ductwork, as well as "static" losses. Due to the uncertain nature of (especially residential) on-site distribution equipment actually installed, it is commonly much easier to just take measurements. The main idea is this: the more restrictive, leaky, and uninsulated the ductwork is, the less heat that will make it to its destination. As with anything, everything you do to improve the system adds up.

As with many other aspects of making a building more efficient, most of the remedy is a one-time expense. After it's done, it just keeps working for free. But it's the work that turns people off. Nobody wants to go crawling around in a dark, dusty, cramped space. Not even once. Don't even mention dragging materials in and out of there!

[Servicetech]

Residential water chillers with "mini split" style fan coils/air handlers could be the way of the future. Each room has it's own small air handler, therefore a way to deal with condensation. No ductwork losses and only one chiller/condenser unit. It's done in commercial buildings all the time on a larger scale.

[ctgottapee]

I would guess spiral wrapping that PVC duct with whatever thickness fiberglass/bluejean/rockwool batting would give you the R values you want.

The PVC provides the air seal so butting it close as possible would suffice.


I like the mini-split idea as a stop gap energy efficiency improvement, and it basically is what I'm doing. It can provide for all your needs most of the year, and then you have the furnace as backup when needed.
I'm not sure it's a homeowner install, although the avg contractor can be just as bad as they generally move the traditional big units. Vacuum the system is not cheap or simple for a DIY'er, and support if problems arise may be non-existent.