EDR versus Heat Loss

Jamie Hall

a case of apples and oranges! The EDR method, which must be used for steam, measures how fast the heating system can condense steam; to make the system work right, the boiler must be able to create steam at least as fast as the system can condense it. This has nothing to do with how fast the building will lose heat (which is the heat loss method) -- a given amount of radiation is able to condense steam at a certain rate, pretty much regardless (well, within reasonable limits) of the surrounding air temperature.

The heat loss method, on the other hand, measures how fast the building will lose heat on a given 'design day', and thus how much heat has to be added to the building to maintain a certain temperature on that design day. The heat is then created by the boiler and transferred to the spaces with circulating water (at least on the Wall!); if there is enough radiation or what have you in the rooms, you will get the desired temperature; each room's radiation is sized on the heat loss from that room and the temperature of the water entering the radiation.

For an old gravity water system? Hmm... not sure which I'd use! Maybe one of the old heads on here has an answer!

Al Roethlisberger

EDR versus Heat Loss


I've seen it posted several times that for boiler replacement/upgrades to existing steam systems one should calculate the total EDR of the radiators to properly size the boiler. And for hot water systems, total structure heat loss calculations should be used.

I've also seen it pointed out that if one uses heat loss for the steam system, the boiler will possibly be undersized. So that tells me that of the two methods, using EDR is the more conservative.

Ergo, if I understand this correctly: If one were to use EDR to calculate the correct boiler, flow, etc. for a hot water system with existing standing iron radiators... you would probably fall on the side of conservatively estimating boiler, flow rates, etc, that is don't risk undersizing as much??

I'm not suggesting doing it the "wrong way" as much as I am just curious if my interpretation is correct. I have an old overhead gravity hot water system that has the new system design based on total EDR of the radiators and wondered if there were any catch-22s or corner cases to be aware of.

Al

Supply House Rick

EDR for steam

isn't about conservative but about the differant method of heat transfer from boiler to radiation. Hot water uses temperature drop of the water to move energy (as does hot air -- just a diffarant working fluid). The less the energy needed to be delivered to the structure, the the cooler the water may be, causing less heat to be delivered by a given size radiator (as long as you deliver at least as much as the losses), with increases in boiler efficiency (especially w/ condensing boilers -- more on that later). EDR will vary with temperature.

Steam delivers its heat at a constant temperature by giving up the heat of vaporization upon phase change back to liquid, not by reducing its temperature (though the condensate will cool further and give up a tiny bit more heat on the way back to the boiler, but this is trivial compared to condensing steam. The EDR of the radiation matters (and it better be at least as much as the bldg losses). If in the case the boiler's output is less than th eEDR, even if much greater than the bldg losses, it can't supply enough steam to keep up with the condensation rate of the radiation so not all the radiation will heat -- not good! EDR remains fixed as temperature is fixed, e.g. 215°F at 2 psig at/near sea level.

Unknown

I can't imagine what EDR has to do with anything as far as a hot water system is concerned, other than affecting what water temperature you need to run to meet load.

Volume of water might have some impact (if you, say, want to make sure your boiler doesn't take 3 hours to get your system up to temp on each call for heat), but volume and EDR are not related and pipe sizes have just as much, if not more to do with that in the end.

Buzz G.

Calculations

of radiation for hot water are not too helpful. The heat loss of the structure is what really counts and the boiler size (I think that was what the question was really about) for the hot water system should be large enough to satisfy the loss. If it satisfies the loss for most days, it will need to be oversized to satisfy the loss on the coldest day. Thats why a modulating boiler is good-it doesn't have to run full out on the average day. Outdoor reset and a pump can really change an old gravity type system. When I did the conversion from gravity and replaced the boiler, calculating the heat loss gave me about the same number as when I used the known temps of the hot water and the "EDR" of the radiators. The deadmen knew what they were doing 100+ years ago. Of course I then had to recalculate the heat loss for modern times with insulation and storm windows etc. That last was what ended up using.