Where did the 20 deg HW split come from

Craig Drabenstadt

Why does the industry use 20 degree split on residential hot water system design.

jerry scharf_2

lazy

a 20 degree delta T can deliver 10 MBH (thousand BTUs per hour) per 1 GPM (allon per minute) of flow. Since the boilers and loads are don in MBH and the flows and pump curves are done with GPM, the 20 degree delta T makes the math easy. It's also a fair compromise between the things that are optimal with higher delta T and those who want lower.

jerry

Bill Nye_2

Do you mean ......

.... Delta T or do you mean the setting on an aquastat?

Madghost

If you didnt know, delta T means temperature differential, so if water is approaching @ say 140 return it should supply at 160. This is typically how HW boilers are designed to operate and is a gerneral benchmark as to thier efficiency and how they are operating vs the manuf. spec.

Betz

What if

you can't achieve the 20 degree delta t? Is there a time when your return is always going to be more than 20 degrees colder than the supply?

jerry scharf_2

shouldn't be a problem

for a given heat source, more flow means lower delta T, less flow means more delta T. As long as the system designs take this into account and the system performs, no problem.

jerry

Mark Eatherton1

In most cases...

you WON'T see a 20 delta T, unless it is extremely cold outside AND inside. THe ONLY time (once) that I saw a true 20 degree split was on a brnd spakin' new hot water baseboard system that was being started in the cold, and the house was cold too...

The only other way you're going to see a 20 degree delta T is to choke the livng crap out of the flow on the circuits, and thats not recommended. If and when it needs to dump a lot of btu's, it will. In the mean time, it does what it needs to to keep the occupants happy:-)

I think a normal expectation would be a 7 to 20 degree rise, and as far as I can tell, the American engineering society uses the 20 degree hash mark (or is that hall mark...) because it makes the math a snap. It's overkill in real time.

Let me ask you this. Which moves more energy 1 GPM at a 100 degree rise, or 100 gpm at a 1 degree rise.

On cold start up you could see MORE than a 20 degree delta T for a few minutes until the roller coaster of heat, or what I kindly refer to as the rolling thermal rollercoaster ride of heat transfer, comes back around. I've got thermal pictures of it. I've seen my supply and return temperatures flip flop for a second. In other words, the outlet of the boiler is colder than the inlet. Only for a few seconds, but nonetheless a real time thing.

Man Mountain Mike is getting to see system thermal equallibrium at its finest. He's got one of the finest thermal generators made hooked up and wired in his own home... And you know the manufacturer is standing back and going "OH YEAHHHhhhh..."

I'm studying it in my own home for a future article on "real time" energy consumption versus theoretical calculations, and the need for the heat source to modulate to the load.

ME

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Mark Eatherton1

It depends...

If it's steady state, before you even get to design conditions, it could pose aproblem. Could be a flow issue. I seriously doubt its a real load issue.

It depends...

When, what and where under what circumstances.

ME

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Mark Walnicki_2

Modulation

Is modulating the heat source to the load the single best way to provide the ultimate in comfort and efficiency?

What if you have an on-off burner/boiler, but the heat delivered to the system is controlled by something like a tekmar control, with vsip, indoor temperature feedback, characterzied heating curves, boiler post-purge, auto boiler differential, etc?

Are both methods close enough to count in terms of comfort and effieciency? If not, why would one be superior over the other?

Any thoughts on the "Beckett Heat Manager", which as far as I can tell is nothing more than an auto boiler differential, packaged as a stand-alone, one-size-fits-all product? I don't necessarily agree with it, but it seems to be better than nothing. Might make my parent's house the guinea pig on that one!

jerry scharf_2

modulation has little to do with comfort

Mark,

If you have a system that runs very hot water and on/off controls, modulation won't do a thing to make it more comfortable. Comfort is a functions of the system controls and the emitters. Imagine a modulating boiler set to fixed high temperature and simple on/off controls. Having the boiler won't help one bit in making this more comfortable.

Areas to be heated are usually relatively massive, so they damp the swings in supply temperature quite well. The problems of overshoot can be worsened by a non-modulating boiler, but I still see this as a controls problem.

Don't get me wrong, I believe in modulating boilers. It's just that modulation is a second or third order contributor to comfort.

jerry

Mark Walnicki_2

Comfort & efficiency

So to an extent they need to be treated as two separate issues?

Get the most energy efficient boiler you can, and then control it as best you can for comfort, to achieve the most efficiency out of the entire system as a whole?

I'm pretty sure I understand what you're saying, but I'm not sure I'm finding the words I need this morning!

jerry scharf_2

systems design

Comfort is a function of systems design. Efficiency is part of the same systems design. They have many areas of interaction and you can't look at them separately. That said, your post was about a specific relationship, and that's what I commented on.

First order contributors to the comfort are the emitters and controls, with the boiler fuctions being secondary. I can always synthesize things like outdoor reset with dumb boilers and smart controls. Everything, including building envelopes are first order contributors to efficiency (you can lose the efficiency anywhere along the chain.)

jerry