Gas Boiler Heat Exchanger Pressure Loss

[Johntheo5]

How do I determine the above or pump head from this, second below, a Vaillant Ecotec 938, the Vaillant Ecotec Plus 438 gives the actual Hx pressure loss.

 

[vulcancontinental]

They're showing the whole boiler head loss not just the heat exchanger head loss.

 

[JonathanM]

Does this mean, if you have a 6m rated pump, it can take 4m just to push the water through the boiler? So there's only 2m left to take it round the radiators?

 

[JonathanM]

This is my 1990 Baxi FS 801 RS. I've often wondered what W.G. means?

 

[Johntheo5]

Water gauge

 

[JonathanM]

Water gauge

Is that an old measurement of pressure? How does it convert?

 

[Johntheo5]

They're showing the whole boiler head loss not just the heat exchanger head loss.

Is this just a pump curve so??

 

[Johntheo5]

Is that an old measurement of pressure? How does it convert?

1 inch = 25.4MM= 0.0254M

 

[JonathanM]

1 inch = 25.4MM= 0.0254M

Thanks. Didn't know what the units were.

My boiler looks like 27 inches on the dotted line, which is about 0.7 metres. That's less than 20% of the Vaillant at similar flow rate. Is that because a cast iron boiler has less resistance?

 

[Johntheo5]

I would think so, like the SE oil fired boiler which had virtually no Hx resistance, its probably still very small with the extra Hx on the HE ones.

 

[fixitflav]

Is that an old measurement of pressure? How does it convert?

Just convert inches to meters (divide by 40 near enough) and you're there.

 

[vulcancontinental]

Compare the sectional view of the Gianonni exchanger as opposed to a large waterway high mass cast exchanger; small bore, manifolds, duplicated tubes equate to high head loss.

No problem as long as you can achieve the mass flow rate you need and maintain the minimum flow rate while still overcoming resistance around the index circuit. If not then a low loss header or some other arrangement with a second pump will be needed.

 

[fixitflav]

Is this just a pump curve so??

You need the total boiler headloss (not just the h/ex, though that will be the bulk of it). Estimate the loss in the rest of the system, also varies as flow squared. At each flow, add the 2 losses to get a total system curve. Of course, there is zero static head in a CH system. Plot that on the pump curve and where they intersect is the flow and head.
I must admit I can't see what your 2nd, pump graph, is telling us. A pump characteristic is usually a single curve sloping down from left to right, sometimes with a hump in the middle. Perhaps a family of curves at a range of speeds.

 

[vulcancontinental]

Is this just a pump curve so??

It shows the residual head

 

[JonathanM]

You need the total boiler headloss (not just the h/ex, though that will be the bulk of it). Estimate the loss in the rest of the system, also varies as flow squared. At each flow, add the 2 losses to get a total system curve. Of course, there is zero static head in a CH system. Plot that on the pump curve and where they intersect is the flow and head.
I must admit I can't see what your 2nd, pump graph, is telling us. A pump characteristic is usually a single curve sloping down from left to right, sometimes with a hump in the middle. Perhaps a family of curves at a range of speeds.

Viessmann have another graph I find confusing, but not quite as bad, and maybe it might help make sense of the Vaillant one. Maybe it's a German thing. It also has a horizontal line across it, like the Vaillant one. Viessmann call it Upper Operational Limit, and I have no idea what it means.