I don't know how (or if) the calculation for residual head is done to give the proper pump speed/output but quite easy to calculate the pump head from a known speed. For example if the pump speed is 46.3% then the pump head (at the pump) is 700×0.463^2, 150mb and the residual head will be a little less, it will be nowhere near the 400mb ABV set pressure. You might take a full set of readings at your present 150mb setting, then increase it to 200mb and take another full set of readings exactly like the ones you took in post #101, would expect a pump speed similar to what you got of 57% = 227mb based on a 7M pump or a max of 260mb if the circ pump is 8M, both far away from the ABV setting of 0.4.
Bypass valve
- Thread starter toneloc
- Start date
Thank you once again John. Presumably the set flow temperature is immaterial for these tests, which I will do as you suggest? I have to solve this problem of why the ABV is passing flow when it should not be.
One thing I have thought of is that, once I have cleaned up the innards of the old valve (which is the same Honeywell one) I can do some pressure tests on it with a little rig I have. Just need some spare time!
Will get back when I've done your suggested measurements. Thanks again.
One thing I have thought of is that, once I have cleaned up the innards of the old valve (which is the same Honeywell one) I can do some pressure tests on it with a little rig I have. Just need some spare time!
Will get back when I've done your suggested measurements. Thanks again.
Hi John. Here are the requested readings:
Pump map 1 (150mb)
Pump modulation 50%
Set flow temp. 60C
Indicated flow temp. 63C
Actual flow temp. 64.3C
Burner modulation 19% (6kW)
Actual return temp. 54.7C
Pump map 2 (200mb)
Pump modulation 56%
Set flow temp. 60C
Indicated flow temp. 61.5C
Actual flow temp. 64C
Burner modulation 19% (6kW)
Actual return temp. 54.5C
The pump characteristic curves in the manual show the 100% pressure up to 1000 l/hour as about 720mb.
The ABV was passing flow throughout the above tests.
Pump map 1 (150mb)
Pump modulation 50%
Set flow temp. 60C
Indicated flow temp. 63C
Actual flow temp. 64.3C
Burner modulation 19% (6kW)
Actual return temp. 54.7C
Pump map 2 (200mb)
Pump modulation 56%
Set flow temp. 60C
Indicated flow temp. 61.5C
Actual flow temp. 64C
Burner modulation 19% (6kW)
Actual return temp. 54.5C
The pump characteristic curves in the manual show the 100% pressure up to 1000 l/hour as about 720mb.
The ABV was passing flow throughout the above tests.
Using the first pump map of 150mb......
The pump head at 50% speed and even using a 800mb pump = 800x0.5^2, 200mb, a new HEX loss at the calculated flow rate of 8.5LPM is ~ 28mb leaving a residual head of 200-28, 172mb to circulate water through the rads and the ABV, no way should the ABV be passing water at its 400mb setting.
The second pump map of 200mb gives a pump head (again assuming a 800mb pump) of 800x0.56^2, 251mb, flow rate calculates the same as above leaving, 251-28, 223mb residual head, again should not cause the ABV to pass.
Could be more crap in the ABV??, your best bet, as you said, is to clean out the old one and do a leak down test.
The pump head at 50% speed and even using a 800mb pump = 800x0.5^2, 200mb, a new HEX loss at the calculated flow rate of 8.5LPM is ~ 28mb leaving a residual head of 200-28, 172mb to circulate water through the rads and the ABV, no way should the ABV be passing water at its 400mb setting.
The second pump map of 200mb gives a pump head (again assuming a 800mb pump) of 800x0.56^2, 251mb, flow rate calculates the same as above leaving, 251-28, 223mb residual head, again should not cause the ABV to pass.
Could be more crap in the ABV??, your best bet, as you said, is to clean out the old one and do a leak down test.
Meanwhile, I have been thinking that I don't actually know how much flow is occurring through the ABV. All I know is that flow temperature water is going around the loop (more exactly now measured at 8.5m of 22mm pipe) and back with very little drop in temperature. However, the whole of the pipe loop is insulated because it is also the primary loop for heating the DHW cylinder (to avoid temperature and heat loss). I now recognise that this is not ideal, as it is supposed to serve as the heat loss circuit during pump overrun. Ie a conflict of requirements for the two functions. (Maybe there should be a separate and uninsulated loop for the ABV and pump overrun?)
So I have now tried to estimate the (unwanted) flow rate around the loop from the calculated volume of the pipe and the time it takes hot water to get around it between temperature measurement points with my IR gun. It's very approximate but the average of several measurements comes out at about 5 litres per minute. How does this sound? (Measurements done at pump at 150mb and valve at 400mb.)
Would there be any practical way of getting a better estimate?
So I have now tried to estimate the (unwanted) flow rate around the loop from the calculated volume of the pipe and the time it takes hot water to get around it between temperature measurement points with my IR gun. It's very approximate but the average of several measurements comes out at about 5 litres per minute. How does this sound? (Measurements done at pump at 150mb and valve at 400mb.)
Would there be any practical way of getting a better estimate?
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
