Central heating flows - why return so hot?

[bigglesuk]

I'm trying to figure out what is going on with our central heating, as I've been looking at ensuring it's optimised. We had some building work completed last year with some of the radiators being changed etc. and the builder didn't rebalance the system so I started looking at things. After looking at the boiler (Worcester System Boiler installed a couple of years ago) I could see that the installer may not have optimised it, with the pump being set on full power, which I have reduced.

I decided to check the temperatures by the valves to see what the flow temperature is at the valves (S-plan), so I turned the heating on, CH valve opened and then after a period of time the pipes started heating up. What I noticed though, was that the return temperature was very similar when the probe was on the middle pipe (1st: supply, 2nd: return, 3rd: CH out) the value was pretty much matching the supply temperature. Nothing should have bene flowing through that as that is the return, only the bypass valve and HW pipework connect to that, CH connects to it under the floor boards. The HW wasn't on, so nothing should flow through it.

I moved the probe to after the automatic bypass valve and it was reading almost identical to the supply temperature, but nothing should flow through that. The valve was fully open, as in the pressure setting. So I completely closed it to see if it made a difference and didn't seem to.

Are my assumptions right in that the return pipework in the cupboard shouldn't really heat up unless the HW was on, or the bypass valve had to open when both the CH and HW are off, but the pump is still running for a bit?

 

[ianmcd]

No the by-pass

Are my assumptions right in that the return pipework in the cupboard shouldn't really heat up unless the HW was on

should be fitted so that it works with either HW only , CH only or both together

 

[Johntheo5]

The valve was fully open, as in the pressure setting. So I completely closed it to see if it made a difference and didn't seem to.

Why was the ABV fully open?, it should be set so as that it opens (as you said) when both CH and/or HW shut down, the circ pump then runs on for a minute or so to cool down the boiler. If its passing when fully shut (with either CH or HW on) then it needs replacing, I take it that fully shut is index 0.5 or max.

 

[bigglesuk]

No the by-pass

should be fitted so that it works with either HW only , CH only or both together

I think I probably worded things incorrectly. The return pipework goes into the floor and back to the boiler, with the CH return connection under the floor (HW is last in sequence). What I was questioning was around the pipework above the floor. If only the CH was active (valve open) then the HW valve would be closed and the water shouldn't be able to flow up the return pipework from under the floor towards the cylinder?

 

[bigglesuk]

Why was the ABV fully open?, it should be set so as that it opens (as you said) when both CH and/or HW shut down, the circ pump then runs on for a minute or so to cool down the boiler. If its passing when fully shut (with either CH or HW on) then it needs replacing, I take it that fully shut is index 0.5 or max.

The valve was as per the pic attached when I took the cover off. I turned it fully clockwise which I believe should have closed it, but it didn't seem to do anything. Could be a faulty valve? If so, I take it the whole system would need to be drained down in order to change it?

 

[Johntheo5]

If you are sure that its installed between the flow and return before the CH and HW motorized valves then yes, you will have to drain before removal.

 

[Mottie]

I turned it fully clockwise which I believe should have closed it, but it didn't seem to do anything.

It will remain hot for some time. Try closing it down then, from a cold system, put the heating on max and let it get really hot. Then gradually open the ABV a bit at a time, waiting a minute or so to see when it gets hot. When you feel heat gluing through the ABV, tighten it up a little. That should be about right. Let the system cool down, when cooled down put it on on max and when the rads are good and hot again, turn room stat down so that the heating goes off and you should then notice the ABV open when the boiler runs on a bit. That’s how I do mine anyway.

 

[Johntheo5]

True but if shut fully then the boiler flow/return dT should increase and decrease if its reopened again as a test.

 

[bigglesuk]

It will remain hot for some time. Try closing it down then, from a cold system, put the heating on max and let it get really hot. Then gradually open the ABV a bit at a time, waiting a minute or so to see when it gets hot. When you feel heat gluing through the ABV, tighten it up a little. That should be about right. Let the system cool down, when cooled down put it on on max and when the rads are good and hot again, turn room stat down so that the heating goes off and you should then notice the ABV open when the boiler runs on a bit. That’s how I do mine anyway.

I was away for a few days, but this morning I fully closed the valve before the heating kicked in. Once up to temperature there was a constant temperature difference of at least 10 degrees, I opened it slowly and after a while it started to decrease. I then closed it slightly and it stabilised again. When the heating turned off the difference between the supply and the return started to equalise as the valve opened.

It had been like that for over 18 months when the boiler was installed. Now all the radiators heat up really quickly (some weren't getting to full temperature) and the house heated up quickly this morning. The boiler return flow is now around 50 degrees., with the flow temperature about 65 degrees. I hate to think how much money we've wasted with the boiler probably not condensating and not working efficiently.

I was going to look into reducing the pump speed, as originally it was on maximum and never reduced by the engineer who installed it. I've reduce it down but now the flow is better, I expect that I may be able to take it down. When I had it one speed lower the radiators could get to temperature, but with this fix maybe they can? Is it worth trying to reduce it a bit? Or just leave as is?

 

[Mottie]

I have mine on the lowest speed - number one out of three. My reasoning is that there is more of a chance for the heat to pass out of the rads, resulting in a cooler return flow. 3 bed semi, sealed system,10 rads and unvented hot water. I have my 18Kw boiler set at 14w max with the water temperature set at 65°. Worked fine during the coldest weather. I have my hot water on constant. If you do change the pump speed, you may have to readjust the ABV.

 

[bigglesuk]

I have my hot water on constant. If you do change the pump speed, you may have to readjust the ABV.

I thought that might be the case, forgot to ask that in my last post. Will try it and see. I need to then look at balancing all the radiators again with it all running efficiently.