Heating woes - is it my pump or something else?
- Thread starter VerticalTobe
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There appears to be different manuals floating around for the same model and GC of the boiler....the early ones have no reference to range rating and maybe the software can't do it either. Wonder if Glowworm finally realised so many 38hxi are massively oversized and decided to implement the BG330 solution...one size fits all and make it range ratable (sort of).
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The Ultracom hxi heat exchanger has a very high resistance - 4.05m @38kW output when the temperature differential is 20C. If the differential is lower, say 10C, the resistance will increase dramatically to 16m, which means the pump will be unable to circulate any water. If the boiler is down-rated to 24kW the resistance of the hex is approx 1.5m @ 20C differential, but still 6m @10C differential.
The OP needs to calculate the differential using d.40 (flow temp) and d.41 (return temp). If, as I suspect, it is about 10C that would explain the problem: the resistance of the hex is so high that there is nothing left in the pump to circulate the water round the system - i.e a "restrictive" system.
The system will need the boiler output adjusted to the appropriate value and then carefully balanced to whatever differential works.
The OP needs to calculate the differential using d.40 (flow temp) and d.41 (return temp). If, as I suspect, it is about 10C that would explain the problem: the resistance of the hex is so high that there is nothing left in the pump to circulate the water round the system - i.e a "restrictive" system.
The system will need the boiler output adjusted to the appropriate value and then carefully balanced to whatever differential works.
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A bit later on with this and I'm still stumped.
The D40 and D41 temps show a difference of 14C and that doesn't alter no matter how much I downrate the boiler. If I downrate to 30kw then the rads struggle to heat up at all. Down to 26kw and the radiators won't warm up at all.
The bypass gate valve also appears to be red hot no matter how much it is opened (even a quarter of a turn). If I close it fully to test then the system starts to make a noise and doesn't seem to like it (which is probably in line with the boiler wanting a constant flow).
I'm thinking of changing the gate valve to an automatic bypass to take it out of the equation, but now cheesed off enough that I'm also thinking of just putting a larger pump in there (ouch, my wallet). Would a 25-80 be ridiculous overkill and likely to cause problems?
The D40 and D41 temps show a difference of 14C and that doesn't alter no matter how much I downrate the boiler. If I downrate to 30kw then the rads struggle to heat up at all. Down to 26kw and the radiators won't warm up at all.
The bypass gate valve also appears to be red hot no matter how much it is opened (even a quarter of a turn). If I close it fully to test then the system starts to make a noise and doesn't seem to like it (which is probably in line with the boiler wanting a constant flow).
I'm thinking of changing the gate valve to an automatic bypass to take it out of the equation, but now cheesed off enough that I'm also thinking of just putting a larger pump in there (ouch, my wallet). Would a 25-80 be ridiculous overkill and likely to cause problems?
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FYI Changing the heat output of the boiler will not alter the resistance of the main hex, it simply limits the burners maximum flame which has no actualy effect on the quantity of water circulating in it.
But you are correct in saying its a very high resistance hex and virtually no big system is going to work on a std domestic pump with a hex that restrictive.
To add if it was fitted correctly it would have 28mm primarys/zone valves/magnaclean valves.
The cheapest and closest to correct way to help it would be to use a header, so one pump circualtes water round the boiler and header at a constant rate regardless of the system, then another pump takes the water from the header and puts it into the system (not loosing any strength through the boiler so a normal domestic 15/60 could well be enough)
True if you don't bother to change the pump speed.
If you reduce the boiler output, say to half what it was, the flow rate required to produce the required differential will also halve. So the resistance of the hex will be 1/4 of what it was. The pump will therefore need to be set at a lower speed.
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So by way of a quick update, this is still (sort of) an issue.
I ended up changing the pump to an Alpha to see if the automatic curve function would help. It didn't (although the pump does run quieter and seems a lot more efficient). But the system would still keep cutting out and was still making a horrible gurgling noise above the boiler / Magnaclean unit.
The next thing to attack was that manual bypass valve. Try as I might I couldn't find a suitable key to turn it. If I tried to bodge it using a pair of long nosed plies (and a firm grip) the thing wouldn't turn. Suspecting it was knackered I replaced it with a wheelhead valve with would allow me to fine adjust it (I know it should be an autobypass but at this point I didn't want to throw anything else into the equation to complicate things - plus there's the potential issue of the Alpha's curve system working against it).
Tried to adjust the new bypass wheelhead valve and no matter how I opened it, it was either just `conductive hot` from the nearby pipework, or red hot.
With that valve a quarter to half turn open, the heating will only stay on if the hot water is on at the same time. Which would be fine with the tank I have, but I was then back to the issue of the rads not warming up.
Tried balancing the system twice - the bottoms of the rads and returns wouldn't get hot, despite the flow and tops being painfully warm to the touch. The rooms and house were struggling to get up to temp with the hall thermostat barely getting above 13 degrees after several hours.
Took all the TRV's off every rad in house the event they were somehow interfering with the system or faulty. Still no change.
So in a fit of desperation I decided to close the gate valve altogether. Low and behold every single radiator in the house got piping hot - the place was like a sauna within an hour, hotter than it had been for balancing the day before when the heating has been on full for 6 hours (the TRV's were still off all the rads which is the difference to what I last posted above).
It seems the gate bypass valve is robbing the whole system of heat but also immediately tricking the boiler into thinking the house is up to temp. The Boiler seemed happy with the valve being closed, however I need to ask the general views on this. Is it a massive no-no or potentially going to damage the system? My plan is not to put TRV's back on some of the rads and leave them fully open all the time (in the hallway as that's the coldest spot in the whole house) which I gather would be okay?
I ended up changing the pump to an Alpha to see if the automatic curve function would help. It didn't (although the pump does run quieter and seems a lot more efficient). But the system would still keep cutting out and was still making a horrible gurgling noise above the boiler / Magnaclean unit.
The next thing to attack was that manual bypass valve. Try as I might I couldn't find a suitable key to turn it. If I tried to bodge it using a pair of long nosed plies (and a firm grip) the thing wouldn't turn. Suspecting it was knackered I replaced it with a wheelhead valve with would allow me to fine adjust it (I know it should be an autobypass but at this point I didn't want to throw anything else into the equation to complicate things - plus there's the potential issue of the Alpha's curve system working against it).
Tried to adjust the new bypass wheelhead valve and no matter how I opened it, it was either just `conductive hot` from the nearby pipework, or red hot.
With that valve a quarter to half turn open, the heating will only stay on if the hot water is on at the same time. Which would be fine with the tank I have, but I was then back to the issue of the rads not warming up.
Tried balancing the system twice - the bottoms of the rads and returns wouldn't get hot, despite the flow and tops being painfully warm to the touch. The rooms and house were struggling to get up to temp with the hall thermostat barely getting above 13 degrees after several hours.
Took all the TRV's off every rad in house the event they were somehow interfering with the system or faulty. Still no change.
So in a fit of desperation I decided to close the gate valve altogether. Low and behold every single radiator in the house got piping hot - the place was like a sauna within an hour, hotter than it had been for balancing the day before when the heating has been on full for 6 hours (the TRV's were still off all the rads which is the difference to what I last posted above).
It seems the gate bypass valve is robbing the whole system of heat but also immediately tricking the boiler into thinking the house is up to temp. The Boiler seemed happy with the valve being closed, however I need to ask the general views on this. Is it a massive no-no or potentially going to damage the system? My plan is not to put TRV's back on some of the rads and leave them fully open all the time (in the hallway as that's the coldest spot in the whole house) which I gather would be okay?
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