Some rads cold - pipe layout/sizing issue?

[AK78]

First-time post, I'm not particularly knowledgeable in plumbing matters...

Just had all the upstairs radiators replaced in our house (4 doubles and 1 1800x500mm towel rail) and the system balanced and boiler serviced. Aside from reasons of aesthetics we also had the work done because one of the four bedroom rads would only get slightly warm (bed 2), and one not hot at all (bed 1). A couple of plumbers had suggested the radiators may have got sludge in them so flushing or changing them should help.

These two now do actually get warm but much more slowly than any others in the house. The plumber who did the work thinks the upstairs in only plumbed in 15mm pipe and this could be the reason, though he did suggest trying a new pump (currently a BG/Grunfoss multihead G). We have a four year old Worcester boiler, hot water tank run from the boiler (both on the ground floor next to each other) and the usual header tanks in the loft. There is a magnaclean on the system but it hardly gets dirty. There are 5 radiators downstairs which heat up quickly, and beds 3 and 4 rads heat up fairly quickly as well).

I've been looking at the pipes to and from the boiler to try and work out how the existing pipes are laid out. The flow (in 22mm) after the pump works its way down into the ground floor, but there is no branch off up the wall to upstairs. Instead there are two 15mm pipe runs up the wall behind the hot water cylinder - they come up out of the floor and go up through the ceiling to the first floor, I cannot see any connections to them in the cupboard. One gets hot after the downstairs radiators have started heating, and the other only warms up after quite a while (maybe 30mins). I assume that these are therefore the flow and return to the upstairs radiators, but it therefore seems that the water has to flow all around the downstairs radiators before it can make its way upstairs? If so I guess that's why it takes so long to heat upstairs and perhaps why the two furthest bedroom rads from the boiler barely get hot?

Can anyone comment:

- is that a normal design, or should there be a split after the pump to the upstairs rads
- could the 15mm pipes upstairs be sufficient if the water didn't have to circulate downstairs first?
- is there a way to improve it without pulling up all the hard flooring upstairs?!
- do I need a better pump?

Any input will be greatly appreciated...

 

[Jadele]

Balancing the system means that all the rads should be getting hot at the same time. I would shut all the downstairs rads off and fully open all the upstairs and see what happens. if they all get hot upstairs quickly then its likely you need to balance the system better. You also need to turn off the rads upstairs individually to see if more than one rad is affected, if that happens then those 2 rads have been incorrectly piped in series. its also not uncommon for idiots to take a pipe off the flow from the boiler to a rad and return it to the same pipe which means there will never be a good flow through the rad which would warm slowly from the bottom or heat up fast if you bled some water from it. This should give you a slightly better idea as to where the problems are.

 

[oldbuffer]

1. There is a limit to the quantity of heat that 15mm pipe can carry. It depends on the flow rate and temperature differential between flow and return, but work on somewhere around 6 to 8 Kw as a maximum. There is of course a limit to the amount of heat that any pipework can carry, but the bigger the bore the greater the capacity for a given speed of flow.
2. Try turning off all the downstairs radiators and see if upstairs gets hot. That would answer your second question.
3. If not already done, the first step is to balance the system to reduce the amount of heat taken by downstairs.
4. You can't really improve it without altering pipework.
5. If you have to alter pipework you could try running 22mm up stairs (replacing 15mm in airing cupboard), "past the nearest radiator", take 15mm feed from the 22 to that radiator, and continue the 22mm as close as possible to the next radiator then reduce back to 15mm. Might minimise disruption. If only partially successful, continue 22mm to past the next radiator and so on.
6. You could try a new pump, but you'd have to be careful that a more powerful pump doesn't cause "pump over" into the feed and expansion tank in the loft, or raise the flow speed to the point of making a noise.

 

[AK78]

Thanks for your replies.

The plumber balanced the system after installing the new radiators. He said he'd got the best overall result he could, but downstairs still gets way hotter than upstairs. I've tried shutting off all downstairs rads and upstairs does get hot quicker but not exceptionally fast. If I turn off beds 3 and 4 and the towel rail, rads in beds 1 and 2 get hotter quicker but nowhere near as hot as downstairs and the pipes coming out on the return side never get above luke warm. I will try experimenting with that a bit more this evening, but it seems as if there's not enough flow for the hot water to get to bedrooms 1 and 2 properly.

Is it odd that the circuit seems to go all round downstairs first and then goes upstairs afterwards? To my untrained mind would it not be better to split the flow pipe after the pump with one branch going upstairs and one downstairs, or am I being too simplistic?!

Do you know the head rating of the British Gas multihead G pump? Would changing it for a Grunfoss UPS2 15-50/60 be worth a try?

 

[AK78]

Is it odd that the circuit seems to go all round downstairs first and then goes upstairs afterwards? To my untrained mind would it not be better to split the flow pipe after the pump with one branch going upstairs and one downstairs, or am I being too simplistic?!

Do you know the head rating of the British Gas multihead G pump? Would changing it for a Grunfoss UPS2 15-50/60 be worth a try?

Is anyone able to comment on these two points please?

 

[oldbuffer]

1. Unless you have a one pipe system, you will have flow and return "back bones" with each radiator across the flow and return, so effectively in parallel. given this it makes little difference where the upstairs flow and return start, provided the resistance of the downstairs spine (not the radiators) is not sufficient to significantly reduce the pressure, and hence the flow, to upstairs.

2. if you look at the thread https://www.diynot.com/diy/threads/bg-multihead-g-pump-replacement-required.195891/ you will see that at speed 3 the BG pump is equivalent to a 15/60. Subject to the proviso in point 6 of my earlier post, you could view replacing the pump as a possible (partial?) cure with minimal cost and disruption. Not guaranteed to work though.

 

[AK78]

Thanks very much oldbuffer

 

[gassymark]

Hi does your system have an air separator to vent air back over the feed and expansion tank if so my guess would be that it is partly blocked, had them before where I've cut them in half totally blocked. Cheers Mark

 

[AK78]

Thanks, but I don't think I have one of those unless it's hidden somewhere other than the boiler and hot water tank cupboards.

I'm going to have a proper bash at balancing the system because turning downstairs rads off does seem to get upstairs hot more quickly. I shall keep experimenting to see if I can isolate the issue.