Balancing Radiators - still too hot...

We have TRVs on all but two rads upstairs; the bathroom towel rail and the landing rad, nearest the thermostat.

When the new heating system was fitted, the rads don't seem to have been balanced at all - some rooms were getting warmer than others, the landing rad was getting warmer quicker than others so turning the stat of etc.

We have a system boiler (Baxi Platinum 32kw) that supplies the downstairs underfloor heating, unvented cylinder and upstairs rads. It's set to 65 degrees (mainly for the hot water).

I bought myself an IR thermometer and attempted to balance them aiming for a delta-T of 20 degrees, however I can't even get a difference of 10 degrees from most of the rads, if I turn the lockshield valve down any further, there's no flow through the rad at all!

My assumption is the flow rate at the boiler pump is just too high so even when the lockshield valves are turned right down, water is being pushed through the rads too quickly and my concern now is that my turning down all the lockshield valves I'm just strangling the pump, wasting more energy - or am I doing something dumb?

It's quite likely that you're boiler is oversized (unless you have something in the region of 25-30 radiators/400m² of UFH?) and yes, your boiler's pump is running too fast. Can't remember whether the pump is adjustable on these but it might be.

muggles said:

It's quite likely that you're boiler is oversized (unless you have something in the region of 25-30 radiators/400m² of UFH?) and yes, your boiler's pump is running too fast. Can't remember whether the pump is adjustable on these but it might be.

Click to expand...

1850's house, solid walls, no wall insulation, approx 300mm rockwool in the loft. 9 radiators (another 5 once the extension's up), 3 UFH zones, 6 loops, approx 550m of pipe in total, (another zone and 600m of pipe once the extension's up). 2 bathrooms (another 2 ensuites once the extension's up)

So if that's all on at once I'm guessing it won't be too far off, BUT, the current UFH is hardly ever on, the slab holds the heat really well and I suspect the newer zone in the extension will be on even less (better wall insulation etc).

From a quick Google, I think the boiler comes with a Grundfos UPS single speed pump... Is there anything I can do but swap that out for a smarter pump that adjusts speed based on demand? (Even the UFH system comes with a 3 speed Grundfos!)

600m of pipe is about 90m². 75w/m² makes that 6.75kW. 1kW per radiator average over the old & new side is 14kW, so 20.75kW total when it's -3°C outside, which halves to 10.4kW when it's a more normal 9°C outside. The Platinum is a solid boiler but from a technical point of view it's coming up to 15 years old and rather behind the times. Its absolute minimum output is 10.8kW which exceeds your likely maximum heat requirement through Spring & Autumn with everything switched on by 400w based on my back-of-an-envelope calculations, and that's when your system is finished which it currently isn't. It's no wonder you can't balance it and everything is overheating.

It'll be 1,150m UFH pipe in total (550m + 600m) and based on the heat loss calc I had done the older part of the house (no wall insulation, high ceilings etc) requires ~ 125w/m² so approx 6.9kW for the original property (550m x 125w/m²) and approx 4.5kW for the extension (600m x 75w/m²) total of 11.4 kW for the UFH. If we assume 14kW for the rads, that's 25.4kW in total, or 12.7 kW on a good day. Another 7-10kW allowance for the unvented cylinder?

The problem I have is, both systems are very rarely on at the same time to reach that sort of demand. The UFH comes on for maybe an hour in the morning to bring the slab back up to temp and maybe an hour on the evening, if it's cold. The upstairs rads are on for longer, a couple of hours in the morning and probably 3-4 hours in the evening, but it's very rare both systems are on at the same time for any period.

I really don't want to swap out the boiler - it's brand new! Is there anything else I can do?

Ah sorry I thought you meant 600m total.

The 75w/m² output figure for UFH is based on the maximum output most hard floor coverings can accept, so while your UFH might theoretically be able to achieve 125w/m² it does depend on whether you have carpet/tiles/vinyl etc. The new side is unlikely to need more than 50w/m² and probably less.

If your existing UFH only runs for an hour in the morning that suggests the flow temperature is too high and the mixer needs adjusting. Low and slow is the way to go - it should tick over for a good portion of the day.

Generally we don't add more than 3kW for a cylinder and nothing if it's set up for hot water priority (unfortunately not an option with the Platinum, which is a shame as it's a much more efficient way to run a system, especially with UFH).

If you don't usually have both systems on at the same time then your boiler is definitely oversized for the majority of the year. Options at this point are unfortunately fairly limited to either a) live with it, or b) go back to the person who specified the boiler, complain that it's not suitable for your property and demand that it is replaced. I'm not aware of anyone who has had any success with option b, but I'm also not aware of anyone who's tried...

I've checked the manual for your boiler and the pump does not appear to be adjustable

I suspected as much

The flow temp is currently 45, which is 5 degrees lower than the UFH design suggested (pipes in Cemfloor screed, tiled finish), I can try lowering it further. - with the boiler set at 65 degrees and the UFH mixer set lower (say 40) cause any problems of their own?

If I lower the flow rate though, the delta-T is all to pot. I currently have it ~ 7 degrees per loop.

You should be able to reduce the temperature without reducing the flow rate. Shouldn't cause any problems. With heating, cooler is almost always better

Makes sense, thanks. Will turning the UFH flow temp down too much not cause the boiler to start short cycling? Modulating down as far as it can, still producing too much heat and turning off - rinse and repeat whilst the UFH is calling for heat?

As soon as the loops are up to temp, the return from the UFH manifold remains low, but the return to the boiler is much hotter - I assume because the bypass valve is then kicking in.