UFH Boiler Return Temp

[jayc89]

I have a S-plan heating system. Condensing system boiler powering upstairs rads and unvented cylinder and the downstairs UFH. Boiler temp is set at 65 for the hot water. UFH is mixing to 40 degrees.

 

When only the UFH is calling for heat, the return temp from the mixer is approx 30 degrees, which is as expected with a 7 degree delta-t across the loops. However, once the loops are up to temp, the return on the actual boiler is far hotter - nearer 55 degrees, so not condensing much at all.

 

I suspect as the UFH demand reduces the bypass valve kicks in, causing the increase in return temp.

 

Is there anything I can do to fix this? I have a feeling my only solution will be a buffer tank for the UFH...

[Temp]

In order to make 40 degree water for the UFH the mixer takes water at 65 and blends it with 30 degree water from the UFH.  The calculator here..

https://www.spiraxsarco.com/resources-and-design-tools/calculators/water-mixing/water-mixing

.predicts that the ratio needed to make 40C will be.. 

 

28.6%  Hot flow at 65C 

71.4% UFH Return at 30C

 

So quite a high percentage is returned to the boiler raising the return temperature.  The boiler return is going to be the same percentages but the other way around so..

 

71.4% Hot flow at 65C 

28.6% UFH Return at 30C

 

and that makes 55C as you are seeing.

 

Do you need the boiler flow to be so hot? Normally >60 C is recommended to avoid legionnaires disease but its sometimes possible to run it a bit cooler and install a timer and stat that heats it to 65C say once a month. Depending on the tank design it can also be possible to dose the system with chemicals but I think this needs a tank that has a heat exchanger on the output rather than the input of the tank.

 

Lowering the boiler flow 5 or 10C will lower the return by roughly the same amount..

 

1) If you lower the boiler flow to 60C the mix ratio to make 40C changes to..

 

33.3%  Hot flow at 60C 

66.6% UFH Return at 30C

 

and the return becomes

 

66.6%  Hot flow at 60C 

33.3% UFH Return at 30C

 

The return temperature should fall to about 50C.

 

Some boilers say <54C is needed to be in condensing mode.

 

2) If you lower the boiler flow to 55C the mix ratio to make 40C changes to..

 

40%  Hot flow at 55C 

60% UFH Return at 30C

 

and the return becomes

 

60%  Hot flow at 55C 

40% UFH Return at 30C

 

The return temperature should fall to about 45C.

 

You could try out these lower boiler flow temperatures for awhile and see if there are any noticable issues such as the DHW being too cold or perhaps a room heated by a rad being too cold when its very cold outside.

 

Our system runs a TS at 55C which is about as low as we want to go before my wife complains the shower mixer is maxed out.

[jayc89]

Interesting. The maths makes sense, but I don't understand why it's only the return to the boiler that increases in heat, NOT the return from the UFH mixer. That's why I assumed it was the bypass valve causing the increase.

 

I'll try dropping to the boiler temp to 60 and see what difference that makes.

[Temp]

38 minutes ago, jayc89 said:

Interesting. The maths makes sense, but I don't understand why it's only the return to the boiler that increases in heat, NOT the return from the UFH mixer. That's why I assumed it was the bypass valve causing the increase.

 

Sorry I didn't notice that distinction in your OP.

 

Some pictures of the manifold and mixer might help. 

 

It does sound like there is another bypass somewhere that is opening to allow hot flow back to the boiler when the mixer shuts down. On our system there is a bypass built into the mixer so there is only one return path. Some mixers have a minimum ratio so always take some hot flow.

 

 

Edited October 12, 2021 by Temp

[jayc89]

Boiler - 4 pipes from left to right;

1. Boiler return
2. Boiler flow (teed; below for UFH flow, above for upstairs rads + water cylinder)
3. UFH return (goes from the UFH mixer return up to the airing cupboard)
4. Cold water supply

 

These pipes run up to the airing cupboard above the boiler.

 

 

UFH is pretty simple. The pipes shown above feed into a a mixer, pump and through to the manifold;

 

 

Airing cupboard, which is far more complicated than I understand, bypass valve can be seen between the 2x 2 port valves and is set to 0.5 bar (max)

 

Edited October 12, 2021 by jayc89

[jayc89]

[Temp]

The Reliance blender is a fairly standard model so should be fine. 

 

If something was switching off the motorised valve feeding the UFH but leaving the boiler running that might explain why the bypass is opening?

[jayc89]

5 minutes ago, Temp said:

The Reliance blender is a fairly standard model so should be fine. 

 

If something was switching off the motorised valve feeding the UFH but leaving the boiler running that might explain why the bypass is opening?

 

The valve's connected direct to the Heatmiser UH8 wiring centre, all stats are Heatmiser NeoStat v2. When the boiler return temp increases, the flow rate on the open loops remains the same so I don't think the valve's turning off. 

 

My assumption was, as the loops get up to temp, the demand for hot(ter) water reduces with the mixer able to recycle more of the water being returned in the loops, as that demand drops, pressure on the boiler feed increases meaning the bypass valve opens and returns that hot water back to the boiler to reduce the pressure. But I'm by no means a heating expert, winging it as I go. I could be talking utter nonsense

[Temp]

2 hours ago, jayc89 said:

My assumption was, as the loops get up to temp, the demand for hot(ter) water reduces with the mixer able to recycle more of the water being returned in the loops, as that demand drops, pressure on the boiler feed increases meaning the bypass valve opens and returns that hot water back to the boiler to reduce the pressure. But I'm by no means a heating expert, winging it as I go. I could be talking utter nonsense

 

Looks like you arecright though. 

 

I thought when the loops were upto temperature the mixer didn't just recycle more from the loops but also sent more unused hot flow back to the boiler. So I expected the return from the mixer to get hot. As that's not happening it must be as you say eg the bypass is opening.

 

I have a Reliance mixer as well but its a different model. 

 

 

 

[jayc89]

What’s the best way to drop the boiler return temp? Buffer tank, or something, between the boiler flow and the UFH mixer?

 

Given I have 550m which will soon turn into 1150m and 16mm PEX carries approx 0.113 l/m (130l in total), I guess any tank would need to be reasonably big?