UFH & Thermal Storage Tank

[David-1712]

Hello, looking for a bit of advice. I’ve recently moved into a new home and the heating system is completely different to my old house (combi boiler) This house has a thermal storage tank that’s heating by a 18kw Heat only boiler and also has the multi-fuel stove connected to it to provide an additional heat source for the cylinder. Cylinder is kept at 63/64°. I’ve no problem with DHW however if I turn my underfloor heating on (15 loops, ranging in length from 110m to 54m + 3 towel rails for in the bathrooms) my Thermal store cools down to around 30° and seems to stay there, even after an hour of running the heating. 

I understand that when the heating has been off and the UFH is cold, I’m asking for a big volume of water in all the loops to be heated, however the UFH mixing valve is set to 45°, the water in the tank is showing 30° so my slow to heat UFH system is now even slower due to the temp of the water getting to the manifolds. 

As mentioned, I have an 18kw Greenstar Ri boiler and a Mcdonalds ThermFlow tank 210L - schematic below of tank and loops for the UFH. 17mm Myson pipe. 

Is the boiler big enough (output wise) to supply the tank hot water quick enough so I don’t drop below 45° This meaning my UFH manifolds have the correct temp water going to them to start with? 

Is the tank big enough in capacity to cope with the sudden request to dump the stored hot water into the floor? 

Combination of both? 

Am I asking to much of the system and just be prepared to bring each room up to temp slowly? 

Appreciate your help with this

Edited September 4, 2022 by David-1712

[bassanclan]

There is approx 0.12l of water in a metre of ufh pipe.

You have 704m + 349m

So 84.48 + 41.88 = 126.36l of water in your ufh add to which you have 3 towel rails

 

 

[JohnMo]

The diagram shows the boiler control stat is set at 75. But is yours set at 65? The trouble could be the temperature is set too low at 65.  So when the UFH calls for heat, the temperature in the cylinder just drops too quickly (not enough capacity in the cylinder at 65) and the boiler cannot keep up, when the temperature drops to far.

[JohnMo]

130l of water heated from 20 to 45 degs only requires 3.8kWh.

 

210l of water start temp 65 deg start temp and end temp 45 has 4.9kWh.

 

At 75 deg start and 45 end has 7.4kWh

 

So the higher cylinder temp has a much larger reserve of energy to play with.

 

[David-1712]

2 minutes ago, JohnMo said:

130l of water heated from 20 to 45 degs only requires 3.8kWh.

 

210l of water start temp 65 deg start temp and end temp 45 has 4.9kWh.

 

At 75 deg start and 45 end has 7.4kWh

 

So the higher cylinder temp has a much larger reserve of energy to play with.

 

Thanks for the reply. I’ll screw up the thermostat on the tank to keep it up at the 75° and run the heating again and see how that goes. Really appreciate the help. 

[ProDave]

First question:  When you turn the UFH on, the boiler should be firing continuously working it's socks off to deliver all of it's 18kW to maintain the tank temperature.  Is it firing continuously or near continuously or not?

[David-1712]

14 minutes ago, bassanclan said:

There is approx 0.12l of water in a metre of ufh pipe.

You have 704m + 349m

So 84.48 + 41.88 = 126.36l of water in your ufh add to which you have 3 towel rails

 

 

Thanks for the reply. I was worried the system was potentially undersized. Reading johnMo’s comments, perhaps it’s just the current thermostat setting that’s the issue. 

[David-1712]

2 minutes ago, ProDave said:

First question:  When you turn the UFH on, the boiler should be firing continuously working it's socks off to deliver all of it's 18kW to maintain the tank temperature.  Is it firing continuously or near continuously or not?

Yes, it fires up and sounds as if it’s working hard to keep the temp in the tank. The flow pipe is always very hot to touch so presume it’s going flat out. I was just worried it was potentially undersized for the heating output required - like it was struggling to keep up.