System boiler modulation with low loss header LLH

[HRP123]

Hi there, thanks for taking the time to read this. I've been trying to diagnose why my gas consumption is so high and been doing various measurements and tests.

With the UFH, my setup is about 10 room zones connected on two separate UF manifolds linked to one UF pump with a return blending valve from the LLH (picture) - i.e. there is no pump on the manifolds, just at the LLH - each room has an actuator opening each zone from the manifold. Total UF area around 160 sqm with UF pipes on average 150mm spacing (some 200 and some 100mm).

In the middle of the night I positioned a camera to see what calls for heat from the boiler at what time. For about two hours in the middle of the night, only the UF pump turned the boiler on, and I believe only 1 or 2 room zones (about 50-80sqm) would have called for heat (I had set the others back on temperature). I think in theory this should be a heat demand of 9kwh. However the gas consumption for that period shows the boiler was going on full 30kw gas burning per hour (I have ranged rated my 40kw boiler down to around 30kw).

My question is if the UF blending temp was turned down to 40/45 degrees, how should the boiler respond when it is set to a flow of 65 degrees? Will it just keep sending out 65 degrees, and because of the low return temp from the UF manifold into the LLH, the LLH will be sending back a low return temperature to the boiler, so the boiler keeps firing fully instead of modulating down to a lower temperature and consuming less gas?

If so, then would any setup with HW, UFH and radiators from one LLH always cause a boiler to fire too much - since the boiler is set to a higher to satisfy the HW but too high for the UFH and radiators maybe, but cannot modulate down because of the LLH distortion?

Thanks for your help... sorry this may be a rookie question!

 

[ShaunCorbs]

Yes spot on tbh your better off running the ufh wide open and maybe look at sizing your rads for the same temp as the ufh then

 

[HRP123]

Thanks for your opinion! I get what you are saying - goes against mainstream advice of going with low flow temperatures for longer.

Basically I need to find the lowest flow temp that will work for the HW cylinders, and push the max temp possible to the UFH (tiles temp maybe 30 degrees). Then set a long boiler anti cycle time (lets say 10-15 mins) and perhaps a 20 degree flow return anti cycle function (vs default 10 difference on WB) on the boiler so basically cycles less when the return starts rising rapidly, giving some time for the heat to flow through into the UFH and rads?

Cheers

 

[ShaunCorbs]

Thanks for your opinion! I get what you are saying - goes against mainstream advice of going with low flow temperatures for longer.

Basically I need to find the lowest flow temp that will work for the HW cylinders, and push the max temp possible to the UFH (tiles temp maybe 30 degrees). Then set a long boiler anti cycle time (lets say 10-15 mins) and perhaps a 20 degree flow return anti cycle function (vs default 10 difference on WB) on the boiler so basically cycles less when the return starts rising rapidly, giving some time for the heat to flow through into the UFH and rads?

Cheers

Depending on your boiler you could go down the option of having a diverter in the boiler so you can run two temperatures

 

[HRP123]

Depending on your boiler you could go down the option of having a diverter in the boiler so you can run two temperatures

It is a 10 year old 40cdi WB, not sure I can install this feature

 

[gmartine]

Do you need a flow temp of 65 or is that just for (2 x 200l) the HW cylinders.

 

[gmartine]

Do you need a flow temp of 65 or is that just for (2 x 200l) the HW cylinders?

 

[HRP123]

Do you need a flow temp of 65 or is that just for (2 x 200l) the HW cylinders?

Yes the flow temp is for the HW cylinders which I have set to about 50-55 degrees - also have a secondary hot water circulator which keeps turning the boiler on for short period throughout the day (every couple of hours).

 

[John.g]

Your system will still only require 9kw, assuming a manifold dT of 8C then the UFH flowrate will be 16.13LPM at 45C, return 38C with 4.61LPM returning to the boiler giving boiler flow/return flows of 4.61LPM and flow/return temperatures of 65C/38C, 9kw in both but the boilr dT at 27C is getting close to the 30C which normally trips the burner.
If you lower the manifold flowtemp to 40C then the boiler will more than likely trip on a high dT of 33C as its flowrate will have dropped to 3.91LPM.

 

[HRP123]

Your system will still only require 9kw, assuming a manifold dT of 8C then the UFH flowrate will be 16.13LPM at 45C, return 38C with 4.61LPM returning to the boiler giving boiler flow/return flows of 4.61LPM and flow/return temperatures of 65C/38C, 9kw in both but the boilr dT at 27C is getting close to the 30C which normally trips the burner.
If you lower the manifold flowtemp to 40C then the boiler will more than likely trip on a high dT of 33C as its flowrate will have dropped to 3.91LPM.

Thanks for this. Sorry, still self educating about the interaction between flow/return temps and rates ... so are you saying basically that whether the temperature is set to 55 degrees or 40/45 degrees - the boiler will consume 9kw? However the lower flow temperature, leads to a lower return temperature, and if this differential is too large (33) then the boiler will trip and cycle?