Boiler modulation with 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 return temp distortion?


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

 

[reds42]

Are you sure your gas usage calculations are correct? If the boiler was running 65 degree water at a full 30Kw constantly for 2 hours that heat has to of gone somewhere into your system, and I'd think that would be noticeable.

 

[HRP123]

Are you sure your gas usage calculations are correct? If the boiler was running 65 degree water at a full 30Kw constantly for 2 hours that heat has to of gone somewhere into your system, and I'd think that would be noticeable.

If only I knew! I have done the calculations correctly, have been monitoring this now for a few months (and paying very high bills). This is why I wonder whether the LLH is somehow causing this high gas consumption, as it acts as a buffer tank, sends out 60 degrees to the UF, which is blended to a flow of lets say 45 with the return temp of 30 - thereby returning cold water to the LLH which returns this to the boiler. So basically the boiler keeps heating the LLH as it registers a low return temp. Just my novice understanding here.

 

[reds42]

LLH can reduce the boiler efficiency as it can cause the return temperature to be a bit higher which reduces the ability to condense, but the heat doesn't just disappear in a LLH - if your calculations are correct 30Kw for 2 hours is a big amount of energy going somewhere. Maybe you have some gravity driven circulation running through other parts of the system like radiators or hot water cylinders even though they aren't calling for heat?

 

[Deleted member 281264]

Your UFH system is old outdated technology

If you want proper efficiency and comfort you need to change your setup, you are half way there with the LLH

You need a proper electronic mixing valve not the garbage one at the manifold which is just taking 65c boiler water and blending it down to whatever you set at the manifold.

My boiler is running at 38c on the boiler 34c for radiators circuit and 30c for underfloor heating and then only shoots to 70 when the cylinder needs charging and shuts off the heating, then back to 35-40 once the cylinder is done. Look into low temperature heating systems/ weather compensation, hot water priority

The low loss header is not optimized in your setup because you are not running at two different flow temperatures which is what the LLH is designed to allow, hydraulic seperation of each circuit

 

[HRP123]

LLH can reduce the boiler efficiency as it can cause the return temperature to be a bit higher which reduces the ability to condense, but the heat doesn't just disappear in a LLH - if your calculations are correct 30Kw for 2 hours is a big amount of energy going somewhere. Maybe you have some gravity driven circulation running through other parts of the system like radiators or hot water cylinders even though they aren't calling for heat?

Boiler in the garage and the only wasted circulation would be to the closed diverter valves you can see for the other circuits (HW, rads, towel rails). Maybe its just heating up the garage?

 

[Deleted member 281264]

@HRP123 do you have seperate pumps for central heating and UFH outside of the boiler?

edit just seen the photo, looks like a very expensive setup. It can definitely be modified to be made much more efficient

 

[HRP123]

Your UFH system is old outdated technology

If you want proper efficiency and comfort you need to change your setup, you are half way there with the LLH

You need a proper electronic mixing valve not the garbage one at the manifold which is just taking 65c boiler water and blending it down to whatever you set at the manifold.

My boiler is running at 38c on the boiler 34c for radiators circuit and 30c for underfloor heating and then only shoots to 70 when the cylinder needs charging and shuts off the heating, then back to 35-40 once the cylinder is done. Look into low temperature heating systems/ weather compensation, hot water priority

The low loss header is pointless in your setup because you are not running at two different flow temperatures which is what the LLH is designed to allow, hydraulic seperation of each circuit

Ah thanks you sound like you know your stuff, it's a lot for me to digest as a novice, but let me repeat in my words to make sure I understand.

The point of the LLH is to allow for two different slow temperatures. So for example if you have a hot water priority boiler, when it detects you need more heat for the HW cylinder, it boosts the flow temperature from lets say 40 degrees to 65 degrees, and then returns back to 40 degrees (with lower gas consumption) assuming that flow temperature can do both the radiators and underfloor heating.

So does the LLH mean that when the boiler is boosting to 65 degrees, you get that 65 degree heat going to your UF and rads temporarily? Or is it the case the boiler shuts the UF, rads and towel rails, and only flows direct to HW at 65 degrees for a while. So during that time you have no heating on?

I've seen the Viessmann, Ideal and Integas have this feature.

Thanks!

 

[HRP123]

@HRP123 do you have seperate pumps for central heating and UFH outside of the boiler?

edit just seen the photo, looks like a very expensive setup. It can definitely be modified to be made much more efficient

Yes the plumber who put the system in put separate pumps for each circuit (10 years ago now). The reason is that the failure of one pump doesn't cause the whole system to stop working. Each circuit (UFH, rads, HW and towel rails) has a pump and diverter valve which link to the pump going into the LLH from the boiler via a relay box.

 

[reds42]

The point of the LLH is to allow for two different slow temperatures. So for example if you have a hot water priority boiler, when it detects you need more heat for the HW cylinder, it boosts the flow temperature from lets say 40 degrees to 65 degrees, and then returns back to 40 degrees (with lower gas consumption) assuming that flow temperature can do both the radiators and underfloor heating.

The point in low loss headers is to allow the use of multiple pumps for different zones without them interfering with each others flow. In your case its so the multiple pumps for your zones can operate independently from each other, and also not interfere with the pump running water through the boiler.

They do introduce losses into the system so for maximum efficiency its best to avoid them, but in some circumstances they are required.

 

[Deleted member 281264]

I'm far from a total expert engineer, but the point of the LLH is to allow each pump to be it's own free circuit and have an independant flow rate. This stops the pumps fighting against eachother and this also allows, in the right setup for each pump to have it's own independant flow temperature.

Yes that's right on a hot water priority setup the cylinder takes priority whenever it calls for heat everything shuts off and it will go full pelt to charge that. Instead of most boilers like yours being set to minimum flow temperature 65-70 if you had a boiler with that setup you could have each pump runnng at it's own different flow rate/temperature. For yours I'd suggest using mixed circuits for the radiiators and UFH and then a unmixed for the towel rads.

With a setup like this you remove all actuators from ufh manifold and the room stats/wiring centre and let the boiler do it all with a wiring centre like the vr71.

Vaillant and Viessman are the best, Viessman is a 4 pipe boiler designe with hot water priorty out of the box and weather compensation

 

[Deleted member 281264]

See this example here

 

[HRP123]

I'm far from a total expert engineer, but the point of the LLH is to allow each pump to be it's own free circuit and have an independant flow rate. This stops the pumps fighting against eachother and this also allows, in the right setup for each pump to have it's own independant flow temperature.

Yes that's right on a hot water priority setup the cylinder takes priority whenever it calls for heat everything shuts off and it will go full pelt to charge that. Instead of most boilers like yours being set to minimum flow temperature 65-70 if you had a boiler with that setup you could have each pump runnng at it's own different flow rate/temperature. For yours I'd suggest using mixed circuits for the radiiators and UFH and then a unmixed for the towel rads.

With a setup like this you remove all actuators from ufh manifold and the room stats/wiring centre and let the boiler do it all with a wiring centre like the vr71.

Vaillant and Viessman are the best, Viessman is a 4 pipe poiler designe with hot water priorty out of the box and weather compensation

That's a very helpful explanation thank you. So basically with lets say the Viessmann (I've read more about it), you wire your HW cylinder directly to your boiler, and when it calls for heat, everything else is shut off. Otherwise it sends heat at a lower flow temperature to your LLH which then allows you to pull it at different flow rates into rads, towel rails and UFH.

You mentioned mixed circuits for the rads/UFH and unmixed for the towel rails - doesn't having two flow temps and needing to mix a higher flow temp down make it less efficient again?

Thanks again!

 

[HRP123]

One more question please, having no room thermostats means you run the entire UFH at one flow temperature which is open all the time - you don't have the capacity to boost heating in one room that is used more often, and reduce in another room (e.g. guest bedroom) - aside from adjusting flow on your manifold right?

 

[Deleted member 281264]

Pretty much yes, Vaillant and Viessman controls are easy wired for hot water priority. WB also do weather comp but your boiler looks quite old so I'm not sure. Which model is it?

No it does not make it less efficient it makes it more efficient with these mixing valves because the electronic mixing actuators sit on top of a three way valve that sends the cool return water back into the system rather than the standard mixing manifold which just blends high temperatures down.

A low temperature efficient heating system is as open loop as possible, with all the UFH running at one flow temperature and same for the radiators, the trvs can be used to limit temperature overshoots in rooms and ufh can be adjusted by the flow setters at the manifold but zoning is bad for efficiency. This is how heat pump systems are designed, esentially with the setup i'm suggesting you are getting as close to heat pump efficiency as humanly possible with a gas boiler.