UFH BUFFER TANK SELECTION (no rads)

[dmitripopov1984]

Good day all,
I would like to get some advise and perhaps the solution to the issues with my UFH system.
We did install UFH in an entire house and got rid of all radiators.
The system is 14 port manifold (only 12 are in use) feeding 7 zones.
Total room area is 97 m2, with the biggest zone being 37 m2 (uses 4 ports).
The volume of the system is approximately 35 litres of water. The systems output around 7.3 kW (as per drawings). System pressure is 2.4 Bar.
The system is fed by the Vaillant Ecotec 838 (reason for such high end boiler is to feed two showers at the time), with direct loop (feed and return).
The Heating Output temperature is set to 60C.
The manifold temperature is set to 50C.
The Delta T between each zone’s Flow and Return is set to 8 – 10 C dT.
The heating kicks in in the morning around 5AM and lasts until around 9AM. Then it kicks in again around 3PM and goes until 9PM (not all zones work in the same time).
Thermostats in the rooms are set between 19C and 21C, depending on the rooms.
The issues what I have are:
1. While there is “call for heat” the boiler is always on, without having a chance for performing “condensing”, until it goes into cooling (possibly cycling?) mode.
2. While boiler is in the mentioned “cycling mode” the temperature can drop down below 30 degrees C and be like that for 20 odd minutes.
3. The consumption of the gas, only for the heating, was initially around £5 per day. This got reduced down to £3-50 (equals to about 100 kW of gas daily) by turning the heating output from AUTO to 10 kW. I did try to run it on 7 kW option, but it does not produce enough heat for the all 7 zones. The Flow temperature can only be achieved of 26C on that setting. To be honest, even 10 kW is not enough to keep the all 7 zones at 50C Flow, but I am sacrificing some rooms to the others.
I am baffled. What we were sold is the “efficient” underfloor heating system. Which suppose to be far more better than ordinary CH system. While we had the central heating in place, even with all the radiators running, the consumption was nowhere near this high (measuring in the kWh).
The supplier could not suggest anything, other than to speak to the Vaillant. After speaking to Vaillant nothing was achieved either.
Reading through the forums some people suggest to install buffer tank. But no further advise is there on what exactly is required. Perhaps there is a “formula” on what size tank is required?
Please could you advise the solutions to bring at least the running costs down and bring the efficiency of the boiler, as from my understanding having it constantly running with dT of only 10C – 12C won’t do any good in the long term, never mind constant cycling.
Many thanks for reading the above.

 

[ShaunCorbs]

What’s your floor covering ?

 

[dmitripopov1984]

#ShaunCorbs
The Upstairs bedrooms are 14mm Bamboo, UFH is installed within grooved chip floorboards (22mm).
Ground floor - all pipes are screeded; living room has 14mm Bamboo, other rooms porcelain tiles.

 

[Sparkgap]

I take it you have a mixing manifold with its own pump? Surprised at 50C flow as I would think this was a bit high for UFH but I am assuming this is required minimum flow temp into the manifold setup. I think your problem is with the small amount of water in the system: by fitting a buffer tank there will be more water to absorb fluctuations in temp from the boiler and it won't be short cycling so much. I didn't notice any minimum system volume requirement in the manual but this seems like the main problem. You might also want to check the settings for the burner cycling.

 

[dmitripopov1984]

I take it you have a mixing manifold with its own pump? Surprised at 50C flow as I would think this was a bit high for UFH but I am assuming this is required minimum flow temp into the manifold setup. I think your problem is with the small amount of water in the system: by fitting a buffer tank there will be more water to absorb fluctuations in temp from the boiler and it won't be short cycling so much. I didn't notice any minimum system volume requirement in the manual but this seems like the main problem. You might also want to check the settings for the burner cycling.

Good morning,
Thank you for your reply.
This is correct, there is a mixing manifold with pump (I am attaching photo).
Would you be able to advise on how to specify the sizing (volume) of the buffer tank?
I have made some research on them in the past and there are so many different ones available on the market.

 

[Ric2013]

Hang on.

UFH is efficient in that it makes you feel warmer than with radiators at the same room temperature. You save energy by running a lower room temperature. Trouble is we get get fixated on 'comfort temperatures' which are really nothing to do with anything and building services engineers moved on to the concept of 'percentage of people dissatisfied' (Fanger) - What Is PMV? What Is PPD? Basics of Thermal Comfort | SimScale - https://www.simscale.com/blog/2019/09/what-is-pmv-ppd/ - many years ago. If you keep the same temperatures, you house will probably lose more heat as the rooms are now more uniformly warm and will be losing more heat to the outside). Or they should be... if you are running a timber floor UFH system intermittently for a good part of the day, that may be well and good, but a screed system really needs to be on more or less constantly (and then you start dropping the temperatures to the minimum you find acceptable).

The other advantage is that the system SHOULD allow your boiler to run in condensing mode most of the time. But it does this by needing cooler water than radiators do. If for some reason you still have to run your boiler at 60°C and then blend down, this might be the problem. Your floor surface temperatures should not be above 27°C or so with the heating running, and a 50°C manifold temperature seems high if you want the floor surface to remain below 27. It seems high for a suspended floor type, and it certainly seems high for a screeded floor type. The idea is that you set the manifold temperature to the temperature that does not cause the floors to overheat even if run constantly. You could drop the temperature further if you find that the rooms start to overheat and switch off on their zone thermostats even in cold weather, though this will of course increase the time required to heat up a room from cold.

Agree with the idea of the buffer, but wondering whether the boiler couldn't be range-rated on the heating side by a gas-safe installer, or if all you have is the setting you are are already adjusting. It seems odd that 7-10 kW is not enough for the area you mention. It is, of course, worth noting that an UFH system will need a good input at startup from cold as the mixing valve on the manifold takes virtually all the water from the flow. But if it gets 26°C, that's shouldn't be a problem as the system temperature ought to increase progessively once the return water temperature ramps up.

Why, incidentally, run the upstairs in grooved chipboard? I'd really rather have seen aluminium spreader plates. Is the chipboard specially designed to diffuse the heat? If not, then it sounds a bit naff.

 

[John.g]

Good day all,
I would like to get some advise and perhaps the solution to the issues with my UFH system.
We did install UFH in an entire house and got rid of all radiators.
The system is 14 port manifold (only 12 are in use) feeding 7 zones.
Total room area is 97 m2, with the biggest zone being 37 m2 (uses 4 ports).
The volume of the system is approximately 35 litres of water. The systems output around 7.3 kW (as per drawings). System pressure is 2.4 Bar.
The system is fed by the Vaillant Ecotec 838 (reason for such high end boiler is to feed two showers at the time), with direct loop (feed and return).
The Heating Output temperature is set to 60C.
The manifold temperature is set to 50C.
The Delta T between each zone’s Flow and Return is set to 8 – 10 C dT.
The heating kicks in in the morning around 5AM and lasts until around 9AM. Then it kicks in again around 3PM and goes until 9PM (not all zones work in the same time).
Thermostats in the rooms are set between 19C and 21C, depending on the rooms.
The issues what I have are:
1. While there is “call for heat” the boiler is always on, without having a chance for performing “condensing”, until it goes into cooling (possibly cycling?) mode.
2. While boiler is in the mentioned “cycling mode” the temperature can drop down below 30 degrees C and be like that for 20 odd minutes.
3. The consumption of the gas, only for the heating, was initially around £5 per day. This got reduced down to £3-50 (equals to about 100 kW of gas daily) by turning the heating output from AUTO to 10 kW. I did try to run it on 7 kW option, but it does not produce enough heat for the all 7 zones. The Flow temperature can only be achieved of 26C on that setting. To be honest, even 10 kW is not enough to keep the all 7 zones at 50C Flow, but I am sacrificing some rooms to the others.
I am baffled. What we were sold is the “efficient” underfloor heating system. Which suppose to be far more better than ordinary CH system. While we had the central heating in place, even with all the radiators running, the consumption was nowhere near this high (measuring in the kWh).
The supplier could not suggest anything, other than to speak to the Vaillant. After speaking to Vaillant nothing was achieved either.
Reading through the forums some people suggest to install buffer tank. But no further advise is there on what exactly is required. Perhaps there is a “formula” on what size tank is required?
Please could you advise the solutions to bring at least the running costs down and bring the efficiency of the boiler, as from my understanding having it constantly running with dT of only 10C – 12C won’t do any good in the long term, never mind constant cycling.
Many thanks for reading the above.

You really IMO shouldn't need a buffer with a modulating boiler even if at times it must cycle when its minimum output (~ 7kw) is greater than the UFH demand.
Unfortunately, the Vaillants don't seem to take too kindly to cycling not helped by their need to fire at ~70%
output for up to 60 secs on ignition irrespective of your range rating.
You might just check a few parameters on your boiler, even though all probably set to default values.
d.001 pump overrun time.
d.002 anticycle time
d.014 pump speed.

 

[dmitripopov1984]

Thank you Ric2013 for your response.

the system SHOULD allow your boiler to run in condensing mode most of the time

But this is where the problem is. The dT of flow and return (at the boiler side) are only around 10C. This is dictated by the dT of the Flow and Return of the zones, which should be around 8C, according to the UFH supplier. According to the boiler manual the "condensing" starts when the set T is around 75C and return is much less (dT should be minimum of 20C?).

Why, incidentally, run the upstairs in grooved chipboard

This is how it was specified by the supplier.
I am attaching some screenshots of their drawing, to ease the understanding.

 

[dmitripopov1984]

You really IMO shouldn't need a buffer with a modulating boiler even if at times it must cycle when its minimum output (~ 7kw) is greater than the UFH demand.
Unfortunately, the Vaillants don't seem to take too kindly to cycling not helped by their need to fire at ~70%
output for up to 60 secs on ignition irrespective of your range rating.
You might just check a few parameters on your boiler, even though all probably set to default values.
d.001 pump overrun time.
d.002 anticycle time
d.014 pump speed.

Hi John.g
Thank you for your message.
Here are the parameters:
d.001 5 minutes
d.002 20 minutes
d.014 speed target AUTO

 

[Ric2013]

Thank you Ric2013 for your response.


But this is where the problem is. The dT of flow and return (at the boiler side) are only around 10C. This is dictated by the dT of the Flow and Return of the zones, which should be around 8C, according to the UFH supplier. According to the boiler manual the "condensing" starts when the set T is around 75C and return is much less (dT should be minimum of 20C?).

I wish installers would bother to explain things to their customers! The mixing valve on the manifold will start by taking full hot water and returning the cold to the boiler. As the set temperature, in your case 50°C (which I've said seems high, but that's for you to check), is exceeded, the mixing valve will mix water from the return from the zones and the flow from the boiler to keep the flow to the zones at 50. The water taken from the boiler flow should be replaced by an equal quantity of UFH zone return water (which, as you say has been set to is 8° below the flow, but bear in mind if you set a dT at a 50°C flow, the dT will drop as the flow drops due to decreased output at lower temperatures). So the return to the boiler will be around 42°C when the flow into the UFH zones is 50°C.

Obviously, when the UFH is running at a lower temperature (at start-up), then there won't be the same Dt across the zones, and thus across the boiler. The dT across the boiler may (or may not) be important for the boiler to be able to regulate itself, but doesn't matter from a condensing point of view. The actual condensing process occurs when the heat exchanger is below 55°C. This is when condensing starts - when the return is at 54 or below and regardless of Dt. But if the return were at 42, that would be even better. Ideally, if the boiler can still heat your DHW properly, and is designed to be run this way, I'd be wanting the boiler to run at 50F/42R. You'd then have the entire heat exchanger dripping with condensation and you'd be extracting the most heat possible from your gas. The 20 degree 'rule' was to allow boilers to run 70/50 when connected to radiator systems - you may be able to go better than that.

This is how it was specified by the supplier.
I am attaching some screenshots of their drawing, to ease the understanding.

Fair enough then. Spreader plates aren't massively effective. I suppose they increase the heat output per sq m a bit, but not sure by how much.

 

[John.g]

Hi John.g
Thank you for your message.
Here are the parameters:
d.001 5 minutes
d.002 20 minutes
d.014 speed target AUTO

They look like the default settings.
d.002 setting of 20 minutes results in a actual anti cycle time of 6 minutes based on a your boiler target temperature of 60C (look up table).
The pump auto setting looking at the boiler/pump characteristic seem to indicate that the minimum flowrate is 500LPH or 8.3LPM at 7kw output which means a deltaT of 12C and ~ 1300LPH or 22LPM at 38kw giving a deltaT of 25C. This will obviously only apply to a UFH (or rad) system that is directly fed from the boiler or where the boiler flow temp and the UFH mixed flow temp are equal which they arn't.
You say that the boiler deltaT is only 10C, by my calcs it should be ~ 19C and give a very economical return temp of 41C (bags of condensing) and if it actually is 10c then the boiler internal (or external)by pass, or is operating, but a boiler return of 50C still is quite OK.
The reason that the boiler isn't able to get away on the first refiring after a anti cycle time of 6 minutes may be because the boiler flow rate is too low initially but will then increase as the ufh temperatures fall causing the boiler & ufh flows to increase.
Changing the pump mode to setting 3 or 4 may help but at the expense of a lower deltaT.

 

[John.g]

Should also ask what is the total of the manifold flow tube rates, if the deltaT is known then quite easy to calculate the heat requirement.

 

[John.g]

Just read through your first post again.

You say that with the boiler output range rated to 7kw that the flow temperature does not exceed 26C,
Is the boiler running continuously during this period or cycling and is the boiler flow temp 60C.

At 10kw range rating, 50C still cannot be achieved, again is the boiler running continuously during this period or cycling and is the boiler flow temperature 60C.

Your energy consumption is ~ 100kwh for a 10 hour heating day which would indicate that the average boiler output is 10kw and is running continuously?

You also say the system volume is ~ 35 litres, this would indicate that the 816 meters of pipework is 12MM.

So, what exactly is the problem as the 100kwh would indicate that the UFH is giving this output and if this seems excessive then that's a different problem? If the boiler is running continuously then the return temp is between 42C and 50C which is quite acceptable.

 

[dmitripopov1984]

Hello,
Many thanks for everyone's replies and inputs in this post.
I would like to make it a little more clearer as it seems that the "intro" does not reveal the actual question.
First of all, I do not complain regarding the heat from the system - it works great.
It works great, but at very high expense. This is where my problem is.
When we moved in into the property (January 20) we were running central heating for 24/7, with radiators being installed in every room. The gas consumption was nowhere near as high (referring to the meter readings, not the rates) as it is now with UFH which was installed during the summer last year. And the house was nowhere near insulated as it is now.
So I am looking for the solution to reduce the gas consumption.
I know for instance that the Air/Water Heat source pumps are using storage tank to contain the heated water.
So why there is no option for the buffer tank with Combi system???
I think that the problem is, like Sparkgap mentioned, that there is not enough volume of water in the system.
I think by having the tank installed the running time of boiler will be reduced drastically. What I mean is that currently boiler heats up very small amount of water which cycles between boiler and mixing manifold (approx 30 meters of combined length of 27mm copper) plus a volume of manifold. And it burns gas all this time, keeping it heated.
But if to put, say, 100 litres tank between the system and set the tank temperature to, say, 75C, and ask boiler to reheat it once temperature of the tank will drop below, say, 50C. With the mixing valve set to 50C there would be "reserved" temperature of 25C in the whole volume of the tank. How long would it take to get it cooled down to the 50C? 30 minutes? 1 Hour? I do not know how to calculate this, so only guessing. But anyway, the boiler would kick in maybe once or twice per hour for smaller period of time?
Please let me know your thoughts.

 

[John.g]

This my view on it.
Assume a 100 litre buffer tank that the boiler heats from 50C to 75C and the cuts out until the temperature falls to 50C again. Also assume that the boiler is range rated to 30kw, (near its max output).
The heating demand is 10kwh/hr or 100kwh/10hr day.
In falling from 75C to 50c the buffer will give up, 100*25/860, 2.91kwh which will last for 2.91/10*60, 17.46 minutes, the boiler will then fire at 30kw, 10kw will be required to supply the 10kw heating load so the other 20kw will heat 100 litres from 50C to 75c in 2.91/20*60, 8.73 minutes and then cut out. So the boiler will fire (at 30kw) for 8.73 minutes in every 26.19 minutes or a firing cycle of 8.73/26.19*100, 33.3%. or 30*33.3%,10kw, exactly the same as firing the boiler continuously at 10kw and not as economical due to cycling and the (admittedly small) cylinder losses. So in one case the boiler is firing 100% of the time and in the (buffer) case only 33% of the time but no advantage whatsoever.
The only need or advantage of a buffer is to avoid more rapid cycling if the heat demand is substantially lower than the minimum output of the boiler.