UFH BUFFER TANK SELECTION (no rads)

[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.

 

[dmitripopov1984]

This my view on it.

Thank you John.g for you response.
When we mention 10 kWh of heating demand, I understand that this is the ALL zones are taken in the account.
Most of the times only the upstairs and the bathroom are on, occasionally the kitchen.
If to assume that the drawing calculations are about correct, then for the bedrooms there is approx 2kWh of heat being demanded.
Could you please advise, if the boiler output is set to 10kWh, but the demand is only 2kWh, doesn't this confuse the boiler and throws it into cycle?
And if that is the case how could we overcome this?

 

[dmitripopov1984]

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

I thought that the setup would be different there? Boiler will heat the tank and then manifold pump will distribute the water between tank and manifold. And like I've mentioned in my previous reply, most of the time there is around 2 - 3 kWh being demanded. Wouldn't this help with situation?

 

[John.g]

Thank you John.g for you response.
When we mention 10 kWh of heating demand, I understand that this is the ALL zones are taken in the account.
Most of the times only the upstairs and the bathroom are on, occasionally the kitchen.
If to assume that the drawing calculations are about correct, then for the bedrooms there is approx 2kWh of heat being demanded.
Could you please advise, if the boiler output is set to 10kWh, but the demand is only 2kWh, doesn't this confuse the boiler and throws it into cycle?
And if that is the case how could we overcome this?

A 2kw load with the boiler range rated to 10kw gives a 20% firing cycle but sadly the Vaillant is virtually unable to deal with this without very long anticycle times so a a 100 litre buffer in this case would be great.
Wont go through the dreary calcs again but the buffer will supply 2kw for 87.4 minutes and the boiler will then fire (at 10kw) for 21.8 minutes giving a total cycle time of 109.1 minutes (20% firing cycle).

 

[John.g]

I thought that the setup would be different there? Boiler will heat the tank and then manifold pump will distribute the water between tank and manifold. And like I've mentioned in my previous reply, most of the time there is around 2 - 3 kWh being demanded. Wouldn't this help with situation?

Don't know if this is the info you're looking for or not.

The buffer will always supply the UFH heat demand whether the boiler is firing or not, when it does fire to recharge the buffer, the buffer is still supplying the UFH heat demand so the boiler must both recharge the buffer and at the same time supply the constant heat "loss" from the buffer. If the UFH heat demand is 2kw and the boiler output is limited to 10kw then there is 8kw net (10-2) available to recharge the buffer.
If the UFH heat demand suddenly increased to 10kw then the buffer would never increase in temperature.

 

[Chuck]

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.

The average temperature (in London) in January 2020 was 7°C and for October 2021 it was 13°C. So, other things being equal, I'd expect the heating element of your gas bill for Jan 2020 to be roughly double what is was in Oct 2021 assuming you like your living quarters to be at an average temperature of around 20°C.

Now, if you are saying that you've burned through more gas in Oct 2021 than in Jan 2020 I doubt that short cycling and/or tweaking flow temperatures are going to provide an answer. If you dump a certain number of kW hours per day inside the thermal envelope of a house you'll get roughly the same average temperature regardless of the short-term (hour to hour) time variation due to different choices of programming for the boiler and thermostats.

Whenever I encounter anomalously high fuel bills, the first thing I consider is the possibility a leaky hot water system. You can lose hundreds of kW hours a day without even noticing it by allowing hot water to run down the drain. High-flow power showers can also use a remarkably large amount of gas and their usage patterns need to be considered.

Next on my list is ground floor UFH. The layer(s) the coils sit on needs to be a really good insulator or a lot of the heat diffuses downwards into the ground. If I've understood the description above, there seems to be a thick layer of wooden flooring above the UFH coils with the result that they need to be operated with a flow temperature of 50°C, which seems very high to me and suggests that a lot of heat must be being lost downwards.

So, perhaps @dmitripopov1984 could summarise the key information about the ground floor UFH construction and operating temperatures? I'd like to understand how much heat the coils are absorbing and where this heat is ending up.