UFH Advise Needed

[Barryscotland]

 

Not really used our heating up till last month and we have managed to use £200 of gas in 1 month and that is with only occassional use and the stats set to 16 degrees when turned on. It seems to take around 2 hours of constant boiler running before the temp starts to lift from 14 degrees then another 2 - 3 hours to get to 16 degrees. Boiler then cuts of and temperature continues to rise to a minimum of 19 degrees which is a fine temp for us. Heating is then switched of untill it gets proper cold in the house a few days later and again switched on till it gets to 16 degrees. 20kw alpha gas boiler 55mm of cemfloor screed with 120mm insulation under it. Is it normal to take so long to heat up? Am I cheaper in the long run to have the boiler running more regularly rather than the slab cooling right down? Boiler is set to 65 degrees, flow at the mixer on manifold is 45 degrees, pump is on number 3 and flow rates are all at about 2.5l a minute, is this as it should be?

 

Its a new build and achieved a low A EPC rating so thought it would be way cheaper and easier to heat than our old stone cottage

 

Looking to install a stove as we have free wood but not going to happen for a few  months yet due to a design cockup

[HughF]

Please post the kWh consumption from your gas meter for the same period, and/or advise on your unit rate.

 

£50/week is £7.14 a day - that's not a lot in 2022

[JohnMo]

I also had quite a few issues when I started using the UFH.

 

Ours is 193m of UFH in 100mm concrete, which I think equates to 56 tonnes.  So heat up times are long.  Our first heat of the season uses loads of energy.  You are doing the same every couple of days.  Couple issues I see

 

1.Your flow temp in the floor is too high if you are overshooting that far.  I have the temp set a 30 and batch charge each night, for a out 6 to 8 hours depending on outside temp, have a single thermostat set at 18.5, which in reality switches the heating of at 18.75 deg.  The heat then settles at 19 an hour or so later. Once charged up the floor keeps the house stable for the next 16-18 hours, the process then repeats.

 

2.  Can you reduce the firing temp of the boiler, it doesn't need to be that high, unless it that high for the cylinder?

 

3. Are you sure you are not short cycling - boiler running on off for short periods of 10min or less?

 

4. Do you have a buffer?

Edited December 6, 2022 by JohnMo
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[Barryscotland]

1 hour ago, HughF said:

Please post the kWh consumption from your gas meter for the same period, and/or advise on your unit rate.

 

£50/week is £7.14 a day - that's not a lot in 2022

 

Not alot if my house was warm but at least 60% of the 4 week period my house was below 16 degrees. Don't no the kwh or unit rate but my bill after submitting a reading was £202.54 for 4 weeks

[Barryscotland]

1 hour ago, JohnMo said:

 

 

1.Your flow temp in the floor is too high if you are overshooting that far.  I have the temp set a 30 and batch charge each night, for a out 6 to 8 hours depending on outside temp, have a single thermostat set at 18.5, which in reality switches the heating of at 18.75 deg.  The heat then settles at 19 an hour or so later. Once charged up the floor keeps the house stable for the next 16-18 hours, the process then repeats.

So turn the temp on the mixer at the manifold down to what temp? Would that not just take longer to heat up and the boiler run longer?

1 hour ago, JohnMo said:

 

2.  Can you reduce the firing temp of the boiler, it doesn't need to be that high, unless it that high for the cylinder?

 

It also runs a half hour in morning and hour at night to heat a pressurised cylinder

 

3. Are you sure you are not short cycling - boiler running on off for short periods of 10min or less?

 

No Definately not, I reckon its running full pelt from the steam and noise out of flue

 

4. Do you have a buffer?

 

Nope

 

[JohnMo]

What boiler do you have?

 

Any idea on what plan it's wired as?

[Radian]

2 hours ago, Barryscotland said:

Boiler then cuts of and temperature continues to rise to a minimum of 19 degrees which is a fine temp for us.

So you can get a comfortable  temperature - but only after an uncomfortable delay. The cost suggests to me that you still have some way to go before you're paying the 'average' price so the heating needs to be coming on earlier and/or not allowed to cool down so much. Have you carried out a heat loss calculation to estimate what your energy demand ought to be?

[Barryscotland]

46 minutes ago, JohnMo said:

What boiler do you have?

 

Any idea on what plan it's wired as?

Alpha tec 20kw system boiler, wired S plan

[Olf]

57 minutes ago, Barryscotland said:

Would that not just take longer to heat up and the boiler run longer

That's the whole point: with large heat capacity of screed, there is a delay between floor surface getting to temperature and air above heating up. I for example have timer set to start heating at 4 am, at 7 it is ok, but peaks midday. Repeated daily. Boiler set to lower flow temperature will be more efficient, and even more so when runnning for longer periods. In other words your £200 will give you more heat than last month.

 

What is your floor finish? You mentioned that the house keeps temperature once heated, so I'm worried that you're blocking the screed from releasing upwards and end up with heating soil below.

 

3 hours ago, Barryscotland said:

Its a new build and achieved a low A EPC rating

Oh, that makes it for a prestige piece of paper to wipe whatever needs to be wiped. May or may not relate to actual thermal performance of the building

[JohnMo]

If you look at your manual for the boiler, section 5.12 allows to set the boiler with independent flow temps, for the cylinder heat and CH.  Not sure if your installer did this or just what they normally do.  If on S plan maybe not.

 

If wired per manual you could lower your CH flow temp and the setting on your mixer.  This would lower the output of the boiler to match more closely the mixer temperature.

 

Also noted from the manual is your min turn down for CH is 4.3kW, this may be ok on a cold day - but any other time you may have some boiler short cycling.

 

Low energy houses and normal plumbers don't mix very well, they try to do 'what they always do'

[Barryscotland]

2 hours ago, Radian said:

So you can get a comfortable  temperature - but only after an uncomfortable delay. . Have you carried out a heat loss calculation to estimate what your energy demand ought to be?

I have no doubt that if i let the boiler run longer we could get the house proper toasty, was often into the mid 30s during summer, sun makes a huge difference to the interior temp bit its been lacking this last month or two.

 

No heat loss calcs done nor would I no how too

[Barryscotland]

2 hours ago, Olf said:

 

 

What is your floor finish? You mentioned that the house keeps temperature once heated, so I'm worried that you're blocking the screed from releasing upwards and end up with heating soil below.

 

Oh, that makes it for a prestige piece of paper to wipe whatever needs to be wiped. May or may not relate to actual thermal performance of the building

Mixture of tiles, carpet and wood. All is meant to be designed for UFH with relevant suitable underlays etc 

[Barryscotland]

1 hour ago, JohnMo said:

If you look at your manual for the boiler, section 5.12 allows to set the boiler with independent flow temps, for the cylinder heat and CH.  Not sure if your installer did this or just what they normally do.  If on S plan maybe not.

 

They haven't but maybe because there is two towel rails that have there own port that comes on with the bathroom stats and needs the higher temperature?

 

1 hour ago, JohnMo said:

 

If wired per manual you could lower your CH flow temp and the setting on your mixer.  This would lower the output of the boiler to match more closely the mixer temperature.

 

Also noted from the manual is your min turn down for CH is 4.3kW, this may be ok on a cold day - but any other time you may have some boiler short cycling.

 

Low energy houses and normal plumbers don't mix very well, they try to do 'what they always do'

 

I will lower the temp on the mixer on the UFH manifold and see if it makes any difference, also the return temp was the same as the feed after the boiler was on for a while so does that mean I need to turn the pump speed down so the screed has time to absorb the heat?

 

[JohnMo]

The mixer always moves excess heat back to the return. You could turn the UFH pump down, so it's taking less from the CH circuit, this would mean less hot water is being returned to the boiler.  Just watch the UFH loop flows to make sure your flow is high enough to cope with loop flows.

 

The UFH loop flow rates should really be defined by the loop length, so shouldn't be adjusted except for fine tuning heat put in to a room.

 

Low flow rate on loop, gives a higher differential temp, but this means the 'mean flow temp' is lower, which equates to a lower heat output from the floor.

[Radian]

5 hours ago, Barryscotland said:

No heat loss calcs done nor would I no how too

 

It's actually pretty simple. You just make a big list of all your external surfaces - walls, floors, roofs, windows and doors and work out each of their areas in square meters. Then you find the U-value for each surface area for which you can either use typical values or for more accuracy a calculator such as this one at ubakus.com. You then multiply (each surface area in m²) * (U-value for surface area) * (temperature difference inside to outside) to get the heat loss in Watts for all your external surfaces. Total up all these Wattages and that's how much power you lose through the building fabric for a given temperature difference... in order to maintain this difference, you have to put in this much power.

 

You can then divide this amount of Watts by 1000 to get kW and multiply by 24 to get how many kWh per day your energy use would be for a given inside to outside temperature difference. You can literally verify this by looking at your energy bill.

 

There are other losses due to ventilation but these would be considerably smaller (unless you have big holes in the walls - in which case you should have included those in your list of surfaces 😁)