UFH economics

[jayc89]

I was rebalancing our UFH system yesterday after adding a couple of new loops, so they're ready for when the weather turns. It was running for a couple of hours as I was waiting for it to reach temp etc.

 

We have 7x loops across approx 70 sqm (150mm centres), all are sitting on 100mm PIR, with 25mm perimeter strips and are connected as a single zone. The manifold is mixing to 45c by a gas system boiler with a flow temp to 55c (for hot water) and return was around 35c during this time (so certainly within the condensing range!). Loops are flowing at between 2 and 2.5 l/m in order to reach a 10c delta. All flooring is tiles and they peaked at 28c. I don't really want to go much higher than that. 

 

According to OVO, I was using approx. 18 kWh whilst the slab was reaching temp.

 

That seems quite excessive to me. Am I missing something or is that just a symptom of our current heat loss? Internal Wall Insulation is next on my todo list, but the weekends seem to be getting away from me this year now!

[JohnMo]

You are trying to heat a big chuck of concrete/screed, if it's 70m2 you could be 8 to 10 Tonnes.  So getting it to temperature takes a fair bit of energy, after that you are managing heat input to heat losses.

 

Would strongly suspect you are running it too hot.  The heat output of the floor is governed by the mean flow temp.  So the bigger the delta T the higher the flow temp needs to be to the same output v a low delta T but the higher the flow rate.

 

Have attached a sheet to help you calculate the mean flow temperature you need.  Ignore the lines already on there.

 

You will need to know the floor area and heat load required.  Then from that calculate the W/m2.

 

 

Have a play.

 

I tried running my floor all day at 30 deg flow temp, massive overheat of the house and used tonnes of gas.

[jayc89]

 

33 minutes ago, JohnMo said:

You are trying to heat a big chuck of concrete/screed, if it's 70m2 you could be 8 to 10 Tonnes.  So getting it to temperature takes a fair bit of energy, after that you are managing heat input to heat losses.

 

Would strongly suspect you are running it too hot.  The heat output of the floor is governed by the mean flow temp.  So the bigger the delta T the higher the flow temp needs to be to the same output v a low delta T but the higher the flow rate.

 

Have attached a sheet to help you calculate the mean flow temperature you need.  Ignore the lines already on there.

 

You will need to know the floor area and heat load required.  Then from that calculate the W/m2.

 

 

Have a play.

 

I tried running my floor all day at 30 deg flow temp, massive overheat of the house and used tonnes of gas.

 

Great, thanks for those. Interestingly, those don't seem to match with what the UFH company sent me when purchasing my kit;

 

 

The City Plumbing Heat Loss Calc suggests I need, on average 100W/m2. Which is works out at approx 7 kWh. Perhaps I need to leave it running for longer to see what the energy usage looks like once the slabs at heat for a period of time? I could live with 7 kWh until I get around to fitting the IWI!

[JohnMo]

Its a pretty crappy calculations tool, it says my house needs 15kW, in reality it needs 3kW - on the coldest day.

 

The output of the tool isn't kWh it's kW, so you will use 7x24hrs per day if the calculation tool is to be believed, to heat a 70m2 area. So 168kWh per day.  In gas terms after Oct that's £17 per day, plus the other house heating.

 

 

 

 

[Radian]

2 hours ago, jayc89 said:

a gas system boiler with a flow temp to 55c (for hot water)

This caught my eye. What controls your hot water temperature? Typical cylinders need a reasonable degree of headroom between the HW setpoint and boiler setpoint to  transfer the heat from the boiler flow in the coil to the water. For example, if you set your cylinder stat to 55^oC and your boiler flow to 55^oC then the boiler would probably cycle on and off for the entire time HW was programmed to receive heat. If you set HW to 50^oC it may still make the boiler cycle on and off for an excessive amount of time.

[jayc89]

2 hours ago, JohnMo said:

Its a pretty crappy calculations tool, it says my house needs 15kW, in reality it needs 3kW - on the coldest day.

 

The output of the tool isn't kWh it's kW, so you will use 7x24hrs per day if the calculation tool is to be believed, to heat a 70m2 area. So 168kWh per day.  In gas terms after Oct that's £17 per day, plus the other house heating.

 

 

 

 

 

Interestingly, I've just had a fiddle with Jeremy Harris' heat loss calc, I included upstairs (radiators) and downstairs (UFH) and it's coming out at approx 17.5 kW (20c delta T). Perhaps that's more accurate and would be a better representation of the impact of installing given insulation. 

 

2 hours ago, Radian said:

This caught my eye. What controls your hot water temperature? Typical cylinders need a reasonable degree of headroom between the HW setpoint and boiler setpoint to  transfer the heat from the boiler flow in the coil to the water. For example, if you set your cylinder stat to 55^oC and your boiler flow to 55^oC then the boiler would probably cycle on and off for the entire time HW was programmed to receive heat. If you set HW to 50^oC it may still make the boiler cycle on and off for an excessive amount of time.

 

Boiler is set at 55, DWH is heating to 50. I've not checked for cycling when only the DWH is calling for heat, I'll add that to my list. 

[JohnMo]

Jeremy spreadsheet is pretty good.  Would trust it over some generic tool on the internet.