UFH running costs

[Jamieh1987]

Hi,

A bit long winded but just looking for some advice. We’ve just moved into a house that we have renovated and just getting used to using UFH and trial and erroring boiler flow temps as well as adjusting the manifold temps and it just seems to be working out a lot more expensive to run the UFH side of the system than the CH side that feeds upstairs.

Is this normal? If you see attached pics, (12-2pm) with boiler flow set to 55, ufh manifold temp set to 45, we always seem to consume roughly 44kw in a 2 hour period, is this normal? (it’s 7 loops all roughly 80m long). Compare this to central heating side (5 oversized rads and a towel rad) uses roughly 30kw over a 7 hour period. Once it gets to temperature it does hold there for hours which isn’t a bad thing.  I’ve read about leaving it running all the time at say 19 degrees but the problem I’m having is it costs the initial heat up (it’s a screed floor) to start budging the thermostat and by the time it kicks back in, the floor slab is cold so you’re having to do the initial heat up all over again.

 

Boiler is a baxi 830, 2 zones, UFH and CH, Floor build up is 150mm celotex with a 40-50mm liquid screed over it. Manifold pump is set to 2 and I have the salus auto balancing actuators.

 

If that seems like normal operating costs then so be it but just looking for some reassurance that I’m not running it inefficiently. I’ve read about buffer tanks, short cycling etc..

 

Any ideas or advice appreciated

[JohnMo]

UFH running is down to how much insulation you have below the UFH pipes and how you operate. So 150mm is good so not down to that.

 

Heating 10s of tonnes of concrete takes quite a bit of energy, so there are two fundamentally different ways to operate the heating regime.  Throw a load of heat at it over 4 to 8 hour period, just like a storage heater, then let is trickle out.  This is pretty much what you are doing.  So it uses a bunch of kWh in a short period. Doing this wrong you can get thermostat overshoot, which is waste full and will use more energy.

 

The other way is a gentle flow of heat into the floor, so heat in, matches heat out, the heating period is pretty much 24/7, this entails using a lower flow temp. If coupled with weather compensation this operation can be automated.  Otherwise, it can be done with a little bit of fiddling manually with the UFH mixer.  As a comparison our flow temps at -5 are about 30 degs - yours 45 degrees, ours is running 24/7.

 

Either way will use a similar amount of kWh other a couple of days.

 

If you can reduce boiler flow temp, there are small gains to be had, as you will be condensing more.

[Jamieh1987]

I did try turning the UFH mixer down to minimum (30) for a few hours but realise I wasn’t patient enough with it. After about 5 hours the floor didn’t feel like it had been on and I’d still clocked up about £7 in gas so I reverted back to 45.

I’m a bit governed with the boiler flow because it still has to feed the rads upstairs so I think 45 is probably my limit, especially with these cold snaps but I’ll reduce UFH manifold to 30 and just leave all stats on 19 for a couple of days and check the usage again. I really need to stop doing my hourly check of the smart meter 😂.

 

And yeah it’s definitely not because of lack of insulation, this is a typical day even when it was -4 outside. It boots in 3-5 and then usually sees us through until 3pm the next day. We’re up and out in a morning and only home 5pm’ish so it works for us. It’s south facing with lots of glass which is why we get a spike in temperature.

 

Thanks for the reply 👍

[nod]

We’ve found our UFH very cheap to run (Gas boiler ) My wife is dreading moving to an ASHP on the next one 

I’d never go back to radiators 

 

[JohnMo]

Think you may be missing the point of UFH, it doesn't need to feel warm.  A radiator feel is hot, because its small surface area needs to be hot to transfer heat to the room.  UFH area is the size of the room, to transfer the same amount of heat to the room it can be cool in comparison.  In some cases almost the same temp as the air temp at the end of a heating cycle.

 

Your bedroom on radiators benefit from heat transfer from the downstairs as well as getting heat from the radiators.  The upstairs heating is being supplemented by the downstairs heat.  

 

Your downstairs is using 40kWh not for 2 hours heating, but really for 24 hrs, because its only on for those hours.  You are really putting an average of 1.6kWh in to the floor when averaged out.  Your upstairs has an average of 1.25kWh.  So the same heat as a small electric convection heater for up and downstairs combined.

 

Don't think that bad, looks quite good to me, its not a spring day, its been well cold outside. As I said two way to operate, long and slow or blast with heat, your doing the latter and as you have to run your boiler at the temp you do, its working ok for you.

[Jamieh1987]

I understand what you’re saying. I suppose it’s just taking some getting used to a different type of heating than we’ve used before and I’ll be happier with it when I’ve stopped constantly checking/tinkering with it.

 

Im gonna run the manifold at 30 for the next 48 hours and leave all the stats on 19 and see if we can get a more stable temperature throughout the day, check the usage and go from there. I think that would benefit us most at weekends when we tend to be home all day.

 

Thanks again for your input 👍

[MikeSharp01]

11 hours ago, Jamieh1987 said:

 

 

Thanks for the reply 👍

It is interesting looking at your trend. The room seems to heat up without the heating running between around 5 and 9:30 in a small way and 11:00 and 12:00 in quite a significant way. I wonder why that is - occupancy perhaps? If so then your building is doing pretty well as I see it? Do you also have MVHR?

[ProDave]

11 hours ago, Jamieh1987 said:

 

If I am reading that chart properly, you only put heat into the room for a short period and then after the heating is off the room temperature carries on rising and over shoots the set point.

 

This means the UFH water temperature is too high.  Reduce the temperature on the manifold mixing valve, it will heat with the floor at a lower temperature over a longer period and hopefully avoid the overshoot.

[MikeSharp01]

35 minutes ago, ProDave said:

short period and then after the heating is off the room temperature carries on rising and over shoots the set point

Yes but this could be a combination or one or other of the heating or some other heating factor such as occupancy. I still think it is curious that the room temperature rises when no heating is applied, in the period before heating is applied and if the same effect is occurring after heating is off then the overshoot may be for other reasons. Having said that the rate of change in the 11:00-12:00 period is higher (approx 2.6 degrees in little over 1 hour) than that in the 16:30 - 21:00 slot (2 degrees over 4 hours) which may indicate, or at least allows for, a combination effect although the somewhat odd time base of the chart with 3 and 4 hours between ticks maybe distorting things.

[Barryscotland]

We are having the same issues and seems changing flow rates, boiler temp, blending valve temp makes no difference to how much gas we use or the spike in room temperature. I had a stat in each room but have now put the beds all on one, the kitchen, living, dining on one an bathrooms on another and just leave them set at 18 degrees. When each room was on its own stat it seemed like the boiler was constantly running. Boiler cuts out at 18 but some rooms still spike at 21, house temp is constantly fluctuating would be nice to get it to just stay at a constant temp. We are using less gas (still £170 a month) now we are just on the 3 stats and leaving them on a set temp instead of playing with them.

[Jamieh1987]

1 hour ago, MikeSharp01 said:

It is interesting looking at your trend. The room seems to heat up without the heating running between around 5 and 9:30 in a small way and 11:00 and 12:00 in quite a significant way. I wonder why that is - occupancy perhaps? If so then your building is doing pretty well as I see it? Do you also have MVHR?

I can’t explain the small jump in temperature in the early hours on that day and it isn’t a common theme when I’ve checked other days. We do get the spike around dinner every day from the glazing at the rear. In mid October with outdoor temperatures around 14 degrees we were hitting 26 inside and very rarely dropped below 21. We don’t have an MVHR system.

[Jamieh1987]

1 hour ago, ProDave said:

If I am reading that chart properly, you only put heat into the room for a short period and then after the heating is off the room temperature carries on rising and over shoots the set point.

 

This means the UFH water temperature is too high.  Reduce the temperature on the manifold mixing valve, it will heat with the floor at a lower temperature over a longer period and hopefully avoid the overshoot.

The problem I’m having is I’ve adjusted the boiler flow from 35-65, adjusted the manifold temp from minimum up to 55. Tried all sorts of combinations and they all seem to consume roughly the same amount of gas in £ give or take a few pence.
 

It just seems like no matter what temps I set the boiler is firing at more or less 22kw for the each hour it’s on (but only with UFH), it’s a brand new Baxi 830.

I’ve been running it at the higher temp for 2-3 hours to get the overshoot and that sees us through until the next day which I understand is not the best way to run it. Seems like if I lower the temp, run it for 2-3 hours, it costs me the same but is considerably cooler. I’m just looking for a more consistent temperature 24/7, more so for weekend.

I’ve reduced the mixing valve to minimum and just set all thermostats to 19 degrees so I’ll see how that goes for the next 48 hours, more importantly I’ve put the smart meter in the cupboard 😂.

Edited December 18, 2022 by Jamieh1987