Max UFH Temp = Lukewarm Rads?

[TheMiddle36]

Hi,

Not long in a new build house and getting to grips with the heating and everything else.

We've a Nibe F4020 heat pump, UFH for downstairs heating and rads/towel rails for upstairs.
The upstairs is on a single zone with a thermostat on the landing (makes no sense to me, but whatever.)

The heating curve on the Nibe tops out at 45degs, which I guess is the maximum for the UFH.

The rads are all warm, but can't say they're hot... Good pressure, and don't need bleeding. Does this mean the max temp of the water to the rads is 45, and is that sufficient?

Cheers

 

[Ben-gee]

I know very little about them but their bumf says temperature up to 58 deg c.
The 45 will be nothing to do with the under floor as this can happily operate with a flow at 80 deg c - because underfloor uses a blending calve to bring the temperature down to what it needs.
( I have under floor operating at 35deg c using a gas boiler with flow at 75).
Presumably the rads have been sized (larger than for traditional system) to allow for a lower temperature?

 

[TheMiddle36]

I know very little about them but their bumf says temperature up to 58 deg c.
The 45 will be nothing to do with the under floor as this can happily operate with a flow at 80 deg c - because underfloor uses a blending calve to bring the temperature down to what it needs.
( I have under floor operating at 35deg c using a gas boiler with flow at 75).
Presumably the rads have been sized (larger than for traditional system) to allow for a lower temperature?

Hmm, Ok.

I presumed the heat pump max flow had been set to 45 to protect the UFH. There’s a minimum flow temp setting but no maximum so it just have been set by the installer. I’ve seen screen grabs of other systems with same control unit that have a higher max temp.

Not sure on the rads, they’re not massive. I look in to how a rad should be sized for rooms

 

[Timmy D]

You can manually adjust the curves, the menus are a pita to navigate.

 

[TheMiddle36]

You can manually adjust the curves, the menus are a pita to navigate.

Yeah, but it tops out at 45?

I can adjust the curve so the temp is 45 inside along the scale for outside temps, but no higher than 45?

 

[John.g]

Heat Pumps are at their most efficient when their hot water output is its lowest and are best suited to UFH. A heat pump working from a ambient temp of 7C with a water outlet temp of 35C will have a COP of ~ 4 (4kw out for every 1 kw in) and between 0C and 45C a COP of ~ 2.5 (2.5kw out for every 1 kw in) a gas/oil boiler has a COP of 0.85 (0.85kw out for every 1 kw in).
A radiator with flow/return temps of 45/40C needs to be oversized by a factor of ~ 2.8 and by a factor of 6 with flow/return temps of 35/30C.

 

[TheMiddle36]

Does this help?

Calc flow temp has been at 46 for a few days now... Am trying to increase the room temps because the mvhr is blowing cold air back in (at 14-15 degrees) and one theory is the extracted air isn’t warm enough once it’s been through the heat exchanger, kind of a catch22.

When you say it’s a factor of 6 or 2.5... A factor of what? I can measure the rads tomorrow and find the room sizes if that would help?

 

[John.g]

Rad rated output is based on a "50 deg" basis which is the mean rad temperature - the required room temp (generally taken as 20C), a rad with flow temp of 75C and return of 65C with a ambient of 20C is a 50 deg rad, (75+65)/2 - 20. A rad with flow/return/room temps of 45/40/20c is a 22.5 deg rad with a output of (22.5/50)^1.3 or 35.4% of a 50deg rad so requires oversizing of 100/35.4 or 2.8.
A rad with flow/return/room temps of 35/30/20c is a 12.5 deg rad with a output of (12.5/50)^1.3 or 16.5% of a 50deg rad so requires oversizing of 100/16.5 or 6. (Fan coil heaters (upstairs) probably a better choice if running at 35C)

 

[Timmy D]

Rad rated output is based on a "50 deg" basis which is the mean rad temperature - the required room temp (generally taken as 20C), a rad with flow temp of 75C and return of 65C with a ambient of 20C is a 50 deg rad, (75+65)/2 - 20. A rad with flow/return/room temps of 45/40/20c is a 22.5 deg rad with a output of (22.5/50)^1.3 or 35.4% of a 50deg rad so requires oversizing of 100/35.4 or 2.8.
A rad with flow/return/room temps of 35/30/20c is a 12.5 deg rad with a output of (12.5/50)^1.3 or 16.5% of a 50deg rad so requires oversizing of 100/16.5 or 6. (Fan coil heaters (upstairs) probably a better choice if running at 35C)

I must say John, I do enjoy reading your technical posts.

Thanks

 

[king of pipes]

You may well live in a passive house these are designed and built with a very high insulation value and need very little heating to warm them often 8- 12 KW , coupled with a air heat recovery unit which should take warmed air from the areas of the home that produce excess heat and use that to warm the incoming fresh air which is then returned to the home through ceiling vents , flow and return temperatures are kept low it's quite normal to be 45° you are only trying to achieve 21° internally, on ufh using air source you do not normally fit a blending valve and 45° is enough, you must change your way of thinking radiators will not feel piping hot like a gas boiler heating system they are often over sized 50% to achieve enough heat at a lower temperature to heat the room, the problems your experiencing are often caused by not commissioning the system to achieve a warm comfortable living space . Kop

 

[TheMiddle36]

That's good information.

However if the room temps where the MVHR are extracting from, bathrooms, en-suites etc aren't warming because of the low temps in towel rails, that's only feeding cool air to the MVHR which is then delivering cold (15deg) air to the rest of the house!

I think the MVHR is working well, so am suspecting the temperatures from the heating system to be suboptimal.

I've since found that several of the UFH need flushing, see pic, and one of the circuits to the d/s bathroom doesn't appear to be working at all which will further lower the MHVR extract temp, and therefore the cold air supplied around the rest of the house.

I have the NIBE app connecting and that charts some data on flow temp, return temp and calculated flow. Would any of those be useful to monitor? Thanks for your on-going help with this

 

[John.g]

HP installations are best run 24/7, also ufh/rads are a poor mix in general but continuous running helps to maintain room temperatures at comfortable levels.
The other challenge for HP is HW production as the compressors have to be run at very high temperatures to give the required temperatures which leads to high running costs and shortened compressor life. There seems to be a number of approaches to this, one is to run at a constant discharge temp of ~45C and boost the HW cylinder temperature once/week to 60/65C for legionella protection with a electric heating element, some systems use a separate HP system for HW production.
Still some years away IMO but HP will be the only show in town but at the present difference in gas/electricity prices (especially in the UK) a HP will have to achieve a COP of ~ 4? to break even with gas boilers.

 

[TheMiddle36]

HP installations are best run 24/7, also ufh/rads are a poor mix in general but continuous running helps to maintain room temperatures at comfortable levels.
The other challenge for HP is HW production as the compressors have to be run at very high temperatures to give the required temperatures which leads to high running costs and shortened compressor life. There seems to be a number of approaches to this, one is to run at a constant discharge temp of ~45C and boost the HW cylinder temperature once/week to 60/65C for legionella protection with a electric heating element, some systems use a separate HP system for HW production.
Still some years away IMO but HP will be the only show in town but at the present difference in gas/electricity prices (especially in the UK) a HP will have to achieve a COP of ~ 4? to break even with gas boilers.

Thanks John,

UFH downstairs, rads upstairs here.
Haven't noticed a problem with hot water production, and have seen the setting to randomly boost the temps for legionella protection so happy that bit is working.

The nibe kit looks great, I'm sure I just need to find the right settings to get the most out of it! There's a lot of numbers reported by the app and I don't understand most of them!

 

[John.g]

That's good information.

However if the room temps where the MVHR are extracting from, bathrooms, en-suites etc aren't warming because of the low temps in towel rails, that's only feeding cool air to the MVHR which is then delivering cold (15deg) air to the rest of the house!

I think the MVHR is working well, so am suspecting the temperatures from the heating system to be suboptimal.

I've since found that several of the UFH need flushing, see pic, and one of the circuits to the d/s bathroom doesn't appear to be working at all which will further lower the MHVR extract temp, and therefore the cold air supplied around the rest of the house.

I have the NIBE app connecting and that charts some data on flow temp, return temp and calculated flow. Would any of those be useful to monitor? Thanks for your on-going help with this

I would think that the MVHR plays a huge part in heating requirements, if not balanced properly. How are the individual air change requirements met and distributed for different rooms I wonder and if there are any air flow measurements taken/displayed anywhere.

 

[TheMiddle36]

I would think that the MVHR plays a huge part in heating requirements, if not balanced properly. How are the individual air change requirements met and distributed for different rooms I wonder and if there are any air flow measurements taken/displayed anywhere.

I have temps and humidity reported by the Zehnder app, and 223m3/h reported when the MVHR is operating on level 2 of 3, but nothing at a room level

 

[John.g]

Its a long time since I did any air volume/temp calcs but off the top of my head it might only take something like 0.8 kwh to heat that vol of air by 10C, not alot.