Should I be concerned that our pending UFH & or ASHP is too powerful?

[Ewan]

We're undertaking a fairly thorough home retrofit on our 1920s 155m2 detached brick narrow cavity house:

 

• Whole house floor out, replaced with insulated concrete 200mm EPS
• New windows & doors all round, u = 1
• Air tightness work on all penetrations, joists, window installs, loft space, eaves & slab
• EWI 150mm EPS
• Attic insulation 400mm fibre
• MVHR

 

so it should be reasonably high performance. I've used a few heat loss calcs can we get 3500w - 4200w (23-27 w/m2) for 20 degrees all round at -2.2, depending on how optimistic I am with the infiltration values. It looks like a Vailant 5kW unit for us (family of 4).

BPC & Robbens have specced an UFH system that puts out about 4kw at 45ºc over a 78m2 heating area downstairs with 150mm pipe centres. Now we'll be aiming to run it lower, 35º or less , but I'm a bit concerned our UFH is too much for the heat loss to the building and the ASHP will cycle a lot, or we'll be too hot!

Then I see most people saying get as much emitter power in there as you can, am I being overcautious and is this level of potential power good for the cold snaps?
 

[ReedRichards]

What about your hot water requirement?

[JohnMo]

15 minutes ago, Ewan said:

but I'm a bit concerned our UFH is too much for the heat loss to the building and the ASHP will cycle a lot

Just turn the flow down, run at mid to high 20s, much better for CoP

[PhilT]

48 minutes ago, Ewan said:

I've used a few heat loss calcs can we get 3500w - 4200w (23-27 w/m2) for 20 degrees all round at -2.2, depending on how optimistic I am with the infiltration values. It looks like a Vailant 5kW unit for us (family of 4).

BPC & Robbens have specced an UFH system that puts out about 4kw at 45ºc over a 78m2 heating area downstairs with 150mm pipe centres. Now we'll be aiming to run it lower, 35º or less , but I'm a bit concerned our UFH is too much for the heat loss to the building and the ASHP will cycle a lot, or we'll be too hot!

Then I see most people saying get as much emitter power in there as you can, am I being overcautious and is this level of potential power good for the cold snaps?
 

Take heed of the recent tales of woe!!! For the sake of a few hundred quid, if you want to be able to maintain that design room temperature, ALWAYS, you will definitely need to go larger. My son has just moved into a new passivhaus with identical floor area to yours and has gone with a 7kW HP which has provided an extremely reassuring bit of headroom during these exceptionally cold last few days (-10degC on a couple of nights). Alternatively you go with the smaller unit to cover 90% of the year and have a few electric radiators for the freak cold days!

Edited December 16, 2022 by PhilT

[PhilT]

Just to follow that up with a bit of hard data, here's my MCS heat loss design calc:-

Indoor average design temp. 19°C

House area 137 sq metres

Peak power/heat loss 8.26kW

At design temperature -2.4°C

And here is how my 11kW HP has been coping on the 15th Dec, operating in WC mode with a degree of manual intervention

Edited December 16, 2022 by PhilT

[JohnMo]

The 5 kW Vaillant does 6.7kW at -3 and a flow temp of 35 and 6.1kW at 45 degrees.  Just make sure the install includes a buffer to keep the heat pump happy on a 10 degree day when heating loads are low.  You also have a good margin available to do hot water on the coldest days. 

 

On full heat demand, will be nicely down its modulation curve, so happy and quiet.

 

The alternative is 3.5kW unit, which is rated at 4.6 and 4.3kW, but that could end up running flat out just doing the heating, and being at full load may make more noise than you want.

 

[PhilT]

8 minutes ago, JohnMo said:

The 5 kW Vaillant does 6.7kW at -3 and a flow temp of 35 and 6.1kW at 45 degrees.  Just make sure the install includes a buffer to keep the heat pump happy on a 10 degree day when heating loads are low.  You also have a good margin available to do hot water on the coldest days. 

 

On full heat demand, will be nicely down its modulation curve, so happy and quiet.

 

The alternative is 3.5kW unit, which is rated at 4.6 and 4.3kW, but that could end up running flat out just doing the heating, and being at full load may make more noise than you want.

 

Absolutely good and fair information on which to base a sound decision - what this forum is all about. I would only emphasize the choice between having the ability to cover temperatures down to -10degC, as I have done here in sunny Aylesbury (unless you are near SteamyTea) and covering -3degC design temp but being prepared to either go cold on rare occasions or have some kind of heating backup plan.

[Ewan]

Thanks all, lots of helpful input there!

I think this will come down to buffer and -10 headroom (plus better DHW reheat power) with 5kW or no buffer 3kW and less headroom (& slower DHW), we don't have a lot of space next to the UVC (210 or 250l) for a buffer that could be 75l plus (is that about right?).

Heat Punk (.co.uk) is saying our UFH rooms need 29w/m2 at -2.2º (Manchester), I can't find any UFH tables that include 35º flow, but at 40º 150mm centre I've seen suggested 55w/m2, so 35º could be ~45w/m2.. Should be fine modulated down a bit for -2.2 and cope with -10, I think, but I'm concerned it will end up cycling too much at higher outside temps.

 

I guess the buffer addresses this, but having watched the Heat Geek vids they sound like something you should avoid unless you really need one (plus we don't have much space), and our installer (who's not done the calcs yet, admittedly)'s first thought was we wouldn't need one, just a volumiser.

 

I don't have all the tools to understand this, Vaillant 5kW can go below 35ºC flow I assume, but what's the lowest it'll do before cycling kicks in? 1/3rd of it's rated output power I've read, but what's that in ºC flow?

How big a buffer might we need? 75l? Things are pretty tight in our utility.
 

Edited December 16, 2022 by Ewan

[HughF]

Don't fit a buffer, it's only there to provide a defrost volume to draw heat from. They sap efficiency and mean you end up with a fixed speed secondary pump.

 

Go direct, much better.

[JohnMo]

26 minutes ago, Ewan said:

Heat Punk (.co.uk) is saying our UFH rooms need 29w/m2 at -2.2º (Manchester), I can't find any UFH tables that include 35º flow, but at 40º 150mm centre I've seen suggested 55w/m2, so 35º could be ~45w/m2.. Should be fine modulated down a bit for -2.2 and cope with -10, I think, but I'm concerned it will end up cycling too much at higher outside temps.

If you use no buffer, make sure you water volume is acceptable for the heat pump manufacturer for warranty purposes, don't zone it to death, one or zones only.

 

Some charts attached

 

[Ewan]

1 hour ago, HughF said:

Don't fit a buffer, it's only there to provide a defrost volume to draw heat from. They sap efficiency and mean you end up with a fixed speed secondary pump.

 

Go direct, much better.

 

I think that's what the installers aiming to do, I'm just concerned about cycling as we have a big amount of area for UFH compared to our heat loss, and also our ventilation heat loss is the biggest unknown variable. Could be very good or just pretty good.. giving 1kW plus on our heat demand, so if we design to the less good values we may end up with oversized UFH, is that a problem for systems with no buffer?

 

Perhaps I'm better not jumping the gun with barely half an idea and see what our installer comes up with.
 

 

47 minutes ago, JohnMo said:

If you use no buffer, make sure you water volume is acceptable for the heat pump manufacturer for warranty purposes, don't zone it to death, one or zones only.

 

Some charts attached

 

 

These look useful, especially the UFH one. Thanks!

[PhilT]

3 hours ago, Ewan said:

Thanks all, lots of helpful input there!

I think this will come down to buffer and -10 headroom (plus better DHW reheat power) with 5kW or no buffer 3kW and less headroom (& slower DHW), we don't have a lot of space next to the UVC (210 or 250l) for a buffer that could be 75l plus (is that about right?).

Heat Punk (.co.uk) is saying our UFH rooms need 29w/m2 at -2.2º (Manchester), I can't find any UFH tables that include 35º flow, but at 40º 150mm centre I've seen suggested 55w/m2, so 35º could be ~45w/m2.. Should be fine modulated down a bit for -2.2 and cope with -10, I think, but I'm concerned it will end up cycling too much at higher outside temps.

 

I guess the buffer addresses this, but having watched the Heat Geek vids they sound like something you should avoid unless you really need one (plus we don't have much space), and our installer (who's not done the calcs yet, admittedly)'s first thought was we wouldn't need one, just a volumiser.

 

I don't have all the tools to understand this, Vaillant 5kW can go below 35ºC flow I assume, but what's the lowest it'll do before cycling kicks in? 1/3rd of it's rated output power I've read, but what's that in ºC flow?

How big a buffer might we need? 75l? Things are pretty tight in our utility.
 

This is all fine in theory and the process my son went through was very similar, with the initial design coming up with a 5kW Midea as the recommended ASHP for his 150sqm passivhaus. Practice and the experience of the last 2 weeks paint a more cautionary picture. There are a number of factors to consider outside of the theoretical calcs. An almost new supposedly over-sized heat pump such as my own was defrosting as often as twice an hour recently during the most humid freezing dark. That's 6 minutes defrost cycle plus 14 minutes to return to flow temperature, 40 minutes out of the hour at below set power level. I suggest that would not be factored into any theoretical calcs. Heat pumps deteriorate with age, even when regularly well maintained. You are designing for the lifetime of the heat pump, not just when it is brand new. There may be exceptional circumstances where you need higher room temps, for example a family I know all went down with seasonal flu during this recent cold spell and needed at least 22degC to feel warm enough to help them get better. You need this kind of flexibility that traditionally has been the realm of the classically over-sized gas boiler. Modern heat pumps have sophisticated variable speed scroll compressors that are much more tolerant to cycling than their reciprocating ancestors, so theoretical over-sizing, and about £400, is, in my view, a small price to pay for all that enormously valuable headroom.

Edited December 16, 2022 by PhilT

[Luke1]

2 hours ago, Ewan said:

 

I think that's what the installers aiming to do, I'm just concerned about cycling as we have a big amount of area for UFH compared to our heat loss, and also our ventilation heat loss is the biggest unknown variable. Could be very good or just pretty good.. giving 1kW plus on our heat demand, so if we design to the less good values we may end up with oversized UFH, is that a problem for systems with no buffer?

 

I don't think you can design oversized UFH. The higher performing UFH the better because it will allow you to run a lower flow temperature from the heat pump. 

 

3 hours ago, Ewan said:

 

I don't have all the tools to understand this, Vaillant 5kW can go below 35ºC flow I assume, but what's the lowest it'll do before cycling kicks in? 1/3rd of it's rated output power I've read, but what's that in ºC flow?
 

 

Your heat pump will be able to heat water to whatever lower limit you require. This makes no difference if its a 5kW unit or a 12kW unit. 

My heat curve starts providing water at 21C. Yes it cycles, but it cycles so infrequently that it's not an efficiency problem.