Maximum permitted flow temperatures?

[sharpener]

I have now got two quotes for a heat pump install. One is for a flow temp of 45C and the other is for 50C (with mean water temps of 2.5C less). The heat loss calculations have various other differences which make a direct comparison difficult, however the total losses are 12.3 and 11.5kW respectively.

 

The exisiting radiator sizes for my boiler CH are mostly too small already. So to get reasonable sized replacement radiators when I move to a heat pump I may need to have a flow temp higher than 45C. (The upper bound of 52.5C is set by the 12.2 kW max output of the HP at the -1.41C design OAT, any higher and it will not meet the total heat loss. In warmer conditions the WC will of course result in lower flow temps.)

 

MCS's publications MIS3005 and its accompanying Best Practice Guide say only AFAICS that flow temps >55C are regarded as "high temperature heat pump" installations, and need to have a standard HP quote alongside for comparison. There is also MCS 021 - Heat Emitter Guide but that sheds no further light other than explaining the star rating applied to MWT.

 

So what are the actual rules about this? The Building Regs Part L have recently been changed to specify 45C for new builds with HPs, and 55C for existing buildings for any complete heating system renewals inc pipework (which is not what I am doing).

 

As a further twist the feed to the rads will mostly come from a thermal store heated at off-peak rates. So the absolute efficiency is perhaps not as much of a driver as might normally be the case.

[Originaltwist]

I like the way your project is shaping up but talk of radiator upgrades made me wonder about a DIY fan-coil unit I came up with a while ago. It's cheap and neat and better for low temperature flow - just needs an open mind to give it consideration.

There's a description and pictures on  https://originaltwist.com/2018/11/14/diy-fan-coil-heater/  so I won't bang on about it here.

[sharpener]

Have thought of fan assistance of various types. But OH will not have a fan of any kind in her study, which is the most difficult of the rads to re-size. I might try and identify a removable fan accessory which could be fitted later if necessary to placate the installer.

 

But the main thrust is still to find out what is actually allowable under MCS and other rules.

[JamesPa]

18 minutes ago, sharpener said:

Have thought of fan assistance of various types. But OH will not have a fan of any kind in her study, which is the most difficult of the rads to re-size. I might try and identify a removable fan accessory which could be fitted later if necessary to placate the installer.

 

But the main thrust is still to find out what is actually allowable under MCS and other rules.

I'm not sure what you are looking for.  It seems you have concluded that building regs don't apply and know the MCS position.  I don't think that anything else regulates flow temperatures, so doesn't the answer boil down to the MCS rules.

[JohnMo]

35 minutes ago, JamesPa said:

I'm not sure what you are looking for.  It seems you have concluded that building regs don't apply and know the MCS position.  I don't think that anything else regulates flow temperatures, so doesn't the answer boil down to the MCS rules.

I would have thought the MCS supplier will basically over egg the whole heat loss thing anyway, so everything will be over sized anyway. If they designed for 50 you possibly wouldn't need anywhere near that on the worst day in reality.

 

Then set the heat pump to min Hz setting to downsize it, when they go out the door.

[sharpener]

28 minutes ago, JamesPa said:

I'm not sure what you are looking for. 

 

I recall being told in the past that MCS specify 45C max flow for new build and 50 or 55 for retrofit situations. But I can't find chapter and verse for that and it may of course have been superseded, or was just a myth from an earlier installer trying to defend his design decision.

 

31 minutes ago, JamesPa said:

It seems you have concluded that building regs don't apply

 

Not entirely, only that Paragraph 5.10 of Part L1 2021 quoted here does not seem to be relevant. Other parts may apply, there are also a CIBSE design guide and BS/EN that may be applicable.

 

5 minutes ago, JohnMo said:

I would have thought the MCS supplier will basically over egg the whole heat loss thing anyway, so everything will be over sized anyway.

 

Would like some arrows in the quiver as I am expecting some pushback if I ask for rads to be spec'd for a flow of 52.5 (which the HP will just do against the predicted heat loss). Much less and we are into rearranging the furniture and massively thick K33s. Yes some of it looks conservative beginning with the min OAT of -1.41 which seems unlikely as the HP is at sea level and 25m from a tidal estuary on the S coast.

 

Someone here must know the facts, maybe the self-builders!?

[JohnMo]

21 minutes ago, sharpener said:

52.5

That's hot - rubbish CoP, but you know that.

[ReedRichards]

Any chance of adding another radiator if you can't resize an existing one to make it big enough?  We added another radiator in a bathroom which my OH loves because it helps to dry/air clothes in winter.

[sharpener]

Well funnily enough we are indeed going to put a radiator in the en-suite which doesn't have one atm and is freezing cold.  And the other cold area is the landing where we can replace the 600mm high rads with wider 700s.

 

On 01/05/2024 at 17:18, sharpener said:

But OH will not have a fan of any kind in her study, which is the most difficult of the rads to re-size.

 

Have now found what I think is the big fix. The problematic study had been entered in Heat Engineer as having ground at 11C underneath it not kitchen at 18. That should bring the room heat loss down by 300W which might be enough to lower the required flow temp to 50 which is less of a psychological challenge.

 

Am hoping installer will also buy the argument that the rads will be fed from the TS which will be charged twice a day at cheap rate to 55C, with as @JohnMo says a rubbish CoP.

 

At -2C the CoP does fall from 2.5 to 2.0, but the electricity costs only half as much so worth using such a high temp to maximise the storage capacity.

[ReedRichards]

23 hours ago, sharpener said:

At -2C the CoP does fall from 2.5 to 2.0, but the electricity costs only half as much so worth using such a high temp to maximise the storage capacity.

But only when you need it.  Most of the time it will be warmer out, you won't need as much storage capacity so you can reduce the thermal store temperature.  This would be  a sort of DIY Weather Compensation, unless you can figure out a way to automate it.  

 

Could you get away with a plinth radiator in the study in addition to replacing the existing one; not the fan-assisted type, just one that is long and low?  Having to modify your entire heating system to conform to the needs of one room seems like the tail wagging the dog.  My retrofit system was designed around 50 C target flow but with hindsight I regret not pushing to make this a bit less, say 45C.  

[sharpener]

3 hours ago, ReedRichards said:

unless you can figure out a way to automate it

 

Current plan as agreed with Vaillant tech is to program the TS as a second HW tank and charge it in parallel with the cyl (to divide the HP output between the two and hence not have to change the existing HW cyl which is v diff.). This will mean to get HW at 50C I will need sthg like 55 in the coil and hence give me 55 in the TS also.

 

In winter all storage is good as what I don't need in the bedroom rads I can use in the rads in the living room.

 

Yes, in shoulder season I can also experiment with changing the TS to be a heating circuit, which will mean automatic WC is available for that circuit too, with its own time/temp schedule. Controls have a Parallel Cylinder Charging option which allows this to be done at same time as HW cyl if necessary (though CoP will be lower).

 

3 hours ago, ReedRichards said:

Having to modify your entire heating system to conform to the needs of one room seems like the tail wagging the dog. 

 

But 42.5 MWT as designed results in several rads being too large for the space available unless K3. In practice as many have suggested the whole heat loss is over-egged. I am not sure if/how he has accounted for the MVHR though the box is ticked on the Heat Engineer printout. I also know MCS will not allow him to account for the AGA which puts out ~2kW 24/7 into the room below the said study. So I don't expect there will in practice be many days when we need a flow temp of 50 let alone 52.5.

[ReedRichards]

In my system the kitchen already had a chunky T3 radiator which my OH liked to put things on to dry.  The heat loss calculation said it needed to be bigger but I thought with cooking and a bit of heat from the fridge and the freezer it would be okay.  I was wrong.  After a cold winter's night the kitchen is pretty chilly first thing.  It warms up during the day but it's a few hours slower than the rest of the house. 

 

Even if your whole heat loss calculation is over-egged you need about the same ratio between calculated heat loss and actual radiator heat output for each room.  Your OH may not like noise from fans but if they use the study in the morning I bet they will also dislike it being chillier than the rest of the house.

[JohnMo]

57 minutes ago, ReedRichards said:

Your OH may not like noise from fans but if they use the study in the morning I bet they will also dislike it being chillier than the rest of the house

Just use a smaller set back or none at all.  The heat input is just about the same set back or not.  You demonstrate well, that the heating system is just playing catch up for the rest of the day, so burning the energy.

[ReedRichards]

I don't think there has been enough studying of set backs to know if they save you money and if so how much.  I know of one individual who worked out his set back saved him about 15% but it must differ in every case.  But I do it because I don't like my bedroom to be so hot at night.  

[JohnMo]

1 hour ago, ReedRichards said:

But I do it because I don't like my bedroom to be so hot at night.  

We just don't heat the bedroom, close the door and open the window (for the last 6 weeks anyway)

[sharpener]

2 hours ago, ReedRichards said:

I don't think there has been enough studying of set backs to know if they save you money and if so how much. 

 

Michael Podesta looked at this in his blog and came to the conclusion "it all depends". There's also some recent discussion in this thread demonstrating that the potential savings are quite modest, around 8% even if you assume setback for 12 hours.
 

To judge by the fact that my room stats click on and off with a cycle time of 20 - 30 mins I think the UFH time constants are quite short. Obvs we will not use it like that with the HP, I will have to wait and see how it all perfoms and whether heating the slab overnight on cheap rate is cost effective or not.

 

Given that we typically do not use the living room until late afternoon it may be that leaving it off or on setback until the Cosy cheap window 1300 - 1600 will be sufficient. We will also charge the TS to 55C during that period.

 

Some years ago I put two big rads in the living room as OH does not like the UFH anyway, the rads give a fast warmup to 20 deg and in future will benefit from cheap heat from the TS.

 

Yes it is unconventional but the calcs all stack up.

 

 

2 hours ago, ReedRichards said:

But I do it because I don't like my bedroom to be so hot at night.

 

We don't heat the bedroom at night either, there is enough heat seeping up from the AGA in the kitchen, also MVHR. It settles to about 16C which is quite enough, any more and neither of us sleep well.

[sharpener]

On 03/05/2024 at 08:44, sharpener said:

Have now found what I think is the big fix. The problematic study had been entered in Heat Engineer as having ground at 11C underneath it not kitchen at 18. That should bring the room heat loss down by 300W which might be enough to lower the required flow temp to 50 which is less of a psychological challenge.

 

Well this turned out to be the solution. Installer has corrected the heat loss for the room in question and this means a K2 will be fine there, with a worst-case flow temp of 50C which he is content with. In turn this will give us a bigger margin overall and mean smaller rads in other places which family are happy with too.

[Dave Jones]

seriously dont bother. Anything over 40C flow and it will cost you an absolute fortune to run.

[sharpener]

Weather comp (and E7 rates) will see to that most of the time. The 50C is worst case at OAT -1.4C, a figure I think in any case highly pessimistic as we are right on a tidal estuary in the SW.

[Dave Jones]

13 hours ago, sharpener said:

Weather comp (and E7 rates) will see to that most of the time. The 50C is worst case at OAT -1.4C, a figure I think in any case highly pessimistic as we are right on a tidal estuary in the SW.

 

if your heat loss is soo bad you need to run at 50c then it will need to run 24x7. If you expect to run it flat out for a couple hours each night and expect that to cover the other 20 hours of heat loss you are going to be in for a shock. 

 

ASHP are not gas boilers that can provide short, fast cheap bouts of very hot water.

 

To not cost a fortune (minimum of 5 x the cost of a gas boiler per month) it needs to run constant and low to avoid spikes.

 

You will be writing to daily mail saying what a rip off ASHP are next winter!!!!

[Dave C]

And yet, my ASHP was installed with a maximum flow temperature of 50c, and it doesn't need to run 24/7 even in the winter, and it doesn't cost a fortune to run either...

 

Much like the OP, the installer set it up to run at 50C, but (a) that was indeed a bit pessimistic - after the first winter it became clear that I could drop the max down to 47 or even a bit lower, and it was fine; and (b) that's only when it's below freezing. Does that happen? Sure, every winter. Does it happen on that many days? In particular, are there that many days when it stays below freezing throughout the day (i.e. during peak electricity rates)? No, not really. A few days every year.

 

That answer might be different if you live in the North of Scotland! I don't, and it sounds like the OP doesn't either.

 

Nothing wrong with suggesting investing in insulation where possible, and being careful not to let an MCS installer flog an inappropriate heat pump, but the idea that (maximum, worst case!) flow temperatures >40C absolutely guarantee it'll cost a fortune isn't true.

[JamesPa]

22 minutes ago, Dave C said:

but the idea that (maximum, worst case!) flow temperatures >40C absolutely guarantee it'll cost a fortune isn't true.

No, but it is the sort of thing that the Daily Mail might claim, or for that matter several other equally regressive media outlets some of which purport to be serious journals.

 

Sofaik heat pumps are the only realistic mass market solution for domestic heating, so we don't have a choice, whatever it costs is certainly less than the cost of our of control global climate chaos.

 

Quite what the proprietors of these outlets think they are achieving now escapes me.  I guess it's to postpone the inevitable action, this making the consequences even more terrible, but that doesn't matter because they, and their slimy friends, are all super rich so will emigrate to New Zealand, turn it into a right wing state to keep everyone else out and exploit the now desperate population of much of the rest of the world to service an increasingly isolated but nevertheless luxurious lifestyle.  

 

It's totally unconscionable.

Edited May 10 by JamesPa

[PhilT]

56 minutes ago, JamesPa said:

whatever it costs is certainly less than the cost of our of control global climate chaos.

Well, whatever you believe on this subject, heat pumps don't need this kind of justification anyway, they easily stand up on their own merit.

[sharpener]

 

2 hours ago, Dave C said:

In particular, are there that many days when it stays below freezing throughout the day (i.e. during peak electricity rates)? No, not really. A few days every year.That answer might be different if you live in the North of Scotland! I don't, and it sounds like the OP doesn't either.

 

Yes. Thank you for posting the logic, I might not have been so polite!

 

On 01/05/2024 at 18:34, sharpener said:

Yes some of it looks conservative beginning with the min OAT of -1.41 which seems unlikely as the HP is ... 25m from a tidal estuary on the S coast.

[ReedRichards]

On 01/05/2024 at 18:34, sharpener said:

the HP is at sea level and 25m from a tidal estuary on the S coast.

 

Any scope to use a water source heat pump?