OVO heat trial - preliminary design

[Spoogster]

Evening all. 

We have moved to the next stage of the now extended OVO sustainable heat trial (I.e. government funded heat pump trial), and have the preliminary system design for our year 2000 detached four bedroom in Kent 130sqm, triple glazing installed two weeks ago.

 

The report shows calculated heat loss is c4.3kw. Don’t know how this compares to others but we have 300mm loft insulation and the triple glazing has significantly reduced air leakage and feels so much warmer than the battered twenty year old double which was replaced. The new doors In particular are much better.

 

New ASHP system is a 7kw Daikin monoblock with a 250 litre Daikin cylinder. Four radiators are proposed to be replaced two of which are in the main bathroom and en suite - the former I am going to resist due to tiling upset.

 

I raised concern with the heat pump firm, who appear to be very experienced with ASHP, over our downstairs rads being served by 10mm plastic microbore, but after the heat loss surveyor called into the office when on site they was happy that our reasonably modern home should cope. I however note that we have not had a buffer tank specified which is my main concern given reading on this forum.

 

I am minded to go ahead without pushing the buffer tank issue, and if short cycling is a problem for the period of the one year trial potentially adding a buffer tank later. Is this a possibility or a complete headache if adding later? I assume it is a case of T ‘ing into the radiators circuit so can be “reasonably” simply added later. Can a buffer go in the loft above the cyclinder location? Assuming I build it into an insulated cupboard?

 

As you can imagine the offer of a free heat pump system is very tempting even if it does mean there is the potential need for future upgrades to get the best out of it, hopefully not at excessive cost...

 

thoughts appreciated 

[SteamyTea]

10 hours ago, Spoogster said:

The report shows calculated heat loss is c4.3kW

Is that the worse case?

Because

10 hours ago, Spoogster said:

7kW Daikin monoblock

makes it quite oversized.

So

10 hours ago, Spoogster said:

buffer tank

should really be fitted to reduce the chance of short cycling, unless you have a large reserve of fluid in the radiators and pipework.

[PeterW]

Can I check then that they are not re-sizing any of the rads ..? The current ones will be sized for DeltaT 50°C (water flow to room temperature) and you’re going to at best DeltaT 30°C with an ASHP so the operating factoring is 0.515 on the output. They may want to replace the bathroom rads as they will be small but adequate now, when they have half the output that may not be the case. Have they provided a room by room heat loss calculation ..??

 

 

[joth]

42 minutes ago, SteamyTea said:

Because

11 hours ago, Spoogster said:

7kW Daikin monoblock

makes it quite oversized

 

Not if it's been sized for DHW reheat time.

 

But, yes... If driving rads you surely want a buffer tank to let it run on a reasonable cycle even if TRVs have shut off most rooms?

 

 

[SteamyTea]

33 minutes ago, joth said:

Not if it's been sized for DHW reheat time.

Yes, I forgot that, only now on my second mug of tea.  I hate the clocks changing.

34 minutes ago, joth said:

If driving rads you surely want a buffer tank to let it run on a reasonable cycle even if TRVs have shut off most rooms

Because the TRVs have shut most of the radiators down.

I just think that all domestic HP systems need buffer tanks.  I suspect that traditional gas combis would work better with them as well.

[Spoogster]

3 hours ago, PeterW said:

Can I check then that they are not re-sizing any of the rads ..? The current ones will be sized for DeltaT 50°C (water flow to room temperature) and you’re going to at best DeltaT 30°C with an ASHP so the operating factoring is 0.515 on the output. They may want to replace the bathroom rads as they will be small but adequate now, when they have half the output that may not be the case. Have they provided a room by room heat loss calculation ..??

 

 

Yes they have done room by room calls looking at existing K1 and K2 rads and appear to have reduced down to delta T based on 50c operating temp (which seems about 5c too high for efficient running). 
 

looking on the stelrad web site we look to have spare to cover 0.515

[Spoogster]

59 minutes ago, Spoogster said:

Yes they have done room by room calls looking at existing K1 and K2 rads and appear to have reduced down to delta T based on 50c operating temp (which seems about 5c too high for efficient running). 
 

looking on the stelrad web site we look to have spare to cover 0.515

 I have also done a spreadsheet following your comment and using 0.515 against stelrad figures for each size confirms majority of existing rads should be fine at lower temps.

 

Two rads will be lock shielded open (hall and landing) so I assume that will assist with short cycling? They aren’t massive but the hall one is being upgraded to K2 600x700. Not buffer tank size tho.

 

has anyone though about my question of adding a buffer a year or so down the line?

[PeterW]

You can add the buffer later but is this all assuming a single circulation pump running it..? If so your buffer becomes a problem as it will allow your ASHP circulation pump to push water round the buffer and back to the heat pump at which point it will switch off and the house won’t be hot as it’s going the route of least resistance.  
 

With UFH this is managed by the UFH having its own circulation / blending pump which is triggered by the timer and room stat, and the ASHP circuit pump triggered by the buffer stat. This means the ASHP stops when the buffer is satisfied, and not necessarily when the floor is at temperature (ie when a single circuit remains open)

 

TBH this is why these retrofit schemes don’t work, and significant problems occur. They are not designed to cope with a radiator based system that isn’t designed correctly, and you’ll find that whilst the heat pump “may” work in your scenario, it will have a seriously shortened life which will only come to light after the scheme has ended. 
 

It is essentially a energy efficiency scheme designed by an accountant. 

[Spoogster]

44 minutes ago, PeterW said:

You can add the buffer later but is this all assuming a single circulation pump running it..? If so your buffer becomes a problem as it will allow your ASHP circulation pump to push water round the buffer and back to the heat pump at which point it will switch off and the house won’t be hot as it’s going the route of least resistance.  
 

With UFH this is managed by the UFH having its own circulation / blending pump which is triggered by the timer and room stat, and the ASHP circuit pump triggered by the buffer stat. This means the ASHP stops when the buffer is satisfied, and not necessarily when the floor is at temperature (ie when a single circuit remains open)

 

TBH this is why these retrofit schemes don’t work, and significant problems occur. They are not designed to cope with a radiator based system that isn’t designed correctly, and you’ll find that whilst the heat pump “may” work in your scenario, it will have a seriously shortened life which will only come to light after the scheme has ended. 
 

It is essentially a energy efficiency scheme designed by an accountant. 

Peter that is a very stark warning if I have ever seen one! 
is it not then a case of adding a second pump and valve to isolate the buffer as you suggest, at either the heat pump or radiator side? Maybe I am being too simplistic.

it would be helpful for me to understand why the HP life would be significantly shortened if no buffer is provided? Five year guarantee on the Daikin parts is being offered...

fascinating and much appreciated.

[SteamyTea]

You also need to make sure there is plenty of room to fit one. If it is too cramped a space the plumber will do a crap job.

With your quote, does it show insulation for relevant pipework?

[Spoogster]

1 hour ago, SteamyTea said:

You also need to make sure there is plenty of room to fit one. If it is too cramped a space the plumber will do a crap job.

With your quote, does it show insulation for relevant pipework?

Not just yet. The contracts manager is coming to our home in two weeks time to talk about position and pipes. I have highlighted that our existing airing cupboard is cramped at 700x800 and offered a handy corner of our breakfast room just below which would give 900x900. A slim type 250 litres Daikin cylinder is likely....

[SteamyTea]

12 minutes ago, Spoogster said:

The contracts manager is coming to our home in two weeks time

Impress on him that you know what the technology is capable of, and the limitations, but you want it to work so that others can learn from it.  A failed system tells people nothing in this circumstance, other than the designer is a twat.

Have a hunt around the MCS site to find the design criteria that installers have to follow.

Get your local weather data as well, Temperatures and RH, for as many years as possible, and at the best resolution you can get.  Then you can do a risk analysis on the system frosting up.

Edited March 30, 2021 by SteamyTea

[PeterW]

1 hour ago, Spoogster said:

Peter that is a very stark warning if I have ever seen one! 
is it not then a case of adding a second pump and valve to isolate the buffer as you suggest, at either the heat pump or radiator side? Maybe I am being too simplistic.

it would be helpful for me to understand why the HP life would be significantly shortened if no buffer is provided? Five year guarantee on the Daikin parts is being offered...

fascinating and much appreciated.

Ok so the constant cycling of on/off heat/cool on the joints in refrigeration systems can cause fracturing of the connections and failure of the refrigeration circuits. These then tend to then run hotter and harder resulting in burnouts of compressors and pump failures, or shuttle valves due to other issues. 

Next off will be running control circuits. Not too difficult to put a second pump and buffer in somewhere and assuming this is all being piped as standard S Plan..? Will need to understand that to add a buffer 

 

[Spoogster]

47 minutes ago, SteamyTea said:

Impress on him that you know what the technology is capable of, and the limitations, but you want it to work so that others can learn from it.  A failed system tells people nothing in this circumstance, other than the designer is a twat.

Have a hunt around the MCS site to find the design criteria that installers have to follow.

Get your local weather data as well, Temperatures and RH, for as many years as possible, and at the best resolution you can get.  Then you can do a risk analysis on the system frosting up.

There appears to be a whole library of documents including best practice guide. I know what I am doing this evening!

[SteamyTea]

20 minutes ago, Spoogster said:

There appears to be a whole library of documents including best practice guide. I know what I am doing this evening!

Good, you can be the forums expert by the morning.

[Spoogster]

14 hours ago, SteamyTea said:

Good, you can be the forums expert by the morning.

Well, the best practice document was well worth reading.

 

page 22: heat pumps not suitable for microbore because of lower flow rates. No in depth reasoning and quite a sweeping statement, and I have found a number of web articles where it has been used. Certainly one for questioning when the contracts manager comes to our house to discuss design. I suppose it is down to how much micro there is and where, fed by what other pipe sizes. 

 

Some of the guidance on buffer tanks is interesting and I don’t admit to fully understanding it, but there is the suggestion it can reduce efficiency through parasitic action. Hhmmm. Sizing is suggested at 10 litres per kW heat pump .  Cycling should be less than 6 times per hour to stop parts wearing out.

Begs some questions with my microbore issue: to stop cycling could it be advised that a number of radiators are lock shielded open so as to keep demand going all the time the main thermostat is calling for heat? Both our existing hall and landing are set up without TRVs for this reason I understand, but I cannot see why bathroom rads could not be also left fully open so as to act a bit like a constant calling loop to lower the potential for cycling (that would be total four or five rads depending on WC too)? Push and pull pumps on flow and return an option? 

Or, is there merit in running the HP temps just low enough to meet the heat demand of the home so as to ensure cycling doesn’t kick in too often, or does that cause issues with delta t? I assume low temp running would be more efficient but rad sizes would need to be further increased. I can see the balance or reasons why or why not being all over the place on this one.

 

What I have taken away from reading the guidance is that good training on design is crucial and to put to one side your existing knowledge on gas heating systems - this stuff is much more complicated and no room for mistakes or slack/lazy design... if you hated maths at school you might struggle.

[dpmiller]

heat pumps need a much smaller differential across their heat exchangers than a boiler, and it takes a lot of flow to make this happen.

[SteamyTea]

1 hour ago, Spoogster said:

if you hated maths at school you might struggle

It is arithmetic, not mathematics.

Arithmetic is pretty boring really, and easy to make a mistake, but we have speadsheets.

 

Always remember that temperature, power and energy are not the same thing.

 

Ideally a system would respond to external stimuli, so if the house only needed 500W to keep it at the desired temperature, the HP would modulate down to this, but when it needs 4 kW, it would deliver it, all be at a lower overall efficiency.  Trouble is, life is not like that and a HP needs to run between fixed parameters of temperature and power.  This is why a buffer is fitted, it allows absorption of a higher power (either though higher temperature or larger delivery volume) before it is needed.  A buffer needs to be insulated just like any storage system, and it does not need to have storage at maximum temperature all the time (can't change the volume so temperature is the only variable).

 

Microbore is the Devil's Vas Difference or Urethra, it gets blocked, and we don't want that.

Edited March 31, 2021 by SteamyTea

[Spoogster]

55 minutes ago, SteamyTea said:

It is arithmetic, not mathematics.

Arithmetic is pretty boring really, and easy to make a mistake, but we have speadsheets.

 

Always remember that temperature, power and energy are not the same thing.

 

Ideally a system would respond to external stimuli, so if the house only needed 500W to keep it at the desired temperature, the HP would modulate down to this, but when it needs 4 kW, it would deliver it, all be at a lower overall efficiency.  Trouble is, life is not like that and a HP needs to run between fixed parameters of temperature and power.  This is why a buffer is fitted, it allows absorption of a higher power (either though higher temperature or larger delivery volume) before it is needed.  A buffer needs to be insulated just like any storage system, and it does not need to have storage at maximum temperature all the time (can't change the volume so temperature is the only variable).

 

Microbore is the Devil's Vas Difference or Urethra, it gets blocked, and we don't want that.

I’m so out of my depth!!!!

[SteamyTea]

19 minutes ago, Spoogster said:

I’m so out of my depth!!!!

Reduce it to basics.

You have a hot end, and a cold end [T].

The energy [J] travels from the hot to the cold.

 

The shorter time it takes to do it, the more power is delivered [W].

Now you have all the units needed in thermodynamics.

J is energy, W is power, s is time in seconds and T is temperature.

A W is a J/s.

All the rest is just metres [m] and mass [kg] to complete the system.

 

Thinking about buffers, I suspect that there is a simple way to work out the volume [m³] from the ratio of power in and power out.

Will ponder that as I stroll along the promenade to Newlyn and beyond. 

 

 

[PeterW]

2 hours ago, Spoogster said:

page 22: heat pumps not suitable for microbore because of lower flow rates. No in depth reasoning and quite a sweeping statement, and I have found a number of web articles where it has been used. Certainly one for questioning when the contracts manager comes to our house to discuss design. I suppose it is down to how much micro there is and where, fed by what other pipe sizes. 

 

Microbore isn't an issue if you manage the flow rates, and a buffer will help here no end. The ASHP will push heat into the buffer (I would go minimum 100 litres) and then the heating system pulls it out.

 

2 hours ago, Spoogster said:

Some of the guidance on buffer tanks is interesting and I don’t admit to fully understanding it, but there is the suggestion it can reduce efficiency through parasitic action. Hhmmm. Sizing is suggested at 10 litres per kW heat pump .  Cycling should be less than 6 times per hour to stop parts wearing out.

 

Parasitic (or syphonic) losses are a misnomer. the heat is lost into the house so its not a problem in reality.

 

2 hours ago, Spoogster said:

stop cycling could it be advised that a number of radiators are lock shielded open so as to keep demand going all the time the main thermostat is calling for heat

 

Not really worth it - use a buffer

 

2 hours ago, Spoogster said:

Push and pull pumps on flow and return an option? 

 

It is a closed system so won't help. Pumps on the ASHP >> Buffer and then Buffer >> Heating are what is needed

 

2 hours ago, Spoogster said:

Or, is there merit in running the HP temps just low enough to meet the heat demand of the home so as to ensure cycling doesn’t kick in too often, or does that cause issues with delta t? I assume low temp running would be more efficient but rad sizes would need to be further increased

 

Nope unless you massively increase the size of the rads

 

 

[SteamyTea]

@PeterW

Can a buffer be sized on the difference between power in and power out. That way you can decide how long the heat source is running for.

You obviously need to know the performance 'curves' of the the source and sink in different weather regimes, but these are generally matched curves, do can be converted to a linear plot quite easily I think.

Then it is just a case of deciding a range in the middle of the new chart to cover 99% of the expected conditions.

Shall try a sketch what I mean later.

 

Thinking a bit more about it, may need to break convention and put time on the y axis and plot heating time against cooling time.

[PeterW]

Possibly ... if you know the minimum input an ASHP can modulate down to, ignore the heat curve based on external temperatures, then you could model something. 
 

One of the issues normally is the binary response of thermostats and their hysteresis. It may be worth implementing a hi/lo type setting that only cuts in when the buffer goes below a fixed temperature and then off when it hits the high with a 10-12°C variation. That should stop

short cycling but will increase temperature fluctuation in the rad flows. 

[Spoogster]

30 minutes ago, PeterW said:

Possibly ... if you know the minimum input an ASHP can modulate down to, ignore the heat curve based on external temperatures, then you could model something. 
 

One of the issues normally is the binary response of thermostats and their hysteresis. It may be worth implementing a hi/lo type setting that only cuts in when the buffer goes below a fixed temperature and then off when it hits the high with a 10-12°C variation. That should stop

short cycling but will increase temperature fluctuation in the rad flows. 

Just called the Daikin tech team who have been very helpful.

 

looks like a buffer or low loss header is definitely the way forward. I am going to have to insist on one I think.

 

 The problem is that the installers on this OVO scheme appear to be budget capped so any increase in specification is coming off their margin, which isn’t ideal for them. I do know that at least one installer on the scheme has asked a customer to pay a £2k contribution as their install came to £15k. I’ve had £10.5k allowed so I’m unsure if a 7kw monoblock 410a and 250litre cylinder and install would leave any slack for a buffer and extra pump/circuit to be included.

im torn here as I would like the trial to be a success and for heat pumps to be supported by the government going forward, but I get the horrible feeling installers are less concerned and could be throwing any old system in with little regard for outcome just to cash in (which in this case is like turkeys voting for Christmas). It is surely in everyone’s interest for the trial to go well....

[PeterW]

So the installer is whining that a £250 buffer and £100 pump will be coming off their £5k budget for the install ..???! My heart bleeds... 

 

You have less than £5k of kit in there - the ASHP and tank are change of £4500 and I’m giving a generous £1000 for other bits which is well over the top. 
 

As I said .. this is a government backed money making scheme for installers ...