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Knowing a system will work before we commit to it

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(@oswiu)
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793 kWhs
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Hi guys,

I just want to start by thanking members for their amazing contributions which have very much increased my knowledge, which as we all know is hard to come by for heat pumps.

My question is in relation to understanding that a system quoted by an installer will actually perform well for me.

I've had a heat loss survey done by a professional installer, but to be frank it had so many issues that it might as well be disregarded, from forgetting an entire room to leaving out external walls and vents. I've therefore done my own heat loss survey in a couple of tools (the official MCS one, Heat Punk, Freedom) and got to about 6kW peak loss for my area and altitude. The living room which is just served by one rad has a heat loss of over 1kW due to two vents for our woodburning stove, and in addition to a normal window, an entire wall of glazing complete with patio doors. A couple of installers have guessed values from our EPC and they've come to about the same, albeit slightly lower values. Our (detached) house is ~140 sq m, and only about 10 years old.

This all seems like an efficient starting point for an ASHP, but there are a few complications. Our piping from the boiler is a mere 22mm, and the piping to our 12 rads (no UFH for us) is 10mm. Problems continue in the placement of the heatpump, which due to a confined alleyway on the side of the house would need at least 10 metres of pipe run to the boiler from where the ASHP would go - assuming it would feed into our airing cupboard which is where the current pipes from the boiler in the garage go.

So our problems/challenges which of course no salesperson has said are actually blockers are:

  1. Heat loss for the house approaches limit of what 22mm piping can provide without hydraulic separation (from what I've read).
  2. Heat loss for the living room approaches or is at the limit of what 10mm piping can provide a radiator.
  3. A large run of piping (20m there and back) adds significant heat loss if piped outside. Using a formula I found on the internet and 55C outward flow with a DT of 5C, and some outdoor rated piping insulation I found, this would seem to add about 350W of additional heat losses at design temp (c. -4C).
  4. This isn't so much a blocker but rather a general grievance that our radiators are woefully undersized for efficient heatpump temps, and almost all of them will have to be replaced. It seems that unlike with every other house in the country, the builders of this one did a heat loss survey and sized the radiators very well for a 70C gas boiler (our current system). We have some comically tiny rads!

I suppose my problem is that I just cannot get through to actual heating engineers to discuss this without paying them £1k deposit. They seem to do quotes without surveys or discussion assuming everything will be fine, but I can't take on a quote if I don't know whether the price will double due to insufficient piping etc. The one quote I did get (for a heatpump from a premium German supplier) suggested replacing the 22mm piping with 28mm piping, and that along with replacing the rads was about £2.5k which I thought was actually pretty reasonable. The trouble was that the rest of the quote was eye wateringly expensive.

So to the wizards of this forum whose answers I have found so insightful elsewhere, I ask:

  1. Will I need to upgrade the piping of the circuit?
  2. What can we do with that 1-1.5kW peak loss living room? I've considered filling in the vents and replacing the woodburner with a direct air version, which would lower air changes significantly.
  3. Will I need hydraulic separation in the form of a LLH or a buffer? The few installers I've had "estimates" from seem to think so.
  4. Is there anything to be done about the long pipe run? Can it go through the house maybe?
  5. Is a split or a monobloc system better for long pipe runs? I've heard splits are, but I don't understand why.
  6. Could I ever get a good SCOP?

 


   
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(@benseb)
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Just for the long pipe run we had this. You can get preinsulated pipe which you bury and therefore it has less heat loss (as underground is warmer than air)

 

but with that run you’ll need to oversize it to ensure good flow rates. 

250sqm house. 30kWh Sunsynk/Pylontech battery system. 14kWp solar. Ecodan 14kW. BMW iX.


   
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(@oswiu)
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793 kWhs
Joined: 2 years ago
Posts: 121
Topic starter  
Posted by: @benseb

Just for the long pipe run we had this. You can get preinsulated pipe which you bury and therefore it has less heat loss (as underground is warmer than air)

 

but with that run you’ll need to oversize it to ensure good flow rates. 

That's an interesting prospect, and I did think of that as the area is appropriate, but most of it would just be round the house so I sort of assumed that conductive heat loss into the ground combined with a longer piperun to go down into it might mean more energy loss than into air round the house. Did your installer talk through implications of this?

Just thinking about COPs, at 6kW peak load at a plausible COP of 3, plus 350W pipe loss, this might turn 2kW of electricity into 2.1kW. Re-arranging to find the COP of useful heat would turn 3 into 2.86, so that doesn't actually seem that bad of a decrease, but again that's just with a conductive equation for heat loss I found, so it's probably not all that meaningful in the real world.


   
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(@kev-m)
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Posted by: @oswiu

 

  1. Will I need to upgrade the piping of the circuit?
  2. What can we do with that 1-1.5kW peak loss living room? I've considered filling in the vents and replacing the woodburner with a direct air version, which would lower air changes significantly.
  3. Will I need hydraulic separation in the form of a LLH or a buffer? The few installers I've had "estimates" from seem to think so.
  4. Is there anything to be done about the long pipe run? Can it go through the house maybe?
  5. Is a split or a monobloc system better for long pipe runs? I've heard splits are, but I don't understand why.
  6. Could I ever get a good SCOP?

 

If your design temp is -3 deg or thereabouts, you're going to be using a lot less than 6kW nearly all of the time, as long as you adopt the 'slow and steady' approach to heating. So, being near the limit at -3 deg may not be a problem.

There is no reason at all why you can't get a good SCOP.  

Have a look at this article on ASHPs and microbore by Graham Hendra aka @grahamh 

https://www.linkedin.com/pulse/heat-pumps-microbore-why-can-excellent-bed-fellows-graham-hendra/?trk=articles_directory

 

 


   
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(@oswiu)
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Topic starter  
Posted by: @kev-m

If your design temp is -3 deg or thereabouts, you're going to be using a lot less than 6kW nearly all of the time, as long as you adopt the 'slow and steady' approach to heating. So, being near the limit at -3 deg may not be a problem.

There is no reason at all why you can't get a good SCOP.  

Have a look at this article on ASHPs and microbore by Graham Hendra aka @grahamh 

removed link

 

Hi Kev, thanks for getting back to me,

I have actually read that, and even have it bookmarked along with Graham's book. Although I thought I ought to finish John Cantor's one before buying too many...

I'm aware that it won't have a peak thermal load very, or at all, often, but also to get the BUS I need a prospective installer to be happy with that hence the question of what they'll say without asking them. If the answer is that they'll be satisfied though, I guess that's OK as we can just light the fire on the odd day that it hits -4C as long as it's fine on more normal temps of a couple of degrees.

Heat Geek suggests that the load should be fine at DT7 without hydraulic separation, but I've seen discussion elsewhere that heat pumps don't want to run at DT7 which has me confused. At DT5, I think this room's load is a bit too high, but part of my problem is I don't know how to account for vents with regards to air changes. If anyone knows what kind of air change figure I can assign to two 400 sq cm grated vents, that'd be helpful. They look like this one removed link so not completely open, but rather 'meshed'.

With a standard 0.5-0.7 air changes/hour we're looking at just over 1kW, but that can go up to ~1.5kW if I assume these vents at one or two air changes. The current radiator has a power of about 1.5kW at 70C gas boiler temps, and whilst it's not felt inadequate, we've only been here for two winters, and I taped up the vents when we weren't using the fire as they really do make the room feel much colder.

All this is to say that if the limit of usable heat in 10mm pipes is 1.15kW (assuming a radiator or fan convector can output every single Watt of that), then an installer might say that's not enough, and I don't know what my best option to ask the installer would be. A second radiator? 15mm pipes just in this room? A buffer tank? The latter is what they all seemingly want to go for.


   
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(@kev-m)
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@oswiu I think your option for the radiator would be (1) bigger pipes then either a bigger radiator or more than one radiator or (2) another pair of pipes fed off the main trunk to feed an additional radiator.  It depends on how easy replacing and adding pipework is in your house.

Have you seen the MCS heat loss calculator? There is some guidance in the Design Tables sheet on likely air changes but nothing specific for your type of vents.

https://mcscertified.com/mcs-launch-new-improved-heat-pump-calculator/

This post was modified 2 years ago by Kev M

   
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(@oswiu)
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Posted by: @kev-m

@oswiu I think your option for the radiator would be (1) bigger pipes then either a bigger radiator or more than one radiator or (2) another pair of pipes fed off the main trunk to feed an additional radiator.  It depends on how easy replacing and adding pipework is in your house.

Have you seen the MCS heat loss calculator? There is some guidance in the Design Tables sheet on likely air changes but nothing specific for your type of vents.

removed link

Do you know if there are any pressure implications on the rest of the system if the pipes are changed for just one radiator? To be honest I think changing the pipes is on the harder end because I think they come from above down through the wall, then curve to go under the window where they both come out in one spot. That said, if it needs doing it needs doing, and we don't like the colour of the walls in there anyway! I just wish I knew how much all of this was going to cost.

Yes I filled out the MCS heat loss calculator. My house is quite new so that gives the living room a default air changes of 0.5/hour, and I then added an open flue with throat restrictor because it seemed like a non-restricted open flue was a bit much, but it's all guess work. It comes to 1.385kW peak load with that setup and suggests 2 air changes.


   
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(@kev-m)
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It was very easy for us as we live in a bungalow and everything goes through the attic and downwards.  Having different sized pipes is OK. You have to balance the system/radiators and make sure the flow is right throughout the whole system, which can be tricky. The pressure will tend to be higher for an ASHP anyway because of the higher flow.


   
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(@batalto)
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@oswiu I tested to see if our house could handle a heat pump by turning down the flow temperature on the boiler. Realistically thats the major test. Just turn it down to 45 and see how your house feels on a cold day

12kW Midea ASHP - 8.4kw solar - 29kWh batteries
262m2 house in Hampshire
Current weather compensation: 47@-2 and 31@17
My current performance can be found - HERE
Heat pump calculator spreadsheet - HERE


   
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(@oswiu)
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@batalto It would be a useful test, and I have been running at low temps recently to try to emulate a steady state heating system*, although I feel that with our setup we'll reach the limit of what the small radiators can output long before we reach the limit of the pipework can carry. Perhaps an approach of replacing the radiators first then testing them at low temps with the gas boiler might work, but then I feel like I'm going round in circles with the risk that the installer doesn't think the rads are the right size and that the pipework is wrong anyway even though it's been tested, and then no BUS grant.

Maybe the answer is to just bite the bullet.

*I say 'try to emulate' and not 'to implement' because whilst I have been doing this for some of the day, I do need to turn the boiler up manually to heat the hot water


   
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 robl
(@robl)
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If you have a smart meter, you can get the information you need from it.  Look back over the last year for the greatest daily gas used in kWh.  Divide this by 24; this figure, now in kW is your peak power requirement.

Next run your system at full for half an hour, so it is at steady state - you will need to crank up the thermostat for this, and turn all radiator controls to max.  Record the boiler flow and return temperatures, house air temp, and crucially the gas power used, from the smart meter.  It may well be that your boiler throttles back, so we need the smart meter to calculate that power -don’t assume it’s the boiler nameplate.  From this test you can calculate how well coupled your radiators are:  GasPower/(Tflow-Thouseair) is what we want most.  This tells us how hot the flow needs to be for a given power - and we already know the required maximum power - so multiply by that and add 21C (Thouseair) , and we have the expected flow temperature you must run at, on the coldest day, to heat your house assuming 24hr heating.  

I went through this exercise a year ago - our old (16kW)boiler cranked up ran at 10kW, Tflow=71C, Treturn=48C, and gave 4.8C/kW - thats how much hotter the rads must be than the air temp to dissipate each kW.  If we needed a big heatpump, that would be disastrous, but we don’t, the most gas we used was 38kWh in one day.  So we need 38kWh/24 =1.6kW (insulation 😀 ).  From this, the minimum flow is 21C + 4.8C/kW x 1.6kW = 29C.

I realise the above glosses over dhw, rad nonlinearities, boiler efficiency, but I expect it will still be much more accurate than a paper excercise.


   
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(@oswiu)
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@robl thanks for your detailed description of - what is to me - a novel method of estimating peak heat loss! Unfortunately I don't have a smart meter despite me repeatedly asking my supplier for one. However, I do have "heating demand" statistics from my heating system going back to January of this year, sadly I wasn't recording it during February 2021 when it was actually cold. I might be able to use this along with some calibration as you described by manually reading my meter to do some calculations as you describe. My highest* "demand" figure seems to be about 30% when we had about 13.5 degree days (from degreedays.net using 15.5C as a base), so that's an average temp of 2C for the day, and we needed 30% of whatever this boiler power measurement comes out to be.

The name plate power of our radiators at DT50 is about 10kW (after I upgraded a couple two years ago and plus a bit for uninsulated piping losses), so if we take that as the boiler power, which I will measure when I get to it, then that would mean that we required 3kW at 2C. An interesting exercise to be sure, albeit with shaky data and a system that was very much not run like a heat pump would be.

Also I commend you on your tiny heat loss!

*The actual highest demand is 44%, but I checked and that's a day when we came back from a trip in mid January (3C outside temp) and we'd had the house set to ~12C whilst we were away, so it probably doesn't count.


   
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