ASHP - using a low loss header and modulation instead of a buffer

[gravelld]

I think I've found an ASHP installer I'm happy with, but one aspect of his design rests a little uneasy on me.

 

Generally we have gone for slightly oversizing the heat pump and having a lower flow temperature to maximise the COP. The pump is a Midea 16kW. The house is a retrofit where we are swapping out an oil boiler. No UFH, all radiators. The heat loss and modelling has shown only one radiator needs to be enlarged.

 

The bit that feels uneasy is that his design doesn't include a buffer tank for the space heating. Having been reading a lot of BuildHub in the past few years, it seems to me that default advice is to have a buffer tank to reduce compressor restarts (improving the life of the pump) and reduce short cycling.

 

Instead of a buffer tank, the installer recommends "low loss headers or plate heat exchangers, both of which act as hydraulic separation".

 

I am not a heating engineer, so I don't really understand the significance of LLHs totally. Reading around, it appears to be a way of allowing two circuits to run at different speeds.

 

I am told Midea heat pumps can operate a second pump, and this is where the LLH comes in. Combined with modulation down to 15%, they think that a buffer tank is not required.

 

Can anyone help me understand this better, or maybe give some ideas to probe a little more with further questions of the installer?

[Marvin]

For me, this is a little confusing. 

 

Low loss headers will, as I understand it, allow circulation of the water even when the radiators have turned themselves off (assuming thermostatic radiator valves are used) however, modulation as I understand it, is the turning on and off of the heating (in this case) to meet demand. It is known that an air to water ASHP should run for at least 10 minutes before stopping to achieve the best COP result. 

 

Having an ASHP and radiators myself I was advised that the water in the buffer tank adds to the system volume thereby acting as a storage so when the cold water returning from the radiators is added the buffer water temperature slowly lowers delaying the ASHP kicking in. Like wise having the buffer means that heating up requires longer therefore aiding the COP.

 

[Marvin]

Try reading pages 61,62 and 68 for info.

 

https://forum.buildhub.org.uk/applications/core/interface/file/attachment.php?id=50188Fetching info...

[ReedRichards]

  On 16/12/2021 at 19:12, gravelld said:

...Generally we have gone for slightly oversizing the heat pump and having a lower flow temperature to maximise the COP. The pump is a Midea 16kW. The house is a retrofit where we are swapping out an oil boiler. No UFH, all radiators. The heat loss and modelling has shown only one radiator needs to be enlarged.

 

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That seems highly unlikely.  If it's true then your house must heat up very rapidly when your oil boiler turns on and once your house is at temperature the boiler probably runs in short on/off cycles.  Be prepared for a very much slower change in room temperatures when you get the heat pump.

[joth]

Will you have TRVs on the radiators? If so you'd be highly advised to get the buffer tank, as the total system volume reduces (probably quite substantially) when several of the TRVs close off, and then the ASHP will very likely short cycle. This will be accentuated if there's any variability in the house e.g. on a mild winter day most rooms keep a nice temperature but one or two get more cold because of draft/wind/exposure or some-such.

Oversizing the ASHP and having it work at the very minimum end of its operating  range further increases the odds of this happening.

 

You can use smart TRVs and have the system only call for heat when some majority of them are on, but that means letting those few cold rooms get even colder before there's enough demand. And often ppl with smart TRVs actually WANT to let some rooms get cold (e.g. if they're unused guest room, or all bedrooms during the day, etc) and again a buffer tank will help a lot with that sort of usage pattern.

 

 

On the flip side, we have LLH and UFH and that works fairly well for the designed mode of operation. In a (retrofit) passive house there's less need to have room-by-room control so we just run the whole ground floor UFH as one zone and it's enough that the 8.5kW ASHP generally does a 20min on / 5min off cycle, which is ~OK, and not sure how much a buffer would help with that.  But I still wish we'd done a buffer tank tough for secondary reasons I'd not considered during initial design (e.g. getting more value out of cheap rate electricity by charging the buffer up to a higher temp, using a (small) FCU for cooling in summer, etc)

 

 

[ReedRichards]

  On 17/12/2021 at 08:41, joth said:

You can use smart TRVs and have the system only call for heat when some majority of them are on, but that means letting those few cold rooms get even colder before there's enough demand. And often ppl with smart TRVs actually WANT to let some rooms get cold (e.g. if they're unused guest room, or all bedrooms during the day, etc) and again a buffer tank will help a lot with that sort of usage pattern.

 

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I have a Drayton Wiser heating controller.  I tried using this with Drayton smart TRVs but if any one of these calls for heat then the system remains on, even if room thermostat is satisfied.  There is no facility for "majority voting" for heat and no facility to give the room thermostat precedence over the TRVs.  In the Drayton Wiser implantation smart TRVs are only useful if you want a room to be cooler some of the time (such as bedrooms during the day).  All TRVs and no room thermostat might work better but then there is more risk of only one radiator calling for heat.  

[J1mbo]

15% is a good ratio. Though the COP would likely be a lot higher mid season meaning the output is likely more like 25% or more?

[gravelld]

  On 16/12/2021 at 20:44, ReedRichards said:

That seems highly unlikely.

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I kind of agree, but that's what the model says...

 

I'll double check it.

[gravelld]

  On 17/12/2021 at 08:41, joth said:

Will you have TRVs on the radiators? If so you'd be highly advised to get the buffer tank, as the total system volume reduces (probably quite substantially) when several of the TRVs close off, and then the ASHP will very likely short cycle. This will be accentuated if there's any variability in the house e.g. on a mild winter day most rooms keep a nice temperature but one or two get more cold because of draft/wind/exposure or some-such.

Oversizing the ASHP and having it work at the very minimum end of its operating  range further increases the odds of this happening.

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We have a fairly even split of traditional TRVs and smart ones (Heat Genius) which can be time programmed. The latter are used in the rooms we use most often. We also have two towel rails that are always open.

 

I was told about 50l of water circulating was about right. I haven't measured the distribution network fully, but could give a ball park estimate. In terms of the rads alone it looks like about 150l but obviously that's only relevant when the valves are open.

 

  1 hour ago, joth said:

On the flip side, we have LLH and UFH and that works fairly well for the designed mode of operation. In a (retrofit) passive house there's less need to have room-by-room control so we just run the whole ground floor UFH as one zone and it's enough that the 8.5kW ASHP generally does a 20min on / 5min off cycle, which is ~OK, and not sure how much a buffer would help with that.  But I still wish we'd done a buffer tank tough for secondary reasons I'd not considered during initial design (e.g. getting more value out of cheap rate electricity by charging the buffer up to a higher temp, using a (small) FCU for cooling in summer, etc)

 

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How does one get a gut feel for the size of buffer tank?

[gravelld]

  On 16/12/2021 at 19:48, Marvin said:

Low loss headers will, as I understand it, allow circulation of the water even when the radiators have turned themselves off (assuming thermostatic radiator valves are used) however, modulation as I understand it, is the turning on and off of the heating (in this case) to meet demand. It is known that an air to water ASHP should run for at least 10 minutes before stopping to achieve the best COP result. 

 

Having an ASHP and radiators myself I was advised that the water in the buffer tank adds to the system volume thereby acting as a storage so when the cold water returning from the radiators is added the buffer water temperature slowly lowers delaying the ASHP kicking in. Like wise having the buffer means that heating up requires longer therefore aiding the COP.

Expand  

 

From what I read LLHs can be used in a number of different scenarios. The installer is saying the two pumps on either side of the LLH can be run at different speeds and in addition the power output can be modulated.

 

Thanks for the rule-of-thumb about running time and also the link to the study - I'll read in more detail later.

[Marvin]

  On 17/12/2021 at 10:36, gravelld said:

From what I read LLHs can be used in a number of different scenarios. The installer is saying the two pumps on either side of the LLH can be run at different speeds and in addition the power output can be modulated.

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Quite right. But hopefully you'll read the info about the effects of the volume of water and modulation....

 

Good luck.

 

Marvin

[Marvin]

  On 17/12/2021 at 10:32, gravelld said:

How does one get a gut feel for the size of buffer tank?

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 You could read the info as it discusses buffer tank sizes and I think hot water tank sizes as well.

[SteamyTea]

  On 17/12/2021 at 10:32, gravelld said:

 

We have a fairly even split of traditional TRVs and smart ones (Heat Genius) which can be time programmed. The latter are used in the rooms we use most often. We also have two towel rails that are always open.

 

I was told about 50l of water circulating was about right. I haven't measured the distribution network fully, but could give a ball park estimate. In terms of the rads alone it looks like about 150l but obviously that's only relevant when the valves are open.

 

 

How does one get a gut feel for the size of buffer tank?

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By working out the power needed and runtimes. Then compare it to what the heat pump manufacturers recommend.

Or put in 100 lt if you must work with a gut feeling. Oversize is better.

[gravelld]

Sorry, yeah, not sure why I said "gut feel" - not much help!

[J1mbo]

Sometimes the manufacturer will specify the minimum buffer size based on defrost requirements.

[ReedRichards]

  On 17/12/2021 at 10:21, gravelld said:

 

I kind of agree, but that's what the model says...

 

I'll double check it.

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If your radiators were specified to work with a condensing boiler with 50 C flow and 30 C return then it's likely they would be suitable for your heat pump.  However if you have an older property where the condensing boiler was a retrofit for a non-condensing boiler then it's highly unlikely that your radiators will be suitable.

[joth]

  On 17/12/2021 at 10:04, ReedRichards said:

 

I have a Drayton Wiser heating controller.  I tried using this with Drayton smart TRVs but if any one of these calls for heat then the system remains on, even if room thermostat is satisfied.  There is no facility for "majority voting" for heat and no facility to give the room thermostat precedence over the TRVs.  In the Drayton Wiser implantation smart TRVs are only useful if you want a room to be cooler some of the time (such as bedrooms during the day).  All TRVs and no room thermostat might work better but then there is more risk of only one radiator calling for heat.  

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Yes, it's a good point that not all smart TRVs are alike. I assumed they all had some kind of sensible call for heat behaviour but obviously not! With loxone it's fully (and infinitely) configurable, but be default a heat source only activates when >30% of zones are open.

It sounds like the Drayton system is functionally no better than dumb TRVs in this respect: can help avoid a room overheating, but not address the "one room is too cold" scenario.

 

 

  On 17/12/2021 at 10:32, gravelld said:

How does one get a gut feel for the size of buffer tank?

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Agree with the other comments on calculating it, but I've seen (but now can't find) a "rule of thumb" sizing on here of 10-15L per kW of heatpump output, which "feels" reasonable?

I stumbled on https://homemicro.co.uk/download/lzc_buffer.pdf which says:

"BS EN 14511 recommends for the purpose of defrosting, and as a guide, a buffer tank should be sized on approximately 25 litres per kW output of the heat pump."

which sounds on the higher side.

 

 

 

[SteamyTea]

  On 17/12/2021 at 15:24, joth said:

 "rule of thumb" sizing on here of 10-15L per kW of heatpump output, which "feels" reasonable?

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Not to me it does not.

You need to work out how long the heat source will run for.

Water takes 0.0116 kWh.kg^-1.K^-1

So 10 lt will take 0.116 kWh.K^-1

So it also depends on the hysteresis you are working to.  5 K would be fine, 2 K would not.

[gravelld]

  On 17/12/2021 at 14:46, ReedRichards said:

 

If your radiators were specified to work with a condensing boiler with 50 C flow and 30 C return then it's likely they would be suitable for your heat pump.  However if you have an older property where the condensing boiler was a retrofit for a non-condensing boiler then it's highly unlikely that your radiators will be suitable.

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I worked my way through the 'model'.

 

Turns out the model was using an assumed W output given Δ50. We are looking at more like a 45C flow temp, so I think more like Δ30 would be more accurate?

 

Given that, I changed the model (it's just a Excel spreadsheet) and this gives over half of the rooms requiring larger rads.

 

So thanks for making me go back to check that...

[ReedRichards]

Thank heavens for that.  If the flow temperature is 45 C then typically the return temperature might be 40 C (heat pumps usually use a higher water flow rate than conventional boilers so the temperature differential across the radiators is less).  So if your room temperature is 21 C and your average radiator temperature is 42.5 C that means Δ21.5!  It's hugely important to get this right or you won't be able to get your dwelling warm enough in winter. 

[J1mbo]

A small four pipe buffer cylinder is just a large LLH really. So maybe that’s the happy medium.

[gravelld]

Just looking at this again and I think I might have made more inappopriate assumptions...

 

The W/m² for radiators in the original model are given as:

 

Towel               320
Single panel        640
Single convector    850
Double panel        1121
Double convector    1480
 

However, sources such as http://simplifydiy.com/plumbing-and-heating/radiators/power suggest this is less than the expected output for Δ50. I found some more sources and the figures are all over the place.

 

Furthering the trouble is that these figures are hardcoded in th emodel so I can't see how they are derived. There's no reference to the flow temperature (and subsequent delta) for example.

 

When I recalculated for a lower delta, I used the above figures as the start point and used a correction factor. But this is invalid if the original figures are not for Δ50.

 

I've found it quite difficult, surprisingly, to find a straight calculator for W given delta and w/h of the radiator. Does anyone know one?

[SteamyTea]

  On 02/01/2022 at 16:06, gravelld said:

w/h

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What is this?

[gravelld]

Width and height - sorry need to be more careful.

[Marvin]

2014 04 09 Kudox Steel Panel Radiators Technical Data.pdfFetching info...

 

This may help.