ASHP + Buffer Tank Questions

[Mr Blobby]

After reading through the very informative and helpful posts on here and other forums I'm left with some schoolboy questions about ASHP and buffer tanks. 

 

So, for background, we will be building a 280 m2 passive house with single zone ufh downstairs heated by (probably 5kw panasonic) ashp with a buffer tank.

 

As I understand it, it is the buffer thermostat that calls for heat from the ASHP, thus eliminating short cycling that would otherwise arise from the low heating requirement.  The room thermostat triggers only the pump in the UFH manifold. (and pump between buffer and manifold?)  (I'm ignoring the DHW loop here and the switching between heating and hot water)

 

1.  Is my understanding of the basics correct? 

2.  Would the buffer thermostat require a wide hysterisis, or maybe two thermostats one at top the other at bottom of tank?

3.  Some buffers have a coil (like @joe90's), while others don't.  Which one is best?  Which one is easiest to set up?

4.  For buffer tank without a coil then should it be configured with a 2-pipe or 4-pipe connection, or does it really make much difference?

5. Does the panasonic range of ASHP allow seperate water temperatures to be set for DHW and heating?

6. Some ASHP require a minimum volume, how does a buffer tank with a coil, hence very low volume, satisfy that requirement?

7. Does a buffer tank with a coil support a defrost cycle ok?

 

Thanks in advance for all comments.

 

Edited November 29, 2021 by Mr Blobby

[SteamyTea]

1 yes.

2 reasonably wide within ideal flow and return range.

3 a coil will hydraulically separate the ASHP from the rest.

4 I think a 2 port is really a volumiser and goes in series with a pipe, 4 port goes in parallel.

Go 4 port.

5 don't know  but would have thought so. No need to go via the buffer for DHW, but I think you know that from your earlier comment.

6 I think it depends on which way around it is plumbed, which would make it more of a thermal store.

It also depends on how fast the discharge and recharge rates are i.e power out v power in.

7 no Idea, but I would have thought so.

 

8 I don't really know much about the nuts and bolts of installations, and I know that some things can seem counterintuitive in plumbing.

Space heating and DWH are different things, at different temperatures and at different times. The more they can be separated the better in my opinion.

[joe90]

Yes I have a buffer with a coil, which means you only need antifreeze in the ASHP/coil part. I use my buffer stat to call for heat and I designed mine so that the buffer tank is heated permanently during the heating part of the year, the room stat calls for heat when the UFH is needed. The downside of this is the buffer is full of heat all the time but means the UFH heats up quicker . My Carrier unit only has one temp setting but the DHW tank stat is set to 48’ and the buffer stat set to 35’. With regard the ASHP/coil being large enough I did buy an expansion tank to put in cct but never got round to fitting it, the pressure can go up and down a bit due to heating.

[PeterW]

1 hour ago, joe90 said:

buffer stat set to 35’

I would set it to 48°C, let the UFH blend off the tank and it will shorten the trigger points between the ASHP calls when it’s ramping down toward the UFH set point. 

[joe90]

2 hours ago, PeterW said:

I would set it to 48°C, let the UFH blend off the tank and it will shorten the trigger points between the ASHP calls when it’s ramping down toward the UFH set point. 

My thinking was the lower the temp the better the COP, ?

[Mr Blobby]

20 minutes ago, joe90 said:

My thinking was the lower the temp the better the COP, ?

 

That's what I would have thoiught too.  ?

 

3 hours ago, joe90 said:

 The downside of this is the buffer is full of heat all the time but means the UFH heats up quicker .

 

This would also be the case with a buffer tank without a coil, would it not?  Both types of buffer tank have a stat to call heat from the ASHP.  So both systems would heat the buffer tank when the heating system is on.  Is that right?

 

 

[SteamyTea]

This topic has got me thinking a about having a hot buffer.

 

Generally, the day tend to start cooler than they finish, e.g. the first 3 hours are, on average cooler than the last 3 hours.

There are obvious exception to this i.e. a warm weather front moving in.

Now if you start with a buffer that is at the slab temperature, pump some energy in until it gets to maximum operating temperature, then start to reheat the slab, while at the same time still putting energy into the buffer when needed, you will end the day with a buffer hotter than it needs to be.

So maybe the answer is to stop adding energy to the buffer at set time, and just let the slab draw what it can.

This could be made very sophisticated with rate of change comparators, but I think maybe a simple thermal lockout is all that is needed i.e. @7PM if buffer is at max temp, stop adding heat to it, if @7PM buffer is not at max temp, reheat for 30 minutes, then check again.

[joth]

A buffer tank makes a lot of sense if you ever want to use cheap rate electricity (e.g. Octopus Go currently gives 4 hours at 5p/unit)

 

A buffer obviously allows maximum runtime during the cheap rate window that can then be pushed through the UFH when needed/more useful later in the day, but also it allows the ASHP to run at a higher energy consumption by storing the water at a hotter temp (e.g. 45ºC as suggested here) which both maximizes the amount of useful work done during the cheap-rate window, and also increases the storage potential of the buffer vessel (i.e. a small tank can store a the same energy as a larger one by holding water at a higher temp).

Skimping on a buffer tank was definitely a mistake in our build, and I'm struggling to make space now to install one. (The plant room layout was not well thought out, and it's bit of a mess to try and retrofit it).

 

 

[joe90]

32 minutes ago, Mr Blobby said:

So both systems would heat the buffer tank when the heating system is on.  Is that right?

In my case ( I designed it so may not be typical). The heating is never “off” during the heating season, buffer permanently at temperature topped up when buffer tank stat calls for heat. Room stat when it calls for heat just runs the pump on the manifold. Any loss of heat from the buffer tank sat there leaks into the house anyway (during the heating season) so it’s not wasted. Because the ASHP does not have to heat the buffer before the UFH gets the heat it’s calling for the warm up time is shorter.

[SteamyTea]

48 minutes ago, joth said:

A buffer tank makes a lot of sense if you ever want to use cheap rate electricity (e.g. Octopus Go currently gives 4 hours at 5p/unit)

Trouble is, unless you only need a handful of kWhs/day, you are into thermal store territory.

Also, if you could rely on a 5p/kWh (equivalent) tariff long term, just fit a resistance heater/s into a large store.

Edited November 29, 2021 by SteamyTea

[Bramco]

We need an student to take on a final year project to trawl through all the really good advice on the forum to come up with some good design patterns or rules of thumb for this.

 

Given a house with x m2, y m3, z heat loss and the price of electricity, cost of a ASHP (parts and installation separately) vs resistive heating (Willis heaters) etc.  then a rating for each design pattern in terms of simplicity of implementation, upfront costs against lifetime costs etc.

 

Anyone here a lecturer at a college (sorry university these days..) with some students up for a challenge?

 

Simon

[SteamyTea]

5 minutes ago, Bramco said:

Anyone here a lecturer at a college (sorry university these days..) with some students up for a challenge

Not any more.  I miss it sometimes, but generally I grew to hate it.  Was not the students, was the piss poor management.

[joth]

1 hour ago, SteamyTea said:

Trouble is, unless you only need a handful of kWhs/day, you are into thermal store territory.

Also, if you could rely on a 5p/kWh (equivalent) tariff long term, just fit a resistance heater/s into a large store.

We need about 25kWh per day, so would be quite doable with a 8.5kW ASHP in a 4 hour window, if I could run it right up at that max power.

EDIT:  although yeah take the point about it being a TS - I'd need a 300L buffer at 55ºC to store 12kWh, assuming I can put the rest of it directly into the slab as it's running. Hmmmm.

 

Definitely agree that there's no point designing a system specially around the existence of octopus go, but my point is if you have the buffer tank it gives flexibility to adapt as tariffs come and go (or indeed, make better use of self generation too), and adds little downside even if you don't need it. E7 has been around a long time and word is dynamic pricing is only going to increase in availability, so sooner or later it would come in useful

Edited November 29, 2021 by joth

[SteamyTea]

10 minutes ago, joth said:

dynamic pricing is only going to increase in availability, so sooner or later it would come in useful

I can see a misselling scandal  coming.

[Nickfromwales]

5 hours ago, joe90 said:

My thinking was the lower the temp the better the COP, ?

Its still pretty good as it can be assumed that, with the correct set points on the buffer stat, the return temp to the ASHP would never be 'cold'.

I think I am a fan of letting the ASHP run 'long and low', eg matching output to actual requirement in real time.

[muhrix]

On the topic of buffer tanks and whether or not they are necessary/recommended.

 

Is there any scenario where a buffer tank is not a good idea?

Lots of posts to sieve through on this topic; apologies if this has been answered elsewhere and I haven't come across that yet.

 

In my case, I have a heat demand of 13.3 kW and will get a 14kW Mitsubishi Ecodan ASHP. It's a monobloc and I was initially told it didn't require a buffer tank (one of the reasons to choose this model since we don't have space).

 

My installer is now suggesting a 50-litre buffer tank with an explanation that it would help with efficiency and reduce the amount of starts needed.

While those are valid points, I believe I can achieve good efficiency (COP) and ensure flow temperature is such that the ASHP does not cycle too often.

 

The downsides of a buffer tank (not a heat a heat store), IMHO and for my case, are:
- One extra pump consuming electricity and making noise (which due to installation location, will be noticeable, particularly during the night)

- The tank will invariably dissipate heat (and at the moment the only place I could have it is the attic, which would be a waste)

- The thermostat and smart TRVs I plan to use would not be directly calling the ASHP for heat (flow/return temp. of radiators would not always match the ASHP)

 

Any suggestions on where I can better inform myself on why and when buffer tanks are required, how to avoid them, or whether I should convince myself I have to have one?

[jack]

On 29/11/2021 at 10:52, Mr Blobby said:

So, for background, we will be building a 280 m2 passive house with single zone ufh downstairs heated by (probably 5kw panasonic) ashp with a buffer tank.

 

For comparison, we have a 289m² house with similar insulation etc, and a 5kW Panasonic Aquarea that's about 6 years old.

 

1.  Is my understanding of the basics correct?

Yes. 

 

2.  Would the buffer thermostat require a wide hysterisis, or maybe two thermostats one at top the other at bottom of tank?

Not sure why you'd need two thermostats.

 

3.  Some buffers have a coil (like @joe90's), while others don't.  Which one is best?  Which one is easiest to set up?

Using a coil means you only need anti-freeze in the loop to the ASHP. You can get away with just corrosion inhibitor in the UFH circuit, which is a lot cheaper.

 

4.  For buffer tank without a coil then should it be configured with a 2-pipe or 4-pipe connection, or does it really make much difference?

Not sure.

 

5. Does the panasonic range of ASHP allow seperate water temperatures to be set for DHW and heating?

Yes, I believe they all do. Mine has a minimum settable heating temperature of 25 degrees, which is about right. I have it set with mild weather compensation - it runs at 25 degrees when the weather is about (from memory) 7 degrees, and then increases the temperature up to (again from memory) 29 degrees when the temperature is a 0 degrees or below. I wasn't sure that this was really needed, but the recent cold spell has shown that it does work. The house was a little cool for the first day or two until the slightly higher water temperature caught up.

 

6. Some ASHP require a minimum volume, how does a buffer tank with a coil, hence very low volume, satisfy that requirement?

The coil acts as a heat exchanger, and anything it exchanges heat with can be considered part of the same hydraulic system. Assuming the system is set up and working properly, the ASHP can't tell whether you're directly heating water in a tank, or transferring the heat into the tank via a coil. 

 

7. Does a buffer tank with a coil support a defrost cycle ok?

Yes, shouldn't really make much difference. 

 

I don't know how big a DHW tank you're planning, or whether you're planning PV. We have a 250L tank, which is arguably a little small for the size of house, especially given we have a lot of PV. However, we also have a decent shower waste water heat recovery unit, and I'm absolutely convinced that this is why we very rarely run short of hot water. They don't cost much and are a very low tech + low maintenance way of reducing energy consumption.

 

On 29/11/2021 at 16:38, joth said:

A buffer tank makes a lot of sense if you ever want to use cheap rate electricity (e.g. Octopus Go currently gives 4 hours at 5p/unit)

...

Skimping on a buffer tank was definitely a mistake in our build, and I'm struggling to make space now to install one. (The plant room layout was not well thought out, and it's bit of a mess to try and retrofit it).

 

We're on the Octopus Go tariff, and I've been thinking about this too recently (because it's gotten cold, basically!).

 

Given the thermal capacity of the slab versus that of a buffer, I'm genuinely unsure what impact this would have compared to intentionally allowing the slab to "overheat" by a degree or two during cold spells. 

 

My initial rough analysis concluded that raising the temperature of a 200L buffer from 20 degrees to something like 40 degrees would require something like the same energy as raising the slab from 20 to 22 degrees. I'd be interested to see what sorts of numbers others are getting for this.

[joth]

18 minutes ago, jack said:

My initial rough analysis concluded that raising the temperature of a 200L buffer from 20 degrees to something like 40 degrees would require something like the same energy as raising the slab from 20 to 22 degrees. I'd be interested to see what sorts of numbers others are getting for this.

This makes sense, but the one thing you need to remember is you only have 4 hours to achieve this (if using Octopus Go cheap rate).

For our house, without a buffer I can only run the UFH flow at ~30º which means the ASHP is only drawing about 1kW. So over 4 hours I achieve 4kWh of "cheap rate" energy draw.  However with a buffer, I can run the ASHP flow at max temp say 55º which means the heat pump is drawing about 3kW, thus would give me about 12kWh of cheap rate usage. This does require a 300L tank though as mentioned ?

Ideally a big TS would act as both DHW and heating store, and over that 4 hour period I'd run ASHP and (if appropriate) the immersion heater at full whack to maximize my energy draw during that cheap rate period.

 

However, that's not what I've tot, so I've given up on this whole idea, and now just looking into batteries  

(And indeed, pushing the slab up 2ºC during cheap rate. The Loxone "Cheap rate heating" input on Climate controller seems to do a good job for this)

 

 

[jack]

27 minutes ago, joth said:

This makes sense, but the one thing you need to remember is you only have 4 hours to achieve this (if using Octopus Go cheap rate).

For our house, without a buffer I can only run the UFH flow at ~30º which means the ASHP is only drawing about 1kW. So over 4 hours I achieve 4kWh of "cheap rate" energy draw.  However with a buffer, I can run the ASHP flow at max temp say 55º which means the heat pump is drawing about 3kW, thus would give me about 12kWh of cheap rate usage. This does require a 300L tank though as mentioned ?

 

I hadn't really thought about those limitations, but I guess 4 hours isn't a lot of time in which to stuff an entire day's heating into a slab using a low-temperature-difference heat source. I'll one day get around to putting energy monitoring in place so I can see what my ASHP is actually doing all day! 

 

29 minutes ago, joth said:

(And indeed, pushing the slab up 2ºC during cheap rate. The Loxone "Cheap rate heating" input on Climate controller seems to do a good job for this)

 

I'd completely forgotten about that option. Definitely worth looking into now that we have Octopus Go.

 

[Dan F]

On 29/11/2021 at 10:52, Mr Blobby said:

As I understand it, it is the buffer thermostat that calls for heat from the ASHP, thus eliminating short cycling that would otherwise arise from the low heating requirement.  The room thermostat triggers only the pump in the UFH manifold.

 

This depends on the ASHP control system I thnk.  Our ASHP doesn't requie a buffer temperature sensor.  It may use the the return temperature from the buffer to decide when to call for heat from the ASHP, I'm not 100 sure, but otherwise the buffer is just adding volume to the system rather than performing any other function.

Edited December 2, 2021 by Dan F

[Dan F]

On 29/11/2021 at 12:04, joe90 said:

Yes I have a buffer with a coil, which means you only need antifreeze in the ASHP/coil part

 

Our whole system;, buffer, UFH any everything was initially going to be all glycol, but heating engineer changed his mind and is now using 100% water with some special valves to prevent damage to the ASHP in the case of freezing conditions with the ASHP off.  It's a simpler approach and water is better at transporting heat than glycol too.

https://www.seconrenewables.com/heat-pump-anti-freezing-valve-7537-p.asp

[Dan F]

On 29/11/2021 at 16:38, joth said:

A buffer tank makes a lot of sense if you ever want to use cheap rate electricity (e.g. Octopus Go currently gives 4 hours at 5p/unit)

 

A buffer obviously allows maximum runtime during the cheap rate window that can then be pushed through the UFH when needed/more useful later in the day, but also it allows the ASHP to run at a higher energy consumption by storing the water at a hotter temp (e.g. 45ºC as suggested here) which both maximizes the amount of useful work done during the cheap-rate window, and also increases the storage potential of the buffer vessel (i.e. a small tank can store a the same energy as a larger one by holding water at a higher temp).

Skimping on a buffer tank was definitely a mistake in our build, and I'm struggling to make space now to install one. (The plant room layout was not well thought out, and it's bit of a mess to try and retrofit it).

 

Don't disagree, but I think in your case the lack of a buffer is more of an issue with you fancoils than with UFH, right? If you just had in-slab UFH then there wouldn't really be that much value in  a large buffer would there, either for storing heat from off-peak electricity or to minimize short-cycling?

Edited December 2, 2021 by Dan F

[joth]

4 minutes ago, Dan F said:

 

Don't disagree, but I think in your case the lack of a buffer is more of an issue with you fancoils than with UFH, right? If you just had in-slab UFH then there wouldn't really be that much value in  a large buffer would there, either for storing heat from off-peak electricity or to minimize short-cycling?

 

Yes undersized fan coil for the ashp is definitely the main reason, but trying to maximize energy use during the short 4 hour cheap rate is a secondary benefit if we had it. If we had fixed rate energy 24/7 it would be much better to just let the ashp run any time it needed to at a low temp to warm the slab.

[Dan F]

 

3 hours ago, joth said:

For our house, without a buffer I can only run the UFH flow at ~30º which means the ASHP is only drawing about 1kW. So over 4 hours I achieve 4kWh of "cheap rate" energy draw

 

How do you get just 1kW? What flow rate is this based on?

[Marvin]

On 29/11/2021 at 16:45, joe90 said:

In my case ( I designed it so may not be typical). The heating is never “off” during the heating season, buffer permanently at temperature topped up when buffer tank stat calls for heat. Room stat when it calls for heat just runs the pump on the manifold. Any loss of heat from the buffer tank sat there leaks into the house anyway (during the heating season) so it’s not wasted. Because the ASHP does not have to heat the buffer before the UFH gets the heat it’s calling for the warm up time is shorter.

This is what I am working towards...