ASHP with 1-2000L thermal store

[Tommytipee]

Anyone know how well a ashp will work with a large thermal store setup?.

 

I’m a uk gasfitter currently living in New Zealand and about to buy a house and have to fit some central heating. Underfloor is not an option for retrofit so I’m left with rads as the only option. 
 

costs per kWh are:

LPG 14p

Diesel 15p

elec 11.75p

 

elec is renewable here mainly hydro and has not risen in cost during the 2 years I’ve been here so hopefully won’t be as inflationary as the others. Diesel has doubled since January and kerosene is dearer than diesel. 
 

Nightime temps go as low as -4 daytime is normally sunny 5-15 degrees. 
 

Because of the daytime temps I’m looking into fitting a heat pump to charge the thermal store can get 1-2000L tanks. Demand will be roughly 15kw over 10 rads. Insulation is not as good as in the uk and timber frame so no thermal mass apart from the concrete slab but generally these are not insulated. Walls are 100mm timber stuffed with rock wall and ceilings rock wall about 250mm thick. House is 200m2. 
 

How will a heatpump work with thermal store effiency wise if I was to run it in the day charging the store then pump around the rads at night and morning. I will need 15kw output for probably 6hrs a day will 2000l heated to 60/70 degrees do this?. I know heat pumps don’t work best at higher temps but looking at the kWh pricing even if it runs at a COP of 1 it’s still cheaper than gas or diesel. We get about 5 days a week of 6/7hr days sun as well in winter so solar is an option although the upfront costs for solar, 2 massive buffer tanks and a vaillant arotherm plus 16kw is going to be £20k plus. Winter is about 4 months long I need about 700kwh a week so about 11,200kwh a year. 
 

Looking online 2000L of water heated from 20-70 degrees takes 116kw how do I work out how long the thermal store will supply the rads for I’m guessing I will get roughly 116kw back out obviously at the end it will be pumping 20 degrees water back out so pretty useless at that point. 

Edited July 9, 2022 by Tommytipee

[PeterW]

You won’t get 70°C out of an ASHP. Why not resize the rads to use the lower temperature of the ASHP, put a 400 litre UVC in and then use any solar if you install to dump to the UVC..? Much better than a big thermal store. 

[MikeSharp01]

Welcome to THE forum for people like us! Can't really see what a big thermal store brings you here unless your electricity has a night rate or some such to allow you to move the demand window. As @PeterW says if you just need DHW to be stored then you only really need that at a worst case volume, rather than vast amounts of losses in a big system, and you can always fit an immersion heater to top it up quickly if needed.

[JohnMo]

Dump your current radiators, double the size at least, by using twin panels, with fins.  Operate the heat pump at a much lower temp to get a decent CoP.  Trying to charge the thermal store to that temp will get a rubbish CoP, but it is unlikely you would ever reach that temp so the heat pump will run for ever trying to a thermostat that is not reachable.

 

The above means you can dump the thermal stores, and get lower running costs.

 

As above UVC for DHW.

[Tommytipee]

I’m not using any of this for dhw this is solely for heating. I was thinking it’s more cost effective to use the ashp to heat in the day when it’s 7 degrees plus temp rather than when I need it and the temps are -4 to 3 degrees.  I was going to heat the thermal store to say 55 via heatpump and top up the rest via immersion. The other issue is installing the rads it’s all done by getting in the loft drilling down through the timber frame with 2m long drill bits and feeding the pipes down cutting a small hole behind the rad and feeding the pipes out. Other issue with larger rads for lower temps is amount of wall space there’s a lot more windows in kiwi houses and can only fit smaller rads not always 1800+ ones. 
 

I don’t like the MLCP pipe here either with crimp fittings so I wanted to use underfloor manifolds so I could run straight from manifold to each rad in one length with no fittings in the wall, this is also not going to fit well with running lower flow temps direct to rads I’m guessing or will it be if I ran the pipe in 20mm rather than 16mm to each rad?. (The furthest run from manifold to rad will be 25m in length shortest would be a 10m run. 

Edited July 9, 2022 by Tommytipee

[Tommytipee]

Edited July 9, 2022 by Tommytipee

[MikeSharp01]

9 minutes ago, Tommytipee said:

down through the timber frame with 2m long drill bits

Respect - getting that bang on over 2m!

[Tommytipee]

3 minutes ago, MikeSharp01 said:

Respect - getting that bang on over 2m!

Haha it’s not too bad. The pics above show it on a new build done at first fix phase but for retrofit it’s clambering in the loft drilling down through the top plate of the wall and any dwangs you encounter. Normally 2 but sometimes 3!.  This further limits where to put rads as can’t normally get onto outside walls as the slope of the roof where it meets the external walls mean that you can’t get a drill into the space let alone a 1m long drill extension. Just remove the plasterboard I hear you all cry, there’s a 6-9 month wait on sheets of plasterboard at the moment it’s causing carnage in the building industry.   🤣 

[Nickfromwales]

5 hours ago, PeterW said:

You won’t get 70°C out of an ASHP.

You can get 80+ out of a split  

5 hours ago, MikeSharp01 said:

Can't really see what a big thermal store brings you here unless your electricity has a night rate or some such to allow you to move the demand window.

I suppose the thoughts were aligned with how ‘we’ charge a passive raft with heat here, just the slight issue of additional waste heat losses from the TS become a deciding factor. Could still reap benefits from a dwelling which has PV + ASHP but little or no medium to store heat energy for slow release.

The TS may well be a good candidate if you went with a low-temp monoblock ASHP, and have already installed smaller radiators? If for space heating only, consider no TS, just a small buffer, a high-temp split perhaps?

Edited July 10, 2022 by Nickfromwales

[ReedRichards]

10 hours ago, Tommytipee said:

I’m not using any of this for dhw this is solely for heating. I was thinking it’s more cost effective to use the ashp to heat in the day when it’s 7 degrees plus temp rather than when I need it and the temps are -4 to 3 degrees.

 

I think most people have a blind spot to the benefit of running your ASHP at times of day when it's warmer outside.  It seems very sensible to me but it's just not a "done thing" here, perhaps because the day and night time temperatures don't vary so much.  Or perhaps because the only thermal store most ASHP owners have is their hot water cylinder.  But why go to all the trouble and expense of making your ASHP perform at peak efficiency and then squander your savings on topping up the thermal store temperature with an immersion heater?  And how are you dealing with DHW if not via the thermal store?   

[Tommytipee]

20 minutes ago, ReedRichards said:

 

I think most people have a blind spot to the benefit of running your ASHP at times of day when it's warmer outside.  It seems very sensible to me but it's just not a "done thing" here, perhaps because the day and night time temperatures don't vary so much.  Or perhaps because the only thermal store most ASHP owners have is their hot water cylinder.  But why go to all the trouble and expense of making your ASHP perform at peak efficiency and then squander your savings on topping up the thermal store temperature with an immersion heater?  And how are you dealing with DHW if not via the thermal store?   

This is what I’m trying to put across the daytime temps here are good so it makes sense to time shift the ashp to produce when it’s most productive. Without a slab of concrete I thought a massive lump of water is next best. 
 

I was only going to use the immersion if it makes sense to if the ashp can get to 70 degrees at a cop of higher than 1.0 then I’d obviously just use that. The hot water I would eventually plumb into the ashp but the standard cylinders here are immersion heated unvented ones and with the house probably being a new build it’s not worth the savings to rip it out and replace with a new cylinder when I don’t use much dhw anyway.  In 10 years when the cylinders gone I’d replace and integrate it then. 

Edited July 10, 2022 by Tommytipee

[ReedRichards]

Back here in the UK even gas boilers are shifting to operating at 50 C output.  Running your rads at 70 C flow is old fashioned and unnecessary, you just need finned radiators with a larger surface area for (say) 50 C flow.

[HughF]

Surface mount the mlcp and box in, no need to mess about with long drill bits. I know it looks a bit council house, but it looks fine in my council house 😂 

 

Ditch the idea of the TS, the losses will be crazy as they’re never insulated well enough. Upsize the rads for a 50 deg flow, job done. Add the odd fan coil unit if you want cooling in the summer time.

[JohnMo]

Or just rip out the rads and replace with fan coils for heating and possibly cooling. Then no need for bigger rads.

 

Possibly no need to do anything in bedrooms as they have a low heat demand anyway.  Leave rads as they are or use same size with fins at the back.

 

I would dig out the ASHP specs, and see what load shifting actually means in dollars over a 10 year period, to see if it makes sense based on the extra capital expenditure.  And add into the equation HT split as Nick mentioned.

Edited July 10, 2022 by JohnMo
Further details

[Tommytipee]

7 minutes ago, JohnMo said:

Or just rip out the rads and replace with fan coils for heating and possibly cooling. Then no need for bigger rads.

 

Possibly no need to do anything in bedrooms as they have a low heat demand anyway.  Leave rads as they are or use same size with fins at the back.

 

I would dig out the ASHP specs, and see what load shifting actually means in dollars over a 10 year period, to see if it makes sense based on the extra capital expenditure.  And add into the equation HT split as Nick mentioned.

What HT split unit is worth looking at?, I’ve only looked into vaillant mainly because it’s the only gas boilers I liked working on lol 

[Tommytipee]

Will running the rads at 50 degrees with a small buffer tank and underfloor manifold with take offs direct to the rads work ok on 16mm pipe over a 25m distance or no chance. I reckon biggest rad size will be 1400x600 t22 that will fit in the wall space. 

[SteamyTea]

Welcome

 

There seems to be a number of things going on here.

 

An ASHP, generally, does work better when the temperatures are higher, but they can take a dip in performance when the OAT (outside air temperature) is between 0°C and 4°C, this is due to the humidity.

 

The next thing is to do a proper calculation of the house's thermal losses, this is independent of the heating system.  It will give you a kW/°C_OAT value.

Without this, it is all guesswork.  Just knowing the worse case is of little usage as that case may only happen for a few days a year, and at 11p/kWh for electricity, you just plug in some fan heater.  OAT is usually normally distributed, so once you know the kW/°C, you can calculate the total energy, the kWh, for a day, a week, a month, a year.

 

Next is the choice and size of ASHP.  Heat Pumps all work the same.  They expand a gas, which cools, that cooled gas then gets warmed up and, during compression, warmed up even more.  Depending on the gases used, the operating temperatures i.e. the min and max, vary a bit, but not much in practical terms.  Never worth trying to work at the extremes, pick the middle 70%.  So if a manufacturer claims that the ASHP works down to -10°C and can deliver to 65°C, then work with a temperature range of 52.5°C, with the bias towards the lower end, so between -2°C OAT and 50.5°C delivery temperature.

50.5°C should heat any store to around 48°C comfortably.

 

Now the tricky bit starts, and highlights why you need to know what power your house uses.

All a thermal store, or buffer, does is allow the ASHP to run to a fixed target i.e. 48°C without having to overshoot that temperature.

While the ASHP is delivering energy, energy is taken out of the store to heat the house.  As the house warms, the return temperature from the radiators reduces.  The maximum efficiency is when half the energy is taken out.  So if you can deliver, after pipe losses, 46°C to the radiators, and knowing that your ASHP is delivering at 52.5°, half the difference is 6.5°, so the return temperature should be around 39.5°C.

The flow temperature may vary a bit depending on what temperature you like your house at.  Say you want your house at 22°C, the radiators have a mean temperature of 43°C, that is a temperature difference of 21°C.  Radiator manufacturers usually have a chart to show what size, for a given power output, their radiators will deliver, at different temperatures.  That will size your radiators (rule of thumb is that just over double the area for halving the supply temperature is pretty close).

That is the radiator sizing sorted.

 

Now the thermal store, or buffer tank.  All this does is allow the ASHP to run for a decent amount of time.  Think of it as the driving to a main road.  As you wiggle though the lanes (or sheep tracks in NZ) your fuel economy is dreadful, but once on the main road, you can drive faster, but much more consistently, and get a decent amount of distance out of a litre of fuel, even when you have to overtake a tractor, you have momentum on your side.

Like anything, there is always a compromise, store too much, at too higher temperature, you loose, store too little, at two low a temperature, you loose, and the ideal spot is constantly varying.

To make matter even harder, the standard units for domestic energy and power are similar, energy is the kWh, power is the kW.

Now you have worked out the power needed to warm your house for any given OAT, and hopefully accepted that an ASHP will not deliver at the lowest extremes.  So how do you size a store.

Well we have already worked out the flow and return temperature difference, 6.5°C, we know the temperature window we are working with.

Heating up a litre, or in real money, a kilogram, of water takes 4180 joules of energy, or 4.18 kJ.  That converts to 0.001161 kWh.  A small amount.

But of you have 1000 litres, or 1 tonne, then at 6.5°C temperature difference, that is 1000 [kg] x 6.5 [Δ°C] x 4.18 [kJ.kg^-1.K^-1].

That is 27,170 kJ, which works out as 7.5 kWh.

Not much for 1 tone of storage.  A 10 kW ASHP delivering at 70% of its performance, will take just over an hour to replenish that.

But of your house only needs 7 kW of thermal energy delivered, there is no need for a 1 m³ of water to act as a buffer, it can deliver that quite happily.

But as you say, there are times when it needs to deliver twice that.  In those scenarios, the ASHP will be working constantly, and the store will be depleted in half an hour, then the ASHP efficiency (the CoP) will start to decline.  Doubling the size of the store, will only buy you an extra half hour in those conditions.  So you would not be gaining much.

 

From that, I can see why you think a huge store is useful, and why using supplementary heating to raise it up to a higher temperature may be useful. But in my opinion, you may as well use that supplementary heating (your cheap 11p/kWh electricity) to heat the air in the house directly.  As I have said, these are the extremes and do not happen often.  If you had 'Canadian' winters, then you would just fit a larger ASHP that can deliver at those lower temperatures, for longer.

 

So, task list.

Work out the house heat losses and get the kW/°C number

Find out the historic temperature profile for your area

(this might help https://www.worldweatheronline.com/auckland-weather-history/nz.aspx)

Find out the power delivery of your existing radiators 

[Tommytipee]

Thanks that explains a lot. I’ll try and get some figures together for the build spec. You can all laugh at how bad kiwi houses are compared to Europe. 
 

Im glad you’ve worked out that the 1000L will only last for 30 mins as I can see that it’s pointless now. Before I didn’t have a clue how long it would last so that’s narrowed it down for me. Just been looking at the daiken altherma 3 HT are they any good?, I can’t find the COP figures at high flow temperatures anyone know what they are?.

Edited July 10, 2022 by Tommytipee

[SteamyTea]

6 minutes ago, Tommytipee said:

altherma 3 HT

May be called something else in UK.

 

10 minutes ago, Tommytipee said:

You can all laugh at how bad kiwi houses are compared to Europe. 

Work on improving it. Makes any heating problem smaller.

[MikeSharp01]

33 minutes ago, Tommytipee said:

You can all laugh at how bad kiwi houses are compared to Europe

On the contrary we are all crying cos we pay 2 or 3 times as much for our electricity as you do and pretty soon it will be even more. So you are the one who should be laughing.

 

[Tommytipee]

8 hours ago, SteamyTea said:

May be called something else in UK.

 

Work on improving it. Makes any heating problem smaller.

https://www.daikin.co.uk/content/dam/dauk/document-library/Brochures/Heating/Heating Installer brochures/Daikin Altherma 3 H HT_767.pdf

[SteamyTea]

@Tommytipee

Had a quick look on their site and can't find a performance curve either.

Pretty poor that is.

[Iceverge]

If dhw isn't a considerable why not use mini split A2A units. 

 

Great COP and cheap. 

[Tommytipee]

11 hours ago, Iceverge said:

If dhw isn't a considerable why not use mini split A2A units. 

 

Great COP and cheap. 

Not keen on them. Dry my eyes out and is hard to explain but it’s never a nice heat as soon as it turns off the room feels instantly cold. 

[joth]

1 hour ago, Tommytipee said:

Not keen on them. Dry my eyes out and is hard to explain but it’s never a nice heat as soon as it turns off the room feels instantly cold. 

Suggests the building is fairly drafty as well as low heat mass and low insulation. When you directly heat (or cool) the air with an a2a or FCU then it's very susceptible to all being blown away the moment you turn the supply off.

 

The fact it drys out the air too is hard to avoid, it's all to do with latent vs sensible heat and stuff I can never explain succinctly 😂: hotter air has lower relative humidity for the same absolute moisture content.

Edited July 11, 2022 by joth