ASHP Sizing

[ProDave]

We run our hot water at 48 degrees. Found by experiment to be the hottest I can hold my hands under, so no point having hot water any hotter than that.

 

As a back up / top up, we have a Stiebel Eltron instant modulating in line water heater.  It is set for an output of the same 48 degrees, so normally it does nothing, but if the HW tank is running low, then it will start heating and maintain the same temperature.

 

If you had a need for say 50 degree water it could be used as the lift to give that.

 

 

[PeterW]

8 hours ago, ragg987 said:

There's no way I would be buying one of those. I mean even if I had a full tank of hot water, but below the magic 50C, they would not work. Barking.

 

I'm surprised the EU has allowed this, it makes ASHP not usable.

 

Our thermostatic Hansgrohe is fine with lower temperature water.

Most thermostatic showers need a minimum hot temperature. Combi boilers run at 58-62 to ensure they run in condensing mode for long periods so it’s not unreasonable to set a minimum of 50c as ASHP is really a niche product for DHW. 
 

1 hour ago, JulianB said:

Are people using any particularly smart controllers on their immersion to take into consideration factors such as outside air temperature, as presumably passed a certain threshold it pays you to top up the final delta T with ASHP rather than immersion?

Nope... your payback would be decades and not only that, CoP and defrost are affected by humidity as well as temperature so you would need some very funky algorithms to work out how it would know not to use the immersion and use the ASHP. 

[dpmiller]

but other than poorly-set defrost parameters, is the COP of the average ASHP ever going to approach 1 never mind dip below it?

[joth]

Unless your CoP is going below 1.0 then the algorithm is simple: always use the ashp and never use the immersion.

 

The slightly funky variation would be to continuously set the ashp to "UVC current temperature +5°C" (say). This way you get a good CoP initially and slowly decreasing efficiency as it works harder to top it off with the final few kJ. (Analogous to the taller you build your pyramid, the more effort is needed to install the final blocks)

 

 

[Ed Davies]

59 minutes ago, joth said:

Unless your CoP is going below 1.0 then the algorithm is simple: always use the ashp and never use the immersion.

 

Yep.

 

59 minutes ago, joth said:

The slightly funky variation would be to continuously set the ashp to "UVC current temperature +5°C" (say).

 

AIUI, that's how they work in practice anyway. Each time the primary water flows through the heat pump it only rises a few degrees so, assuming the return temperature is pretty much the tank temperature the flow will be UVC +5 °C so you'll get the better CoP anyhow.

[ragg987]

1 hour ago, joth said:

The slightly funky variation would be to continuously set the ashp to "UVC current temperature +5°C" (say). This way you get a good CoP initially and slowly decreasing efficiency as it works harder to top it off with the final few kJ

This is the way our's works by default, the built-in controller has a parameter to control rate of heat. When set to efficient, it modulates to lowest level initially (which gives a deltaT of about 3 degC) and then increases progressively to reach about 50C. At setting standard it heats harder and faster.

 

Ours is a small 7kW unit (nominal), so recommended temperature is 50C max - I would guess this is to prevent it struggling to heat the last few degrees given the low (relatively) heat input at the top. Larger units from the same range are able to heat to 55C.

[ragg987]

3 hours ago, JulianB said:

On E7 it does sound like a worthwhile route to top up with immersion...Are people using any particularly smart controllers on their immersion to take into consideration factors such as outside air temperature, as presumably passed a certain threshold it pays you to top up the final delta T with ASHP rather than immersion?

The ASHP controller has the proportional control as standard and I use it to good effect. It monitors external, set and actual temperature to adjust flow temperature and circulator speed. There is an option to use the built-in electric heater to raise the temperature above what the unit is capable of, though I do not use it. I believe you can set it to come on at off-peak periods - again not tried so do check.

 

Our unit is from Hitachi. I looked at the usual suspects, Panasonic, Daikin, Samsung and some others. Two things drove me to this - first was the Hitachi controller was capable to running 2 separate heat zones natively (annexe for elderly parents needs to be warmer), and second was the support I received from Hitachi was head and shoulders above the others, at enquiry, design, installation and commissioning.

 

The down-side is local tradesmen are not familiar with this brand - Daikin yes, Hitachi no, so servicing might become an issue.

[JulianB]

22 hours ago, joth said:

Unless your CoP is going below 1.0 then the algorithm is simple: always use the ashp and never use the immersion.

 

The slightly funky variation would be to continuously set the ashp to "UVC current temperature +5°C" (say). This way you get a good CoP initially and slowly decreasing efficiency as it works harder to top it off with the final few kJ. (Analogous to the taller you build your pyramid, the more effort is needed to install the final blocks)

 

 

 

Thanks, my thoughts exactly. Could I ask what UVC stands for?

 

20 hours ago, ragg987 said:

The ASHP controller has the proportional control as standard and I use it to good effect. It monitors external, set and actual temperature to adjust flow temperature and circulator speed. There is an option to use the built-in electric heater to raise the temperature above what the unit is capable of, though I do not use it. I believe you can set it to come on at off-peak periods - again not tried so do check.

 

Our unit is from Hitachi. I looked at the usual suspects, Panasonic, Daikin, Samsung and some others. Two things drove me to this - first was the Hitachi controller was capable to running 2 separate heat zones natively (annexe for elderly parents needs to be warmer), and second was the support I received from Hitachi was head and shoulders above the others, at enquiry, design, installation and commissioning.

 

The down-side is local tradesmen are not familiar with this brand - Daikin yes, Hitachi no, so servicing might become an issue.

 

Thanks for the info...will definitely look into the Hitachi...I've heard that the Mitsubishi ecodan isn't too bad either? Would any brand lend themselves more towards DIY install? One aspect I forgot to mention is that the unit won't be terribly far away from the house so I'm hoping it won't be too loud!

 

23 hours ago, ProDave said:

We run our hot water at 48 degrees. Found by experiment to be the hottest I can hold my hands under, so no point having hot water any hotter than that.

 

As a back up / top up, we have a Stiebel Eltron instant modulating in line water heater.  It is set for an output of the same 48 degrees, so normally it does nothing, but if the HW tank is running low, then it will start heating and maintain the same temperature.

 

If you had a need for say 50 degree water it could be used as the lift to give that.

 

 

 

Thanks! I take it that this heater is primarily to combat your hot water running out in the tank rather than it being a cheaper way to get to the higher temps relative to ASHP CoPs? Would you say that by going to the lower 48degC storage temps, you'd lose any pressure / flow rate at the 'user' end i..e shower head based on the argument of typically one is able to hit their desired temp by mixing it with the cold feed and hence raising the overall flow rate? From memory, the last time I checked what the minimum flow my better half would be willing to accept, it was somewhere down at 11l/min?

 

Re. ASHP sizing, to make sure my sums are adding up...the spreadsheet suggests that Jan is my worst month according to mean lowest OATs...2213KWh. That gives me 71.4kWh per day. I then factor in that I have 3 women in the household (a wife, 4 yr old, 2 yr old, and 1 more on the way), I've set aside an extra 15kWh per day for them. The question is do I now divide by 7 and assume I will only use E7 tarrifs, or should I divide by 12 and assume half a days running? The difference in resultant Kw is 12.3 vs 7.2...and I have not applied any safety factor on that w.r.t. CH demand, however I assume the fact that the calcs are based off the worst case annual OATs inherently places some safety margin 

[ProDave]

No problem with flow rate from showers.  It's possible to empty the 300L tank in just over half an hours showering, which with 2 females in the house, is not a rare occurrence.

 

We don't have E7 but the ASHP is available to heat the house for roughly 12 hours.  I run ours in the day so it can soak up any PV generation and because I prefer a silent house at night (no UFH circulating pumps running)

[ragg987]

1 hour ago, JulianB said:

One aspect I forgot to mention is that the unit won't be terribly far away from the house so I'm hoping it won't be too loud!

We can't hear it from inside the house, it is approx 2m away from the north wall. We don't open windows on this side.

 

Our neighbors oil burner is much noisier than our ASHP when we are in the garden. When modulated down, you only hear it when close to the unit. Noisier at full power.

[ragg987]

1 hour ago, JulianB said:

The question is do I now divide by 7 and assume I will only use E7 tarrifs, or should I divide by 12 and assume half a days running?

The pump will be more efficient when not at full power. I run mine 24x7 and it usually just ticks over at 30% capacity, though when external temperature gets to freezing it will boost to 50 or 60%. We have a passiv standard build.

 

I also have solar PV and time DHW heating during daytime to maximize the advantage. I did the calcs and E7 was not cost effective for our usage.

[joth]

3 hours ago, JulianB said:

Could I ask what UVC stands for?

 

Unvented cylinder.  The 'unvented'  is not important here, I was just being lazy and not wanting to type Hot Water Cylinder / tank.

 

 

[JulianB]

Morning All,

 

Firing up this this discussion again as I've nearing the point of having to press the 'go' button on it all.

 

I've been doing a bit of research into what the system will look like with the buffer tank in series. From what I've read a buffer tank in the region of 70-90lt is the way to go, and this wants to be  heated to around 35-40degC as it is primarily supplying the UFH and in a temperature band which produces good COPs from the ASHP. I've stumbled across examples where the buffet tank is direct and indirect - when indirect the ASHP is plumbed to the coil, presumably to save on anti freeze, however would this also likely caused more frequent 'pulsing' from the ASHP seeing as there is a smaller volume of anti freeze compared to the direct cylinder?

 

 The other aspect I'm still slightly puzzled on is getting the temperature up to DHW temperatures (45-48degC?); If using an indirect tank, the water + inhibitor side would switch between the UFH and UVC. I can't quite see a simple way for the ASHP to provide increased temperatures for the UVC without either bringing the entire buffer tank to the same temperature or having some complex alternative ASHP supply which would by pass the buffer tank and directly feed the UVC. In by passing the buffer thank, I presume you would then experience ASHP issues due to the system pulsing due to the reduced volume of anti freeze? The other thing that doesn't sit well is the fact that you would not be using any of the lower temperature buffer tank to prime the UVC with this by pass, which would be a bit of a waste (unless there was an additional DHW timer, which 'pre-heated' the UVC prior to the alternative route at higher temperature). 

 

Alternatively, I could use the buffer tank and immersion heater to bring the UVC up to the DHW levels - however this will be solely powered by the grid and I'm not sure if this would be entirely cost effective? The last option is to run the buffer tank at DHW levels (seems a bit wasteful to keep the buffer tank at the higher temperature?). 

 

I've also been thinking about whether SA would be beneficial cf. UVC, or even just for the buffer tank - I'm not sure if the steadier temperature delivery of the SA would also enable me to drop from a 300lt UVC to their 210lt equivalent? I like the idea of reduced heat losses, especially as the tanks will be stored in a cold loft.

 

Am I over complicating matters?

 

 

[ProDave]

An ASHP normally runs to either heat DHW or heat the house, never both at the same time.  The details vary a bit from one ASHP to another, but mine has a different "water leaving" set point temperature depending on whether it's heating the house or the hot water.

 

Motorised valves determine where the water from the ASHP goes, either to the HW tank or to the heating (with or without buffer tank) and only one motorised valve is open at a time.  so when heating the DHW nothing is going to the heating or buffer tank so you won't over heat it.

[joe90]

Mine is a bit different in that the controller was sooo complicated i only managed to set it to deliver 48’ to either DHW or buffer tank fir UFH. The manifold blends  down to 24’ fir heating and 48’ is more than enough DHW.. 

[JulianB]

2 hours ago, ProDave said:

An ASHP normally runs to either heat DHW or heat the house, never both at the same time.  The details vary a bit from one ASHP to another, but mine has a different "water leaving" set point temperature depending on whether it's heating the house or the hot water.

 

Motorised valves determine where the water from the ASHP goes, either to the HW tank or to the heating (with or without buffer tank) and only one motorised valve is open at a time.  so when heating the DHW nothing is going to the heating or buffer tank so you won't over heat it.

 

Thanks for the reply, Understood... along the lines of a 'Y -  plan'? My concern was with the buffer tank only being on the UFH loop with this sort of layout, as the UVC loop would not benefit from any temperature stored in the buffer tank plus the potential adverse COP effects from not having a buffer tank in series with the CH heating loop. That is unless we're saying ASHP pulsing is not as much of an issue on the UVC side as the volume of water we're heating up is a lot greater than the UFH side? 

 

40 minutes ago, joe90 said:

Mine is a bit different in that the controller was sooo complicated i only managed to set it to deliver 48’ to either DHW or buffer tank fir UFH. The manifold blends  down to 24’ fir heating and 48’ is more than enough DHW.. 

 

What sort of COPs are you achieving on average with that set point for both DHW & UFH? 

[ProDave]

I can't say I see an issue with the ASHP short cycling.  When heating DHW it seems to try and maintain about 5 degrees between flow and return temperature and modulated the compressor power to achieve that, and the compressor does not seem to stop while it is heating DHW.

 

When heating the UFH then the compressor generally runs a lot slower and does stop from time to time, so it can't always modulate the power low enough so it stops, and re starts a bit later.

[joe90]

43 minutes ago, JulianB said:

What sort of COPs are you achieving on average with that set point for both DHW & UFH? 

unfortunately I don’t know (not a tecky type) but i have never seen my ASHP defrost and my bills are very small. As a rough guide I was running on immersions before I commissioned the ASHP and our bills were triple at least so a COP of 3 or more ?

Edited May 23, 2020 by joe90

[SteamyTea]

1 minute ago, joe90 said:

unfortunately I don’t know (not a tecky type) but i have never seen my ASHP defrost and my bills are very small.

I wonder if your controller lets you know the CoP.

Maybe have a look next time I am over.

Trouble is, playing with the buttons can inadvertently set something wrong.

[joth]

49 minutes ago, JulianB said:

My concern was with the buffer tank only being on the UFH loop with this sort of layout, as the UVC loop would not benefit from any temperature stored in the buffer tank

 

Why do you want the UVC to benefit from the stored energy in the buffer tank - for speed of heating the UVC, or feeling it will be 'wasted' energy if not used?

To the former, the buffer tank is lower temperature so not sure it will make much of a dent in heating the UVC unless it has been totally drained and is stone cold. To the latter, so long as the buffer tank is inside the thermal envelope of the building, the energy in it should not go to waste: this  escaping energy is usefully heating the house. This is how I'm reasoning about it anyway ? 

 

[Andrew]

5 hours ago, ProDave said:

 

An ASHP normally runs to either heat DHW or heat the house, never both at the same time.  The details vary a bit from one ASHP to another, but mine has a different "water leaving" set point temperature depending on whether it's heating the house or the hot water.

 

Motorised valves determine where the water from the ASHP goes, either to the HW tank or to the heating (with or without buffer tank) and only one motorised valve is open at a time.  so when heating the DHW nothing is going to the heating or buffer tank so you won't over heat it.

 

I’ve been looking at this setup recently as need to start looking for someone to install the ASHP and DHW systems. The heat pump I have my eye on (Samsung) does similar to this but the controller uses a single 3-way diverter valve to route between UFH and DHW.

 

The bit I’m struggling with is how the UFH circulation pump is controlled. The UFH has a timer / thermostat which connects to the ASHP controller to make a demand for heat. Presumably the UFH circulation pump connects through the Thermostat and diverter valve and only runs when there is a demand for heat and the diverter valve is in the heating position?

 

is that how it’s normally wired?

[Stones]

 

@Andrew  @JulianB  Some additional reading for you both in respect of ASHP feeding UFH, and a 300 litre UVC using a preplumbed Mitsubishi Ecodan:

 

 

 

[ProDave]

43 minutes ago, Andrew said:

I’ve been looking at this setup recently as need to start looking for someone to install the ASHP and DHW systems. The heat pump I have my eye on (Samsung) does similar to this but the controller uses a single 3-way diverter valve to route between UFH and DHW.

 

The bit I’m struggling with is how the UFH circulation pump is controlled. The UFH has a timer / thermostat which connects to the ASHP controller to make a demand for heat. Presumably the UFH circulation pump connects through the Thermostat and diverter valve and only runs when there is a demand for heat and the diverter valve is in the heating position?

 

is that how it’s normally wired?

You can use a 3 port 2 position valve, but I preferred to use two 2 port valves instead,. Do NOT use a 3 port mid position valve.

 

Yes there is a bit of an anomoly in the control system for mine.  The time clock activates the UFH manifold controller and that controls the UFH circulating pump and calls for heat from the ASHP.

 

But because the ASHP decides for itself when to switch between space heating and DHW heating, the ASHP controls the motorised valves.  This leaves the UFH circulating pump circulating ever cooling water while the ASHP is heating DHW.  Not ideal but it does no harm and I could see no easy way to stop it doing that.

[Andrew]

1 hour ago, ProDave said:

This leaves the UFH circulating pump circulating ever cooling water while the ASHP is heating DHW.  Not ideal but it does no harm and I could see no easy way to stop it doing that.

Thanks for explaining. I hadn’t considered having the UFH circulating pump going without heated water from the ASHP. Like you say other than a bit of wear on the pump it wouldn’t do any harm. 
 

Looking at an apparently suitable 3 port / 2-way valve it looks like there is a micro switch that could be used to sense the position of the valve - https://salus-controls.com/files/PMV3238-Quick-Guide.pdf . Presumably this could be wired up in such a way to control the circulating pump to only come on when the ASHP is supplying water to the UFH?

[joe90]

4 hours ago, SteamyTea said:

I wonder if your controller lets you know the CoP.

Maybe have a look next time I am over.

Trouble is, playing with the buttons can inadvertently set something wrong.

I don’t think so, even our @Jeremy Harris had problems understanding his (virtually same as mine). I am loathed to change it as it works fine ?