Our DHW capacity was dictated by the MCS guidelines; they would see our house as a ‘4 bedroom house’, (in fact, 3 bedrooms and a 48 square foot room used as an office). As such, MCS guidelines recommend 200 litres plus; there are two of us living here these days and we had a 99 litre DHW tank that served our purposes without ever a cold shower or washing up bowl. What we actually use is a Sunamp Thermino eP210 system that is equivalent to 210 litres; on a few days when the rates have been very high, we have declined to re-charge the cabinet and left it for another day before using solar or grid power and have not run short ever.

I don’t have experience of using our heat pump to heat the water but have seen numerous comments on RHH about using a heat pump to heat a cylinder from cold being more efficient than ‘topping up’ a part warmed store of water.

We didn’t have a suitable space for a DHW tank without redesigning the whole household and a lot or pipework changes so the Sunamp suited our needs far better as it is only a quarter the volume of a conventional tank of similar capacity - the losses are good too, ~0.75 kWh per day or less and that heat is in the airing cupboard. Regards, Toodles.

Posted by: @jwilliams89

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Afternoon all,

 

I'm in discussion atm with our ASHP installer about what size cylinder to go for along with the pump. We are having solar and a battery fitted alongside these. There are two adults and two children in the house, and we don't tend to use large amounts of hot water. He originally specced a 250lt cylinder, and it is between this or a 200lt.

I know the arguments for having more capacity, especially with the solar (and and Eddi), in terms of keeping the water at a slightly lower temp and storing energy from the solar in the water. My uncertainty lies in the fact that if we usually use less than 200l a day - lets say around 150lt, overall are we wasting energy and money by heating water we don't go on to use, or would the extra capacity in a 250lt cylinder mean the water was warmer the next day, and therefore cheaper to heat up to back up to the temp threshold.

The difference in quote cost is only £130, so the main factor is the ongoing running costs. Would one cylinder have a more positive effect on the SCOP of the pump?

As Toodles stated, MCS state a minimum capacity of DHW cylinder, so this would be the minimum size that you could have installed.

As far as heat loss is concerned there should be data available from the cylinder manufacturer, but it is normally quite small.

Unless you intend to export your solar PV generation at a suitable export tariff, then heating your DHW by solar PV using an Eddi, or similar, is obviously more cost effective.

For 9 months of the year our DHW is predominantly heated by our solar PV system to a temperature of 60C, which is adequate for legionella protection. If you live in a hard water area then heating the DHW above 60C may cause scaling within the cylinder and pipework.

During the Winter months the DHW could be heated to 45C to 50C using the heat pump, with a weekly boost to 60C for legionella protection.

@derek-m Thanks for that -

The minimum with regards to the MCS regs for the four of us would be 180, so both are over that.

I intend to run the cylinder at 45c, unless as you say it makes sense to heat it hotter with the eddi. Adam's (HG) perspective is that the best thing to do is to use as large a cylinder as possible, enabling the DHW to be kept at a lower heat, however the issue (as far as I can see it and which I'm trying to decide upon) is that if we use less than 50% of the capacity regularly then the cylinder should be kept at 50c instead of 45c (for legionella due to less cycling), which would surely negate any savings from the larger cylinder size, and would lead to the increased costs etc. Basically would we be heating water we didn't need or would the extra capacity enable us to decrease the regularity of which we would need to heat the DHW, and potentially stop the need for using the ASHP for that throughout most of the year. I just can't work out which situation would lead to higher efficiancies.

Interestingly the two two points made by HG could be seen as contridictory. The first is that going big is best as this will allow DHW to be stored at closer to usage point, the second is that if you should match cylinder size as close to 75-100% daily usage so that frequent cycling occurs and that you could therefore keep the water at close to 40c without any issues.

I do intend to export, and know that it's cheaper to export then import at lower rates on Agile etc to heat the water, however independence within the system is important to us so I'd still rather top up the DHW before exporting to the grid.

@toodles We thought about the sunamp but decided against it eventually. Mostly because of the increased cost upfront, along with the fact that we have a large loft to put the cylinder in. You don't find the fact that it has to heat the water up so much hotter than for a cylinder causes much lower efficiancy from the ASHP?

Interesting point about it being more efficient to heat up cold water with the ASHP than prewarmed. I would have thought it would have been the other way around, just like with space heating. Can you provide me with a link to that discussion? I can't seem to find it.

I know that claims are made about heating DHW from cold is more efficient and there may be some truth to that statement, but it needs to be considered that hot water may not necessarily be available when needed if you are trying to use all or most of the hot water before producing more.

It also may depend upon the size and location of the heating coil and the location of the temperature sensor. The water within a DHW cylinder tends to stratify, so there could be quite hot water in the upper region and quite cool water further down.

When heating the DHW using a heat pump it would probably be more efficient if the heating coil is down near the bottom to ensure the maximum volume of water is fully heated to the desired temperature. The location of the temperature sensor is more problematic, since if it is too high in the cylinder then it will probably stop the heating before the full volume is heated, though it will indicate when you are starting to get low on hot water. Having the temperature sensor lower down in the cylinder will help ensure that most of the water is heated, but when water is drawn from the cylinder it may indicate a low water temperature when there is still quite a large quantity of hot water available. I suppose in an ideal world there should be several temperature sensors at different heights.

One of the benefits of using an Eddi or similar is the the immersion heater would only be powered by the excess solar PV generation, so even when the solar PV is producing less than 3kW, the immersion heater will still be heating the water without importing from the grid.

@jwilliams89 I had a quick search for the discussion about heating DHW from cold but could not find anything specific but as Derek M mentioned, the idea has been talked about.

Something more informative about the Sunamp unit we have; the Thermino eP210 is the model that has electrical element heating from solar or grid depending on one’s setup. The unit heats up a solution of ‘salt ‘n’ vinegar’ basically. There are models that can be heated with an immersion heater element or ones that can heat the ‘Phase Change Material’ (PCM) using a high temperature heat pump. Having solar PV panels, we opted to provide solar / grid power to provide the energy required. As to the temperature of the working system, the PCM is heated to approx. 65-70 C. and when you request hot water, the cold water flows through the internal piping and exits the unit at approx. 55 C. I think. I am a bit vague about the actual temperature at exit as I have the Sunamp regulater in line to ensure we are  not at any risk of being scalded. So water is effectively heated on demand rather like it is in a combi boiler.

There is virtually no risk of Legionella having a field day in this system so no additional measures need be taken. For the two of us to have daily showers plus all the washing up needs catered for, we use ~4.5 kWh in winter and ~3.5 kWh in summer. Regards, Toodles.

@jwilliams89 Can I throw in a left of field alternate option?

If you are considering export and TOU tariffs for import, consider a Mixergy cylinder heated via an immersion (You may need to have it plumbed to the heat pump for MCS, but could leave it unheated or marginally heated via the heat pump).

Although this on the face of it sounds nuts, the advantage is that no matter what size of cylinder is foisted upon you, you only heat the water that you need because these cylinders heat from the top downwards.

I have one and two of us use about 3kW of power overnight to heat our DHW at 7.5p a unit (Octopus Intelligent Go) and although I have an Eddi, I disconnected it because I can export at 15p a unit during the day.

7.5p a unit is equivalent to a COP of about 3.5 compared to daytime prices and although you could argue heating the cylinder via a heatpump at night would give a much higher equivalent COP, you would have to heat the whole 200-250 litre cylinder rather than just the volume you actually need.

You dont say how many are in your house, but the two of use use about 60 litres of hot water a day.

@bontwoody Indeed, the cheaper rates available via Agile are normally used to heat our Sunamp Thermino as the solar PV kW’s are being sold at a beneficial difference (15p p kWh) during the day. I prefer to heat the Sunamp straight from the grid rather than via the battery thus avoiding the ~10% loss during conversion to DC then storing and reconversion to AC again. Regards, Toodles.

@bontwoody I do like the look of the Mixergy cylinders, and especially like the look of the new HG one, however the installer is wedded to fitting a vaillent unistor cylinder, partially because of the fact it bumps the warrenty of the pump up from 5 to 7 years. I have to say I also find that appealing. However, as you say the ability to only heat what we use seems ideal. There are two adults and two children in the house. The issue I have is that some days we might only use 50lt or less, whereas if we've got a guest or two and one or two of us have a bath I can easily see that being over 200. On the Unistor cylinders the drawdown amount is something like 173 for the 200lt and 236 for the 250lt.

The issue I have is that numbers for showers etc wildly vary, and atm we have an oil fired boiler which provides instance hot water so it's very hard for me to work out standard usage atm.

@jwilliams89 I would imagine that the MCS would insist on fitting a heat exchanger to the Mixergy anyway, so if you knew when people were coming you could heat the cylinder that way. If you just need water for a shower, you can boost the Mixergy in 10% increments. This is very quick.

I guess there is no perfect solution.

Have you looked at the new heatgeek cylinder made by Newark? Its supposed to be the most efficient. (edit, sorry I think you meant Heatgeek by HG)

We just replaced our oil boiler with an ASHP.

We use solar panels (not PVs) with a rooftop 160lt solar cylinder for DWH and as backup (for the cloudy days) an in house 100lt cylinder that used to get heated with oil (using an internal coil for heat exchange). 

The coil is too small to operate with the ASHP (its water volume is below the minimum 40lt required by the ASHP) and we were of course told to replace it with a suitable 200lt cylinder, which we’d like to avoid.

The current cylinder resides in a separate dedicated loop (for the time being not connected to the ASHP). Would it work to add a volumizer at the return of this loop to supplement the liters required by the ASHP? Has anyone come across such a set up ever? When operating in DHW mode, the ASHP can deliver the 60 to 70 degrees that the oil boiler was delivering and which were more than enough to heat our current cylinder - so hopefully no worries on that front.

Our installer dismissed the idea as non-standard. So we either do the replacement or operate with the internal electrical resistance our cylinder has. 

Note: in our part of the world, there are no obligatory guidelines regarding the size of the DWH cylinder. And we can get by with solar DWH for circa 8 months a year.