ASHP and insufficient hot water

[sinaloa]

dancinplumber suggested that I should post this in the renewable enegy forum. It started of as a question about our immersion heater but is linked to the installatiion of an ASHP - see below. Hope I have done this correctly and am not breaching any forum etiquettes.

I am a complete novice so forgive any very silly questions. Can anyone help me get to the bottom of all the conflicting advice I am getting about the potential volume of hot water my immersion heater could produce. We have just had an ASHP installed and need to rely more on our immersion heater for the volume of water we need. The ASHP seems to be at it limits to keep the house warm enough in really cold spells so to use it to heat our hot water during the day will result in the house getting cold while the water is being heated. We were advised and had hoped and expected that we could solve this problem by putting the immersion heater on a timer to reheat the water following the large usage for showers etc in the mornings. (When we had the oil boiler we used to rely entirely on the immersion heater for hot water during the summer months and did not have a problerm with shortages of hot water).We now run out of hot water if we want to shower in the afternoon/evening as well as the morning. The insulation on the tank seems to be very effective with a temperature drop at the sensor point of only a couple of degrees over12- 15 hours.

The temperature sensor on the hot water tank is towards the bottom of the tank. The firm who supplied the ASHP etc have fitted a new 29" immersion element but this has had no effect. They say that an immersion will not heat the water below the bottom of the element and certainly the temperature sensor shows that after 5 hours of the immersion being switched on the temperature of the water at the sensor point has only risen about 2 degrees (from 18 to 20 degrees). They tell me that the 29" element is the longest available and I can only expect hot water from the top 29" of the tank. The only other potential solution is to change the tank in order to have two side elements.

Other plumbers I have spoken to say that I should be able to expect an immersion heater to heat a complete tank within about an hour and a half - albeit at a high cost in electricity. That seemed to have the ring of accuracy as I understand that an immersion heater is needed with an ASHP in order to periodically boost water temperature to above 60 deg to prevent the danger of legionnaires disease.

My water tank is 1500h X 500 Diameter which I believe gives a 245l volume.

I have seen mention of dual element immersion heaters when I have looked on the internet and also mention of thermostats on the element needing to be at the bottom rather than the top but I have no idea whether this is contributing to our problem - or if I just have unrealistic expectations.

Any advice will be very much appreciated. This forum seemed to be the ideal way for me to get unbiased and reliable and expert advice.

Read more: http://www.ukplumbersforums.co.uk/p...lying-too-little-hot-water.html#ixzz2IPozcPz5

 

[Si H]

Hi. What size ASHP do you have? The heat pump should be able to do both heat and hot water even at these low temps even down to -15/20 depending on manufacturer and be topped up with the immersion when required.
The system uses the immersion to boost both heating and hot water when temperatures reach below -3 as specified by MCS 3005 design guide.
Does your system have a buffer tank for storing water for the heating circuit etc?

 

[sinaloa]

Hi Si H, Soory for delay in response. The ASHP is a 14 Kw Ecodan. We don't have a buffer tank - I don't even know what one is!
The heat pump works OK but is pretty much flat out providing space heating until we get the cavity wall insulation which I hope will provide enough spare capacity to enable us to heat the hot water twice a day - which should work? I had hoped to use the immersion heater in the meantime to heat a tank of water after the morning power showers have used most of the tank full heated overnight.

We are being told by some that a 29" element in the tank will only heat water to a depth of 29 inches so this will never give us sufficient hot water after the morning usage. Others tell us that a 29 inch element should heat a complete tank of water given time and the inevitable costs. I just don't know who to believe. Logic tells me that the latter should be true as I understand that an immersion is necessary with an ASHP system in order to periodically drive water temperatures for the WHOLE of the water tank above 60 degrees. If the immersion element will only heat water to the level of its own depth surely this wouldn't be effective in combating the threat of legionaires disease?

 

[chris watkins]

Sinaloa, Your immersion being a top entry type will only heat the water down to the tip, so only the top 29" will be heated I am afraid. This is because the water that is heated by the immersion will rise to the top of the cylinder, being lighter, it cannot circulate lower because it is always less dents i.e. lighter.
Can you post a picture of your cylinder what hight is the coil from the ASHP system ?

the time taken to heat the water in cylinder down to 725mm (29") can be found by using the following (very approx) volume of heated water in cylinder (forgetting dome top) dia = 0.5M Pi R sq x Height 0.7M = 0.1374 M3
= 137 Litres.

Time in sec = SHC x Litres x C x 100 = 4.2 x 137 x 50 x 100 = 2877000 = 11,839.5 sec x 60 x 60 = 3.28 Hours to heat 137 Litres from 10 deg C to 60 deg C with a 2.7kW running a 90% ...... ....efficiency Kw x efficiency . . 2.7 x 90 243

Hope that helps

 

[Si H]

Hi Sinaloa as above comments from Chris that is correct the only other way would be to install a cylinder with 2 immersion heaters one top and one at the bottom an economy 7 type cylinder.
The Ecodan is invertor driven so in theory doesnt need a buffer tank but you really should be getting 100% heating and hot water from your ASHP down to -3 deg c as stated by MCS 3005 before any immersion back up is used.
How big is your house and what is the heat load for your building? 14 KW heat pump is probably gonna be to small if its an old house with no insulation etc to heat and provide hot water to -3 deg c.
Was the installation carried out by an MCS accredited installer?

 

[sinaloa]

Chris, Thanks this really helps. If I interpret this correctly I can only expect to heat 137 litres of our 245 litre tank using the immersion element I have installed (29") no matter how long I leave it on. Am I correct in assuming that if I want to heat a larger volume of water using the existing tank I will either have to install a longer immersion element or install a further element in the side and towards the bottom of the tank?

Thanks for the formula for calculating I can now work out the time needed to raise the temperature from 20 deg to 40 deg (which I believe will be the range we need as that's about the level it drops to after morning showers and I believe 40 deg is sufficient for baths and showers in the evening)

 

[sinaloa]

Thanks Si H, Thanks for being so helpful. You'll see my post to Chris which deals with the immersion heater and the need for two elements in order to heat more of the water in the tank.

The house is about 2700 sq ft, 20 years old and has had UPVC double glazing installed about 4 years ago, 300mm Loft insulation. It is not particularly drafty. We are awaiting the installation of cavity wall insulation in the next couple of weeks.
When the ASHP was sized we used the Mitsubishi Sizing model which showed a total heat loss calculation of 12.82 Kw before the insulation is carried out to the walls and 11.12 Kw after the cavity insulation. The installation was done by an MCS accredited installer. The Ecodan output at -3 is said to be 13.54 Kw.

 

[chris watkins]

Chris, Thanks this really helps. If I interpret this correctly I can only expect to heat 137 litres of our 245 litre tank using the immersion element I have installed (29") no matter how long I leave it on. Am I correct in assuming that if I want to heat a larger volume of water using the existing tank I will either have to install a longer immersion element or install a further element in the side and towards the bottom of the tank?

Thanks for the formula for calculating I can now work out the time needed to raise the temperature from 20 deg to 40 deg (which I believe will be the range we need as that's about the level it drops to after morning showers and I believe 40 deg is sufficient for baths and showers in the evening)

sinaloa, there is a simple way of increasing the amount of water your cylinder can hold that is heated by the immersion & that is to use a small (bronze) hot water circulator connected from the hot outlet of the cylinder & back into the cold feed. This arrangement is know as an anti-stratifcation pump & basically mixes up the hot water so it does not just sit above the heating element & you should get ruffly the same temp water right across it given enough heating time. The other idea is an external heater which will do the same thing they are commonly used in N.I. but I can't remember the details someone will no doubt help.
It is not a good idea to store water at 40 deg C despite what they tell you about sterilising once a week with an immersion, as you can see in this case the bottom of the cylinder will not get up to the required temps of 70 deg C for this purpose. Water should if at all possible be stored at 60 - 65 deg C (minimum of 55 C) this both stops legionaires disease but also more usable hot water to be stored (as it will be blended with cold).

 

[sinaloa]

Chris, Thanks the anti stratification pump seems to make sense for us for the time being. Do you think that is a better solution than fitting another element lower in the side of the tank using the Essex Immersion Heater boss that someone else has mentioned?

Your comments about legionaires disease I understand and the need to store water at 60 deg plus. This has been adding to my confusion. The literature and advice I had when we were considering the ASHP said that it was necessary to have an immersion heater that is programmed to regularly boost the temperature in the water tank to above the level that the ASHP will heat it to. It seems however that without modifications (ie the anti stratification pump or an external heater or a lower side installed element or a longer top mounted element) that would never be possible. Surely this flawed advice leaves people exposed to the dangers of legionaires?

 

[Nostrum]

Seems about right, allowing for some system losses, slight inaccuracies of calculations and temperatures below those used in the design (-3) and you won't have much spare to play with.

The problem with the calculations are the data they are based on is averaged over the previous 20 years. It's evident that we are experiencing periods of cooler weather more recently, so at times the systems may not be able to supply 100% of the heat loss for the building, even though its sounds like the system had been designed correctly and in accordance to the industry standard formula.

as Chris said, a de-strat pump will help although its an expensive solution both in installation costs and running expense for what is essentially an undersized heat pump.

 

[Si H]

Hmm not a big fan of whole house calculators prefer to use room by room calcs bit old fashioned but you get a better result.

IMHO id say its probably undersized if your not achieving 100% for your heating and hot water, dont know where you live but its 0 deg c where i am and it should be doing all your Heat & HW needs with a boost above the level the heat pump temp is providing with your immersion for Legionella protection.

According to MCS 3005 you should be sterilising HW once a week using the immersion and letting the heat pump do most of the work in between.

The Ecodan used to come packaged with a Range cylinder which had a bottom immersion element for top up, do they not supply that cylinder now?

 

[Si H]

Any chance you can post a pic of your cylinder and pipework Sinaloa?

 

[Nostrum]

Thinking about it, you may be better off supplementing space heating with another source and using the heat pump to heat the cylinder through the coil. At least you'll get a whole tank full of hot water.

 

[chris watkins]

Thanks to Mr Croppie for reminding me of this from N.I. albeit not recommended for hard water areas - Willis Renewables - Solasyphon - How it works but it is an option.
I would still stick to my circulator, something like a Lowara EB-V 15-1 / 65 @ around £120, add a 7 day time clock so it can be set to come on with the immersion as required. 15mm copper pipe work with a couple of isolation & a check valves, all insulated, it's not going to break the bank nor will the running costs (min 3 max 7.5 Watts) at least you will be able to keep the existing cylinder.
IMHO the cut-in immersion bosses are not a good idea they are prone to leakage & not that easy to install.

Cavity wall insulation will make a lot of difference but I would still think that the hot water production should if at all possible be done over night when the heating is either not required or not so critical. Heat pump to pre-heat cylinder up as far as possible then immersion (might be worth looking at economy 7 tariff) to raise temp, down to you & cylinder size as to what temp but as cylinder is not that large might be better to get it up to 60 -65 which should mean it will last all day. (with only a boost if required) depending on your water usage.

P.S. Where do I send the consultancy fee to. LOL

 

[sinaloa]

Hi sambotc, Thanks. Am I correct in thinking that the pump is undersized because whilst the calulations appear correct the model's historic basis makes the industry standard calculations understate the size of pump needed due to the colder weather we have been having in recent years?
We have to hope that the cavity wall insulation gives us sufficient spare capacity in the space heating to use the ASHP for the water twice a day. Given your comments about a de-strat pump perhaps I should live with what we have at the moment and see if the wall insulation does what we hope and we can divert the pump from space heating.