Immersion heater supplying too little hot water

[sinaloa]

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.

 

[dancinplumba]

Hi and welcome, might be a good idea to post this in the renewable energy forum on here and maybe Change title ?

 

[kozak1968]

Peoples expectations often exceed the government guidelines. We are now experiencing the reality of the real world. ASHP work better with fossil fuel back up in my opinion.

 

[snowhead]

If it's a 3kw element you've got no chance of heating 245lts in 1 1/2 hrs.

 

[sinaloa]

Hi Snowhead, Thanks very much. Would that element EVER heat the whole of the 245 litres. We didn't have a problem with hot water previously when we relied on the immersion for water during the summer and 6 of us lived here. We are now only two.

 

[sinaloa]

Hi Kozac, Thanks a lot. Fortunately - it turns out now - I was not confident enough to allow them to remove the old oil boiler so we still have a back-up although we went into the ASHP project in the hope that we could get rid of the oil!!

 

[Roger Welton]

Agreed, this is my concern with ashp, they are poor imho with sub zero temps. I have considered one to supplement our oil boiler at home but would never want to fully rely on one.

 

[Roger Welton]

If it's a 3kw element you've got no chance of heating 245lts in 1 1/2 hrs.

If my (very poor) maths is correct you require 11.9 kW hours of energy to heat that tank to 60deg form 18, assuming no losses, so it would take you single immersion nearly 4 hours.

 

[Ray Stafford]

They tell me that the 29" element is the longest available

Unless its a non-standard imm heater, you can still get 36" immersion heaters.

 

[sinaloa]

I have tried leaving the immerdsion on for 5 hours and monitoring the water temp at the ASHP control panel. The immersion only raised the temp from 18 to 20 degrees in 5 hours. When I challenged the suppliers they say that the immersion element will NEVER heat the whole of the tank only the water above the bottom of the 29" element. I guess what is happening is that if the themostatic control is at the top of the tank the water at the top is getting to temperature and turning off the element before the bottom "half" of the water has warmed up?
If this is the case how can it be an effective way of avoiding the danger of legionaire's as presumably the whole of the water in the tank could not be heated to above the required temperature?

 

[kozak1968]

I personally have my unvented cylinder set to 55 deg C because of the kids. I think there is less chance of legionnairs desease than with a cold water storage cistern open to the atmosphere.

 

[dancinplumba]

temp should be 60c and your dhw outlet should have a blending valve on, set that to 55. legionaires doesnt live in cold water

 

[kozak1968]

I agree, i should of said open vent hot water is more of a problem, but dancinplumba is correct.

 

[lame plumber]

it seems stupid to have an ashp and be relying on an immersion, surely you ve looked at solar hot water heating as a main source, backed up by immersions, and on a 245l cyl one should be economy 7 at least. When you looked at an ashp did the calcultions on pump size get worked thro correctly, as if your not getting enough hot water and heating your undersized. Was your house properly insulated to allow an ashp to work properly? you may wish to take further advice fm an independant source to see if your system needs improving!!

 

[sinaloa]

Thanks all. We did not look at solar hot water as we had been assured that the ASHP would provide all of our heating and hot water needs and we were looking primarily to replace the high running cost oil boiler. The calculations for the ASHP did seem to be correct and the 14Kw system seemed to be sufficient. To be fair we are still waiting for the cavity wall insulation and it is likely that the time my wife takes in our power shower (10 - 12 mins - it gives her some pain relief) was not factored in. This seems to be the main issue in that if her shower uses say 130l of water at say 40 degrees which is then replaced by 130 l at say 5 degrees that is not heated again until 2am the following morning we will have a problem. This is why we were advised to set the immersion to heat for a couple of hours after her morning shower. Unfortunately it only seems to heat a small proportion of the water in the tank (we are being told by some, the top 29 inches only).
If after we have the cavities insulated the ASHP keeps the house warmer we will be able to set the hot water to come on for an hour or so during the day and that will probably provide sufficient hot water.
In the meantime or if the cavities dont make the difference we are told they will is there an immersion heater element that I can fit to our existing tank that will heat the whole (or a substantial proportion) of the water in our tank to say 40 degrees from say 20 degrees - even if that takes 2 or 3 or 4 hours??
Thanks for your patience with my obvious ignorance.

 

[lame plumber]

if your house isnt insulated properly then an ashp will never cope, your installers need shooting, all you will have at present is huge electricity bills!

:banghead:

 

[TBServices]

Ashp in my experience are only good for houses insulated to current new build standards. They also work much better with low temp heating, such as underfloor rather than rads.

I once fitted a load of oversized rads (sized by heatpump installer, supplied by customer) for a customer in a large, detached Victorian house with no insulation. Needless to say, it was my fault the house was cold, not the muppet who took a load of their money for an ASHP, fitted to a 30 year old heating system which was never up to the job in the first place. In a house that size and build, the rads need to be working at 65-70 degrees, not 50.

 

[lame plumber]

long live fracking to give us gas and save the need for going green

 

[sinaloa]

Love the head and wall attachment !

The house was fitted with PVC Double glazed units four years ago, has 300mm of loft insulation and is not drafty. The cavity wall insulation should be done in the next couple of weeks so we are hoping that will help. Are there other insulation measures that I should consider?
The system sizing was done using the Mitsubishi model and the inputs were accurate as far as I can tell although it showed the 14 Kw output to be sufficient even WITHOUT the cavity insulation. The space heating is just about OK but I wouldn't say that there is enough "spare capacity" when it is really cold to divert the pump to heat the water again during the day. Perhaps this will come when the cavities are insulated and solve the problem.

I was hoping to be able to deal with the water shortage problem by installing an element in the immersion heater that would heat the whole or a substantial part of the water in the tank when needed - albeit at some cost.

 

[lame plumber]

the immersion you have is for booosting the cyl temp, relying on the fact the main heat source will bring the whole tank back to temp in a longer period, if your having to use the immersion to heat the whole cyl your system design is wrong and needs looking at or youll be paying a fortune in elec in due course!! which is why ashp and gshp should imho only go in new builds or total renovations to ensure they work to spec

 

[snowhead]

If your cylinder is copper or stainless and depending on where the coil is you could fit an Essex flange low down and have a second horizontally mounted element. No need for a new cylinder.

It it's an enamelled steel cylinder you'll wreck the cylinder cutting the hole.

Essex 2.25" E2/SX Immersion Heater Boss Flange CURVED surface cylinder tank e2/r | eBay