ASHP to heat water for a shower or electric shower?

Ken2000

Hi, 

My hot water needs are very low, at most, it's a bit for washing up and then a shower for one or two people per day. I have an Air Source Heap Pump for my heating and for hot water, although I'm not using the hot water tank at the moment. For showers, I'm just using an electric shower. What is likely to be cheaper; either using the ASHP to heat my water or to leave it off and just use the electric shower once or twice per day?

matelodave

If your ASHP has a COP of even just 2 it will cost you half as much as an electric shower.

Sea_Shell

Does a heat pump system get the water hot enough to kill nasties like Legionnaires disease??  60⁰c+

Or is that only a risk in pipes that are "stagnant" rather than in daily use?

shinytop

Ken2000 said:

Hi, 

My hot water needs are very low, at most, it's a bit for washing up and then a shower for one or two people per day. I have an Air Source Heap Pump for my heating and for hot water, although I'm not using the hot water tank at the moment. For showers, I'm just using an electric shower. What is likely to be cheaper; either using the ASHP to heat my water or to leave it off and just use the electric shower once or twice per day?

If you want an idea of the cost of heating the tank as opposed to the water you use, we are the same as you but we use electric showers because we don't yet have any fed off the tank.  However, we heat the tank 24/7 for the few sinkfuls of HW we do use.  Since April 1 to now we have used less than 2kWh per day, to do not much more than keep 250l of water at 50 degrees. Not a lot really. 

matelodave

Sea_Shell said:

Does a heat pump system get the water hot enough to kill nasties like Legionnaires disease??  60⁰c+

Or is that only a risk in pipes that are "stagnant" rather than in daily use?

My Daikin heatpump has a fixed sterilisation cycle, once a week to heat the tank to 60 degrees. I know that some other manufacturers have a similar function but with the advantage that you dont have to have it do it weekly.

TBH as my hot water tank is mains fed and unvented and our water smells like a swimming pool it pretty unlikely that anything nasty will grow in there but I cant turn off the sterilisation cycle. However that said, as the tank stays pretty hot it doesn't heat for as long the next day if at all, so it probably doesn't cost much if anything extra.

We only have our hot water function activated for two hours a day and it takes around an hour to reheat it to 45 degrees and uses around 1.5-2kwh a day (a bit more in the winter). That gives us plenty of hot water for both our showers, some washing up and other ablutions. Most of our washing up goes in the dishwasher.

A 10kw electric shower for five minutes would use 1.66kwh, so two showers a day would use twice as much leccy as our heatpump.

BUFF

matelodave said:

A 10kw electric shower for five minutes would use 1.66kwh, so two showers a day would use twice as much leccy as our heatpump.

by my reckoning a 10kW shower for 5 minutes (1/12 of an hour) uses 0.833kWh so 2 such showers a day would use 1.666kWh or similar to your heat pump, not double.

Ken2000

Sea_Shell said:

Does a heat pump system get the water hot enough to kill nasties like Legionnaires disease??  60⁰c+

Or is that only a risk in pipes that are "stagnant" rather than in daily use?

It has a function whereby the hot water tank is heated to kill off nasties every so often, which has got me thinking, as I've switched the hot water off at the moment, so there's a chance that it could be getting a bit nasty in there. I probably should turn it back on, in which case, it might be safer to keep the hot water tank on permanently and therefore it might make more sense to use that hot water to have a shower.

matelodave

BUFF said:

matelodave said:

A 10kw electric shower for five minutes would use 1.66kwh, so two showers a day would use twice as much leccy as our heatpump.

by my reckoning a 10kW shower for 5 minutes (1/12 of an hour) uses 0.833kWh so 2 such showers a day would use 1.666kWh or similar to your heat pump, not double.

sorry, finger trouble

Reed_Richards

Sea_Shell said:

Does a heat pump system get the water hot enough to kill nasties like Legionnaires disease??  60⁰c+

The gas boiler in my old house had this feature; it would raise the tank temperature to 60 C for an hour every Sunday morning, allowing us to keep the usual storage temperature lower.  There is also a brand of immersion heater controller that will do this.  There is no point in keeping the hot water tank hotter than you need and it is potentially dangerous if you don't have a blending valve on the output or on all the hot taps.

With a heat pump you need to use an immersion heater for the final 5 to 10 degrees to reach 60 C.  My heat pump will do this once a week but you can program this feature on or off.

We used to have a combi boiler with a very long pipe run so you had to run off a bucket of water before it ran hot out of a hot tap.  I would far far rather have hot water on tap when I want it, even if this were more expensive.  

wittynamegoeshere

I'd say you definitely need to ensure that the tank and pipework are extremely well insulated.  My feeling is that if it's not then the heater shower may well cost less than the heat pump.

Also even with good insulation ideally it needs to be heated reasonably soon before use.  Don't heat it at 5am if you're not having a shower until 8pm.

We've yet to start looking at the ins an outs, we're going to get a system installed from scratch.  I'm going to look at thermal storage, I've no idea whether it makes physical or financial sense, and it may be too early in its development to be practical, but it's on the list of things to ponder.  I think this is the main supplier at the moment...

https://sunamp.com/

The idea of this is that it's a way of storing heat so that it doesn't constantly lose energy - once it's in its locked away.  Then it gets released to heat the water on demand whenever it's needed.

We've got two showers, and one is a long way from the rest of the house.  So we'll have an electric shower there for definite, but the other is currently a big fat question mark.

Reed_Richards

wittynamegoeshere said:

 I'm going to look at thermal storage ....The idea of this is that it's a way of storing heat so that it doesn't constantly lose energy - once it's in its locked away.  

No that's completely wrong.  It will still constantly lose energy, anything above ambient temperature will.  Now because a phase-change thermal store is more compact than a tank of hot water (which is also a thermal store) then you can insulate it very well, although I strongly suspect that you still lose a lot of heat down the pipes.  The main virtue of the Sunamp product is that you can fit one in a smaller space than a hot water tank of equivalent heat storage capacity.