ASHP and running out of hot water

[Ben100]

Hi,

 

I'm looking at the option of replacing my oil boiler with an ASHP, but I'm worries that I'll run out of hot water more often. I have a new large hot water cylinder ~300L, but we're a family of 5 living here with 3 bathrooms. I'm worries that if I move from oil to ASHP the water will be cooler, so we'll use more hot water when we shower and run out.

 

I'm interested in others experiences of a similar situation?

 

Thanks! 

[IanR]

We're a family of 5 with a 500l tank, and never get close to running out of hot water.

 

For swapping over I'd be more concerned about the size of heat emitters. If you are on rads, they need to be oversized.

[Ben100]

Good to know. I'm replacing the existing radiators on the first floor and installing UFH on the ground floor, so I can oversize as needed.

 

I'm running 100% off of my cylinder element at the moment, so maybe that's why I'm running out of water...

[ProDave]

What size of ASHP are you planning?

 

This is one issue with them, they don't heat water as hot, and they take longer to do so than a gas or oil boiler, depending of course on the size of your ASHP.

 

Ours is only a 5Kw ASHP with a 300 litre tank so it does take a while to re heat if all used up. We heat the HW to 48 degrees with the heat pump so it will use more hot from the tank than it would if say heated to 60 degrees with a boiler.

 

I have "solved" this issue by fitting a modulating instant electric water heater on the output of the HW tank.  It is detailed in this thread

 

Most of the time the electric heater does nothing, but if the hot water starts to run cold, it powers up and tries to maintain 48 degrees output.  At the very worst case, if the tank has gone completely cold and is only supplying cold water, it will still maintain some warmth or if you turn the shower flow rate down, it will maintain full temperature at a low flow rate, like showering  with a 10Kw electric shower.

 

Another point worth making is the positioning of the temperature probe in the tank is important.  Most ASHP's are provided with a temperature probe to put in the tank.  I did not give this enough thought and even the lowest pocket is about 1/3 of the way up the tank, so the tank could be down to just 200 litres of hot water before te ASHP even knows it needs to turn on and start heating it.  If I were specifying the  hot water tank now I would have a thermometer pocket lower down, probably several of them near the bottom of the tank so you can experiment which one works best.

[PeterW]

48 minutes ago, Ben100 said:

I'm worries that if I move from oil to ASHP the water will be cooler, so we'll use more hot water when we shower and run out.

Why will it be cooler ...?? Assuming your oil stat is set at 70°C you could replicate this by using the ASHP to 50°C and then top up using the tank immersion to 65°C which will be more than adequate. 
 

In terms of cost that adds about 45p on E7 to your daily DHW electricity bills. 
 

 

[SteamyTea]

40 minutes ago, Ben100 said:

running 100% off of my cylinder element at the moment, so maybe that's why I'm running out of water

Where is the element? Top or bottom of cylinder.

5 minutes ago, PeterW said:

adds about 45p on E7 to your daily DHW electricity bills. 

But how much if you are on E7 and run the ASHP on the day rate, which may be close to 25p/kWh now.

[PeterW]

10 minutes ago, SteamyTea said:

Where is the element? Top or bottom of cylinder.

But how much if you are on E7 and run the ASHP on the day rate, which may be close to 25p/kWh now.

I am assuming running an ASHP at night - wouldn’t necessarily run during the day so I agree I’ve put my figures down as night rate as 9p or thereabouts. 

[ProDave]

29 minutes ago, PeterW said:

I am assuming running an ASHP at night - wouldn’t necessarily run during the day so I agree I’ve put my figures down as night rate as 9p or thereabouts. 

That comes back to where is the thermostat pocket.

 

With ours 1/3 of the way up the tank, the ASHP will come on not long after a decent length shower as been used.  If we risked it not coming on, that is when you get into dangerously low for the next shower.

 

My philosophy is slightly different that I try to not use the ASHP at night, preferring it to run in the daytime when there is PV generation to help it along.  I don't have dual rate electricity though I always keep that option open.

[Ben100]

1 hour ago, SteamyTea said:

Where is the element? Top or bottom of cylinder.

But how much if you are on E7 and run the ASHP on the day rate, which may be close to 25p/kWh now.

 

The element is around 1/3 of the way up from the bottom.

 

[Ben100]

1 hour ago, ProDave said:

What size of ASHP are you planning?

 

This is one issue with them, they don't heat water as hot, and they take longer to do so than a gas or oil boiler, depending of course on the size of your ASHP.

 

Ours is only a 5Kw ASHP with a 300 litre tank so it does take a while to re heat if all used up. We heat the HW to 48 degrees with the heat pump so it will use more hot from the tank than it would if say heated to 60 degrees with a boiler.

 

I have "solved" this issue by fitting a modulating instant electric water heater on the output of the HW tank.  It is detailed in this thread

 

Most of the time the electric heater does nothing, but if the hot water starts to run cold, it powers up and tries to maintain 48 degrees output.  At the very worst case, if the tank has gone completely cold and is only supplying cold water, it will still maintain some warmth or if you turn the shower flow rate down, it will maintain full temperature at a low flow rate, like showering  with a 10Kw electric shower.

 

Another point worth making is the positioning of the temperature probe in the tank is important.  Most ASHP's are provided with a temperature probe to put in the tank.  I did not give this enough thought and even the lowest pocket is about 1/3 of the way up the tank, so the tank could be down to just 200 litres of hot water before te ASHP even knows it needs to turn on and start heating it.  If I were specifying the  hot water tank now I would have a thermometer pocket lower down, probably several of them near the bottom of the tank so you can experiment which one works best.

 

I've not planned which size ASHP to go for yet, but I like the idea of an electric top-up!

[Ben100]

1 hour ago, PeterW said:

Why will it be cooler ...?? Assuming your oil stat is set at 70°C you could replicate this by using the ASHP to 50°C and then top up using the tank immersion to 65°C which will be more than adequate. 
 

In terms of cost that adds about 45p on E7 to your daily DHW electricity bills. 
 

 

 

Good point. I was thinking ASHP heats to a lower temp, but wasn't considering the heating element.

[SteamyTea]

54 minutes ago, Ben100 said:

The element is around 1/3 of the way up from the bottom.

This is what happens to my E7 heated cylinder over an average day.  The element is as close to the bottom as it can be.  Temperature probes are at the top and the bottom.