Heat pump hot water cylinder + boiler

anon_ymous

70sbudgie said:

Reed_Richards said:

waqasahmed said:

So realistically I should just save up to get it for both? 

That would be my recommendation.

70sbudgie said:

If you had a hp integrated cylinder for dhw would it enable a smaller air to water hp for the central heating? Would this enable increased efficiency?

Unless your house is really energy efficient you will use a lot more power keeping it warm than you will in heating your hot water.  So your heating heat pump could only potentially be a little bit smaller if it was not required for hot water.

Last time I calculated the contribution of hot water to gas consumption for our house, it was about 15%. 

I appreciate that wouldn't translate directly to a 15% reduction of a heat pump, but I'm trying to get my head around where a threshold might make sense for it being worthwhile or not. I guess it would come down to individual assessment for a property, how much hot water, how efficient the central heating (radiators) etc are, and as you say, how energy efficient the house is in general.

But another upside to a hp integrated cylinder is that it could allow a staged investment. Rather than having to save up to pay for a cylinder at the same time as overhauling the entire heating system, a hp cylinder could be installed ahead of time and with some thoughtful implementation of tou tariffs, may allow savings in the short term that would help with saving up for the bigger purchase of the central heating hp. 

That was my rationale too ie:  staged investment but sadly it seems that the best bet is just to "save up" 

70sbudgie

Have you done the calcs for your own situation? Do you have smart meter consumption data? That would help you work out how much of your gas use goes to DHW (summer gas usage). Then dig out the specs for one of the HP cylinders. 

This is something that I will be doing soon, but I have a relatively new DHW cylinder, so am not looking to replace it, but wondering if there is a way I can incrementally transition off gas. As my DHW cylinder is dual core, I am wondering if there is a way I can make use of the second core.

When doing the calculations, don't forget to bear in mind potential under capacity of your house - ie if you have more bedrooms than people, you probably want to size your system for the number of bedrooms. The way I do this currently is that I have the hot water on a lower temperature, heating for only a couple hours a day. If we were to have guests for more than one night, I would heat the water for longer, so there is more hot water in the cylinder. The water temperature is noticeably warmer if we've been away for a few nights.

Alnat1

@70sbudgie using summer figures to calculate gas use for hot water needs to allow for a couple of things. Incoming water in winter is colder so more gas is used to get it  up to the same temperature as in summer and people also tend to have hotter showers in winter.

70sbudgie

Alnat1 said:

@70sbudgie using summer figures to calculate gas use for hot water needs to allow for a couple of things. Incoming water in winter is colder so more gas is used to get it  up to the same temperature as in summer and people also tend to have hotter showers in winter.

Would there also not be some efficiencies in winter from the boiler already operating for the central heating? 

Do you have a better method for working out the approximate % of gas usage that goes towards DHW? Or how do you factor in the above? I also use gas for cooking, so that affects the accuracy of my calculations.

I have my cylinder set to heat for 2 hours a day, but I still haven't worked out how to take the smart meter consumption data from that and turn it into a more reliable % gas use for DHW.

Alnat1

I can see from my stats that averages for Jun/Jul/Aug are lower than the other non-heating months, guessing incoming water hotter, cooler showers and tank losing less heat as the weather was generally warmer.

In May/Sep/Oct I averaged 5kWh a day heating water but Jun/Jul/Aug was 3.5kWh. I heat for 40 minutes a day from 8am. I can see from Bright stats that I'm using around 5kWh in the 8-8.30 slot each day in Dec but TBH I don't know if my boiler is heating the tank and radiators at the same time. 

anon_ymous

70sbudgie said:

Have you done the calcs for your own situation? Do you have smart meter consumption data? That would help you work out how much of your gas use goes to DHW (summer gas usage). Then dig out the specs for one of the HP cylinders. 

This is something that I will be doing soon, but I have a relatively new DHW cylinder, so am not looking to replace it, but wondering if there is a way I can incrementally transition off gas. As my DHW cylinder is dual core, I am wondering if there is a way I can make use of the second core.

When doing the calculations, don't forget to bear in mind potential under capacity of your house - ie if you have more bedrooms than people, you probably want to size your system for the number of bedrooms. The way I do this currently is that I have the hot water on a lower temperature, heating for only a couple hours a day. If we were to have guests for more than one night, I would heat the water for longer, so there is more hot water in the cylinder. The water temperature is noticeably warmer if we've been away for a few nights.

I've got the "loop" app which doesn't necessarily get as granular as hot water heating but it does go as granular as "heating" in general

I'm not sure how you could even get as granular as saying "x gas is for heating and y gas is for water"? 

70sbudgie

waqasahmed said:

I'm not sure how you could even get as granular as saying "x gas is for heating and y gas is for water"? 

I look at my usage in the months when the heating isn't on and extrapolated it across the whole year. It gave me a rough figure of about 15% of gas usage. 

But I think @Alnat1 has done a more in-depth analysis of their data, so they can probably advise better than me.

Alnat1, do you think your DWH gas consumption would go up again in the winter months compared to May / Sept / Oct?

Alnat1

You got me thinking about it earlier. Is it worth me altering the time the heating comes on tomorrow and freezing my butt off in the morning just so I can tell you how many kWh of gas I use heating my water for 40 minutes. 

Or maybe estimate of 15% is good enough? 

anon_ymous

70sbudgie said:

waqasahmed said:

I'm not sure how you could even get as granular as saying "x gas is for heating and y gas is for water"? 

I look at my usage in the months when the heating isn't on and extrapolated it across the whole year. It gave me a rough figure of about 15% of gas usage. 

But I think @Alnat1 has done a more in-depth analysis of their data, so they can probably advise better than me.

Alnat1, do you think your DWH gas consumption would go up again in the winter months compared to May / Sept / Oct?

Thanks.. I've only been in this house since August or so, so I don't even have a true comparison to go by! My smart meter was installed in September and that's when heating was just started to be needed 

anon_ymous

70sbudgie said:

Reed_Richards said:

waqasahmed said:

So realistically I should just save up to get it for both? 

That would be my recommendation.

70sbudgie said:

If you had a hp integrated cylinder for dhw would it enable a smaller air to water hp for the central heating? Would this enable increased efficiency?

Unless your house is really energy efficient you will use a lot more power keeping it warm than you will in heating your hot water.  So your heating heat pump could only potentially be a little bit smaller if it was not required for hot water.

Last time I calculated the contribution of hot water to gas consumption for our house, it was about 15%. 

I appreciate that wouldn't translate directly to a 15% reduction of a heat pump, but I'm trying to get my head around where a threshold might make sense for it being worthwhile or not. I guess it would come down to individual assessment for a property, how much hot water, how efficient the central heating (radiators) etc are, and as you say, how energy efficient the house is in general.

But another upside to a hp integrated cylinder is that it could allow a staged investment. Rather than having to save up to pay for a cylinder at the same time as overhauling the entire heating system, a hp cylinder could be installed ahead of time and with some thoughtful implementation of tou tariffs, may allow savings in the short term that would help with saving up for the bigger purchase of the central heating hp. 

I guess you could also potentially use the heat pump cylinder whilst still on gas? And when the time comes, pop it onto an ASHP?