ASHP RHI timing out in Mar 2021

[SteamyTea]

1 minute ago, dpmiller said:

almost...

as others have pointed out in the past, the store temp of a TS drops as you draw off, so it's hard to say what capacity you actually have. With a normal cylinder, if it's all hot, it all comes out hot.

Is there some sort of different physics going on then.

[dpmiller]

you do remember the TS has a heat exchanger in it? So the output temp varies with flow rate, and the body of water is cooled as the cold water flows through the coil.

 

/BTW  I've fitted one, and I'll be using an ASHP. Convention be damned..

[SteamyTea]

19 minutes ago, dpmiller said:

you do remember the TS has a heat exchanger in it?

Yes, and this is why I am claiming that sizing is important.  There will be a slight temperature drop due to the heat exchanger efficiency, but that leaves more energy in the store.

If space heating load is also being drawn off at the same time as DHW, then sizing is even more important.

[ProDave]

28 minutes ago, dpmiller said:

you do remember the TS has a heat exchanger in it? So the output temp varies with flow rate, and the body of water is cooled as the cold water flows through the coil.

 

/BTW  I've fitted one, and I'll be using an ASHP. Convention be damned..

Do let us know how you get on.

 

With say a 200L UVC, you fill the tank with water at say 50 degrees, then turn on the hot tap and you will get 50 degree hot water up to the point you have drawn all 200 litres out of the tank and it will then quickly go cold.

 

Now fill your 200L thermal store to 50 degrees.  Turn on the hot tap and you will get 50 degree hot water.  but by the time you have drawn say 100L from the hot tap you will have removed half the stored energy from the tank, so the water that is left in the tank will be a lot less than 50 degrees now and so the hot water temperature will be lower.

 

So you either need a very much larger thermal store, or you need to store the water at a much hotter temperature with a thermal store.  They usually have a thermal mixing valve to regulate the delivered water temperature.

 

As Heat pumps don't do much over 50 degree hot water, the results with a thermal store are likely to be disappointing.

 

[SteamyTea]

11 minutes ago, ProDave said:

With say a 200L UVC, you fill the tank with water at say 50 degrees, then turn on the hot tap and you will get 50 degree hot water up to the point you have drawn all 200 litres out of the tank and it will then quickly go cold.

Isn't this looking at the problem from the wrong perspective.

 

Rather than say 200lt at 50°C, if you calculate it as the effective energy stored, say 17 MJ, then within a tiny bit, a UVC and a TS, or even a traditional WH cylinder of the same size and the same temperatures, store the same amount of energy.

If you extract water (energy) at the same rate, I am really (not being difficult) struggling to see the difference.

 

Now you can 'average' the stored temperature out by allowing turbulence within the store i.e. constant pumping, but that is not the same thing.

Edited October 31, 2020 by SteamyTea

[LSB]

gosh, this is all starting to go above my head, more studying needed.

The only thing I thought was that an ASHP struggled to provide enough hot water, which is why I thought I also needed PV.

 

[SteamyTea]

11 minutes ago, LSB said:

only thing I thought was that an ASHP struggled to provide enough hot water, which is why I thought I also needed PV.

That is tackling the same problem, but with different technologies.

First thing to know is your DHW demands. Do you need 50, 100, 1000 litres a day. Without knowing that, it is just speculation based on guesses.

And don't get temperature, energy and power mixed up. They are different things.

[Bramco]

2 hours ago, dpmiller said:

as others have pointed out in the past, the store temp of a TS drops as you draw off, so it's hard to say what capacity you actually have. With a normal cylinder, if it's all hot, it all comes out hot.

 

We have a thermal store and heat exchanger in our current house on which we have a number of sensors. There is always a gradient on the tank (210l), so v hot at the top, getting gradually cooler as you go down the tank. As you draw off hot water, the hot water is extracted from the top through the heat exchanger and returned lower down. If the temperature at the middle drops enough, then the backup boiler kicks - we have solar thermal and PV divert to harvest as much solar energy as we can.  I'm guessing that an ashp or gshp wouldn't be able to kick in with the same amount of energy as the gas boiler does, so wouldn't maybe be able to give you several showers and a bath.

 

Having said that I can't see how a how water tank would be any different from our thermal store. The hot water generated by the heat pump surely enters the tank at the top as cooler water is drawn off at the bottom to be heated up. So the tank won't be all hot, there will be a gradient, so as hot water is drawn off at a shower or tap it will eventually get cooler as again the heat pump won't be able to keep up.

 

So it would seem that each solution will behave in a similar manner and depending on sizing might be behave exactly the same as each other.  The reason we went for the thermal store in our current house was to make use of solar thermal and diverted PV and to have mains pressure potable hot water through the heat exchanger without having to worry about legionnaires. 

 

For the new house we'll have to decide whether we go the same way again or go for a hot water tank and make sure it goes through a hot cycle for the legionnaire treatment.

 

It would be interesting to know the relative sizes of a thermal store and standard cylinder to get the same behaviour, i.e. amount of hot water before it begins to run cooler. Assuming I'm right about the heat pump not being able to keep up.

 

Simon

 

 

[LSB]

42 minutes ago, SteamyTea said:

That is tackling the same problem, but with different technologies.

First thing to know is your DHW demands. Do you need 50, 100, 1000 litres a day. Without knowing that, it is just speculation based on guesses.

And don't get temperature, energy and power mixed up. They are different things.

Most of the time there is just 2 of us needing daily showers.

But, we have 4 kids plus partners and grandchildren, so when they visit the requirement is much higher, but this is not that often.

 

[SteamyTea]

18 minutes ago, Bramco said:

We have a thermal store and heat exchanger in our current house on which we have a number of sensors. There is always a gradient on the tank (210l), so v hot at the top, getting gradually cooler as you go down the tank.

Any chance you can log some numbers when the tank is in different states i.e. heated but unused, after use, in use and being heated.

Be interesting to see how much turbulence is created.

 

18 minutes ago, Bramco said:

guessing that an ashp or gshp wouldn't be able to kick in with the same amount of energy as the gas boiler does,

It will if the thermal outputs are equivalent, not if the ASHP is smaller.

 

8 minutes ago, LSB said:

But, we have 4 kids plus partners and grandchildren, so when they visit the requirement is much higher, but this is not that often

During those times you can heat the tank up to a higher temperature with a normal immersion heater. Just remember to turn runoff when they leave.

Edited October 31, 2020 by SteamyTea

[A_L]

2 hours ago, Bramco said:

It would be interesting to know the relative sizes of a thermal store and standard cylinder to get the same behaviour, i.e. amount of hot water before it begins to run cooler.

 

Here is some information on thermal stores, however they are probably rather better than those routinely available in the U.K., certainly tank B is.

 

https://forum.buildhub.org.uk/applications/core/interface/file/attachment.php?id=17632

 

 

[newhome]

So totally not scientific but I heat my 475l thermal store to 68 degrees C at the top of the tank. That then provides enough hot water to fill a large bath and have enough hot water for the rest of the day for other uses. If I shower then it needs to be at least 50 degrees or the water is too cold before I've finished. Ok so that includes using various hair products too so not a uber quick shower. It's an expensive way to have a shower in summer though with no need to use the water for heating. I wish that I had installed an electric shower in one of bathrooms as that would be much more economical in the summer (I'm still considering doing this).  No ASHP though. The one quote I got was crazy money (£14.5k) but mostly I can't get anyone to quote at all. I just switched to EDF and saw that they install ASHPs so thought I might be able to get a quote from them. 'No EDF approved installers in your area' it said.

 

[PeterW]

12 hours ago, Bramco said:

For the new house we'll have to decide whether we go the same way again or go for a hot water tank and make sure it goes through a hot cycle for the legionnaire treatment.

 

Legionnaires in sealed domestic hot water systems is a myth. It requires an open connection to air to contaminate the water and oxygen to continue its growth. If you do feel the need to “disinfect” then once a fortnight you can hit the 72c boost on the immersion  

[SteamyTea]

6 hours ago, newhome said:

So totally not scientific but I heat my 475l thermal store to 68 degrees C at the top of the tank.

Does your space heating system also draw from the thermal store?

If not, do you have any idea what the temperature gradient is.

I heat my basic E7 cylinder to 40°C now (used to be 50°C but reduced it for an experiment) and can eke out two baths, just,

[Nick1c]

Isn’t one of the key benefits of a Sunamp that there is availability of water at a constant temperature for the vast proportion of the energy stored? It only behaves like a conventional thermal store once all the material has changed phase. 

[SteamyTea]

59 minutes ago, Nick1c said:

Isn’t one of the key benefits of a Sunamp that there is availability of water at a constant temperature for the vast proportion of the energy stored? It only behaves like a conventional thermal store once all the material has changed phase. 

Yes, and when it has changed phase there is not a great deal of energy stored.

[newhome]

6 hours ago, SteamyTea said:

Does your space heating system also draw from the thermal store?

 

 

Yes it does. The temps above were based on how I use it in summer when the heating isn't on. When the heating is on I just let it do its own thing as there is enough hot water to shower / bath then anyway. The heating is only on once a day and then set back as the floor temperature is in comfort range during the day and I don't need it to switch on again until the following morning. 

 

[SteamyTea]

9 minutes ago, newhome said:

The heating is only on once a day and then set back as the floor temperature is in comfort range during the day and I don't need it to switch on again until the following morning. 

You may find that in the summer, you are better off setting the TS heating period to switch off just before your normal usage time.  This reduces the thermal loss time.

It does seem barmy that you are heating, to a certain extent, half a tonne of water for a 30 litre, or even 150 litre shower.

 

I would fit a simple electric shower.  Or get that ST sorted, which is probably part of the reason you have the set up you have.  You could junk the ST and fit a simple PV system that just dumps into your thermal store.  ask @ProDave to sort it, he is always in need of cash to pay for the losses on his build.

[newhome]

5 minutes ago, SteamyTea said:

You may find that in the summer, you are better off setting the TS heating period to switch off just before your normal usage time.  This reduces the thermal loss time.

It does seem barmy that you are heating, to a certain extent, half a tonne of water for a 30 litre, or even 150 litre shower.

 

I would fit a simple electric shower.  Or get that ST sorted, which is probably part of the reason you have the set up you have.  You could junk the ST and fit a simple PV system that just dumps into your thermal store.  ask @ProDave to sort it, he is always in need of cash to pay for the losses on his build.

 

In the summer I tend to just heat up the water before I want to shower / bath rather than having it on at a set time each day. As it's only me here that tends to work out fine as I don't have anyone screaming that there is no hot water. And I invariably don't shower immediately after the water is up to temperature.  

 

I think the way to go is an electric shower as PV is not a viable option due to the long payback period and the ST system is still screwed and no one will fix it. 

[SteamyTea]

7 minutes ago, newhome said:

I think the way to go is an electric shower as PV is not a viable option due to the long payback period and the ST system is still screwed and no one will fix it. 

An good electric shower is the simplest and easiest solution.  You need to calculate i.e. read the meter before and after the shower heating period, to work out a payback.

But then payback is an odd concept.

Do you calculate the payback of different food types, different plants in your garden, different types of pets.  No.

So why should choosing a long term reduction in your overall energy bill, be compared to not getting your system sorted (if you pay the right amount, someone will fix it).

[newhome]

15 minutes ago, SteamyTea said:

(if you pay the right amount, someone will fix it).

I haven’t even got as far as anyone wanting to give me a quote. As soon as I say it needs roof access they’re not interested. I don’t blame them in truth as I wouldn’t want to go on the roof either! 

Payback means different things to different people. I don’t mind paying for things I value but getting the same energy using 2 different methods, if the outcome is the same why would you want to pay more using one method over another unless one way of deriving the outcome has value to you? 
 

[SteamyTea]

7 minutes ago, newhome said:

I don’t mind paying for things I value but getting the same energy using 2 different methods, if the outcome is the same why would you want to pay more using one method over another unless one way of deriving the outcome has value to you? 

That is a circular argument and the kind of thing an accountant would say.  An economist would say that you have not taken into account the external costs.

[Bramco]

On 31/10/2020 at 11:19, SteamyTea said:

Any chance you can log some numbers when the tank is in different states i.e. heated but unused, after use, in use and being heated.

Be interesting to see how much turbulence is created.

 

Here's a graph of a sunny day. I should say that our thermal store in now 10 years old and is a DPS store - they were notorious for not being lagged well - it does make for a great drying room though!!

 

The zig zags are the boiler kicking in. So at 7:30 or so because the tank temp has dropped low enough, then there's another one, when one of us took a shower. The big drop in the afternoon is both of us taking showers - it's a rain shower, so uses a lot.

 

From 9am the rising red, grey and orange lines are due to the solar PV divert and the blue line due to the solar thermal going into the bottom of the tank.  The PV divert goes into the middle, so only heats the top half.  Graphs are all from the open energy monitor software emoncms which is excellent for this.

 

If you look to the left you'll see it with the heating on the previous evening - the boiler only fires when temperatures drop enough. There's actually two setpoints on the tank controls, so the backup boiler is set to fire at a higher setpoint when the heating is on, i.e. the tank is a bit warmer - this prevents the flow to the rads dropping too much (we have both rads in the old part of the house and UFH in the extension.

 

Hope that is of use.

 

Simon

[Bramco]

On 31/10/2020 at 13:58, A_L said:

Here is some information on thermal stores, however they are probably rather better than those routinely available in the U.K., certainly tank B is.

 

https://forum.buildhub.org.uk/applications/core/interface/file/attachment.php?id=17632

Thanks I'll take a look.

 

Simon

[LSB]

19 hours ago, Bramco said:

 

Here's a graph of a sunny day. I should say that our thermal store in now 10 years old and is a DPS store - they were notorious for not being lagged well - it does make for a great drying room though!!

 

The zig zags are the boiler kicking in. So at 7:30 or so because the tank temp has dropped low enough, then there's another one, when one of us took a shower. The big drop in the afternoon is both of us taking showers - it's a rain shower, so uses a lot.

 

From 9am the rising red, grey and orange lines are due to the solar PV divert and the blue line due to the solar thermal going into the bottom of the tank.  The PV divert goes into the middle, so only heats the top half.  Graphs are all from the open energy monitor software emoncms which is excellent for this.

 

If you look to the left you'll see it with the heating on the previous evening - the boiler only fires when temperatures drop enough. There's actually two setpoints on the tank controls, so the backup boiler is set to fire at a higher setpoint when the heating is on, i.e. the tank is a bit warmer - this prevents the flow to the rads dropping too much (we have both rads in the old part of the house and UFH in the extension.

 

Hope that is of use.

 

Simon

 

we are planning on ASHP and PV, do you think we need to also consider a back up boiler.  We are off grid here so it will have to be oil or biomass or possibly LPG