Slope weather curve and questions

matt_drummer

If you know about HA you are not very far from cheap monitoring.

Look up ESPAltherma

ninjaef

matt_drummer said:

If you know about HA you are not very far from cheap monitoring.

Look up ESPAltherma

my occupation is software developer... I know a bit.😁 however I'm not a thermally dynamic sponge 😂

michaels

ninjaef said:

matt_drummer said:

I am with Octopus too.

It is an ev tariff. I have one but had no charger at home but got my wife an ev so needed a charger for her because I can't always take it to work to charge for free! They say they don't mind but I think it's taking advantage.

But it gets me electricity at 7p for 6 hours a day which is enough to charge my batteries, I have 38kWh of battery storage.

It's enough to run all day and more with no sun, I have a lot of panels too, and that is why I like to run my heat pump low but all day and night, it means I rarely draw more than my inverter can supply, you know, if we are cooking and heating it's a problem with a heat pump using loads of electricity.

trvs limit how much radiator capacity you have to get rid of the heat produced by the heat pump.

Not really a problem when it is cold but you will encounter a time where without enough radiators turned on (open and flowing water) you won't keep the heat pump running long.

It will short cycle and a Daikin will short cycle badly. They have a very aggressive start up procedure where for the first 20 minutes of a heating cycle is at full circulation pump speed. This results in a load of heat and if radiators are close the return temperature will rise so quickly that the cycle will last only a few minutes.

The heat pump will stop and then try again a few minutes late, but it doesn't start where it left off, it starts again from scratch with full heat output, and this will just continue.

It is one of the reasons they don look't like low flow temperatures when it is warmer, the start is so aggressive that most houses won't have radiators big enough to deal with it.

I have huger, really huge radiators, so much that it is actually a problem. and I still cannot get it to run when it is warm at low flow temperatures, it produces so much heat it is impossible.

I do have a way to deal with it now by restricting the flow rate to the absolute minimum on start up but it also has other consequences.

Really, just be wary that whilst your trvs do no harm now they may well cause short cycling (very short cycling) when it is warmer and you still want heating, just keep an eye on it.

wow, where do you learn all this. super.

ah ok EV and 38kw makes sense. I've no EV and 10kw battery , they will redirect you to be charging an EV every day hence 7p 😁

when ambient is around 16C the heating is off.

So now, should I open all the TRVs ?. they did replace all the rads of all different sizes .. for "equilibrium" they said.. whatever that meant. 

Have a look at the Tomato (energy supplier) tariffs, there were bad reports re CS and billing but they now seem to be getting their act together.  Potentially up to 6 hours at 5p overnight but then usefully a 2 hour at 14p slot in the morning (and again 10pm-midnight) that might mean you can top up your battery and run entirely on 5p with a bit of 14p electricity.

ninjaef

@matt_drummer QQ please if I may consult your expert brain..

1.
today is zero C ambient. LWT is 49C. Octopus set the max LWT at 50C at -3C ambient on the weather curve. House raised from setback of 18C to 20C in 2hrs today.
any views on this? 

and...
2.
At 49C my system is loosing about 27.5% efficiency. (2.5% every 1C). 
At what point is it better to dial in a negative offset  on the curve (so applied across all temps) to reduce the LWT to run at a lower temp but for longer, and if so, isn't any efficiency gain lost because the system is heating for longer? Is there a formula or "quick way" to test/run if so?

and finally, 
3.
I guess there's a LWT temp at which the house just will not heat... so it would take days/weeks of adjustment to discover the optimum  Or small I just leave everything? 

ta

matt_drummer

OK, dealing purely with efficiency and ignoring electricity costs varying throughout the day.

Efficiency is really the key, you want to generate just enough heat to keep the house at the desired temperature using the least amount of electricity possible.

This generally happens by running the heat pump continuously at the lowest possible flow temperature to produce the heat required.

This isn't always true as my data shows but without full monitoring it is impossible to say what will work in every home, what happens in my house may not be what happens in yours.

49c is hot and as you are suggesting running at a lower temperature for longer, I would guess that your heat pump is not running continuously at 49c flow temperature?

I don't know what flow temperature you need to keep your house at 20c but lets say it is 40c

Let's say the heat pump is 400% efficient at 40c and 250% efficient at 49c

Let's say you need 4kW to keep your house at 20c when it is 0c outside

So you need 96kWh of heat

If the heat pump produces 4kW at 40c then running all day will keep your house at 20c and consume 24kWh of electricity at a COP of 4.0

If the heat pump produces 8kW at 49c then it only needs to run for 12 hours a day.

At a COP of 2.5 the same heat pump will consume 38.4kWh of electricity to produce the required 96kWh of heat


So, I made the figures up, but I wouldn't think they are wildly out. Replace them with what you think you have.

There are other considerations, higher flow temperatures result in more chance of defrosting, start ups may consume proportionally more electricity than continuous running. electricity tariff, solar contribution and batteries.

The lower the instant power draw the more chance of covering it from solar and/or battery storage.

It will always use the least amount of electricity to just run continuously matching the heat output to the heat loss of the house.

Vary as required to suit comfort and electricity tariff to get the lowest running costs.

ninjaef

that's great Matt but I have absolutely no idea what much of that was about or what figures I have/use. Lei me all another way please...

I hear a lot about driving at a constant temp but I don't want a house at 20c all day ..17c at night is best for me. So 6am to 9pm is 20C, 9pm.to 6am is 17c. 
I'm confused because if I lower the LWT then it's going to take longer to get from 17c to 20c so the heating will have to ramp up earlier say 4om and that is going to affect my sleep, to warm.
I don't know how best to set up my system in the MMI *in principle* as I know the figures will be different than you. 

my pump is 4kw, 12kw solar, 8kw battery, 89m² floor space bungalow.

matt_drummer

I can't tell you how to set it up, I can only tell you what is most efficient.

You can't have a home that is 20c from 06.00 to 21.00 and 17c at other times without some gradual arming up and cooling down, it's a heat pump, not climate control.

Really, your radiators should/could have been sized to give 20c in the living areas and 17c in the bedrooms is that is what you like.

You could leave the heating running and increase the heat loss at night by opening a window or two?

You could just get used to it?

Or accept that it is going to take some time to heat up from 17c to 20c when it is zero outside.

I would leave it running at a lower flow temperature all day and night and either get used to it or open the windows at night.

At 4kW I doubt your heat pump will be capable of getting the bungalow warmer any more quickly.

I don't know, but I would suspect that being off for most of the night the heat pump will consume more electricity at 49c than it would have done if you had just left it running.

The Madoka does have modulation which you can adjust in the MMI but if the heat pump is already running at 49c then it is probably already close to its maximum output and it won't be able to modulate any higher.

Reed_Richards

I too really like a night-time setback.  The problem is that if you are running at the limits with your weather compensation finely tuned to give you just the daytime indoor temperature that you need (and not using any other type of compensation) then it can, and indeed should, take a very long time to raise the temperature from a lower night-time temperature to a higher daytime temperature.  Personally I don't mind this because I feel I don't need it as warm in the morning, when I am fairly active, compared to the evening when I become more sedentary and need a bit more heat.

What I would recommend is that you tweak your weather compensation settings until the time it takes to get back to the daytime temperature becomes unacceptably large.  This will vary according to the outside temperature so this may be quite a long process.  I recommend you make notes of the changes you make in case you accidentally head off in the wrong direction and want to go back a few steps.  

ninjaef

I understand that thanks Matt. I. would love to be able to dial down TRVs in the sleeping rooms but my daughter won't sleep with the door shut (nightmares) and my dog sleeps in the bedroom with us and needs to go in and out of the bedroom. for water and sometimes just lays somewhere else or sits by the rear doors watching badgers and foxes. so I can't shut the bedroom doors. Consequently heat from the house is going to radiate into bedrooms. it's only a bungalow. I think this I've answered pay of my own question... have to accept the hit bringing up temp at 6am.

I've also applied  -2c offset on the curve , it's 2C outside now and LWT has dropped to 42. I don't think it's realistic to expect the "perfect" 38C to heat a home when it's 2c outside and to match the heat loss thereof... so I guess 42 isn't too bad but I'll need to run for 24hrs to get a good test yes?

matt_drummer

Hi Reed,

My other point was not very clear but it is important.

If the home takes a long while to heat from the night time set back, doesn't it also take quite a long time to actually cool down.

Let's say heated to 20c until 21.00 as above, how long does it take to get to 17c?

Perhaps the bedroom isn't actually at 20c? Maybe it just feels right to turn the heating down at night? Efficient with gas or oil but probably not the most efficient with a heat pump.

My radiators are sized to give 2c cooler upstairs than downstairs, so I just leave the heating running. It may be 20c downstairs when we are in bed and don't need 20c but it is cheaper than letting it cool down.

My house would take all night so by the time the house had reached the temperature I wanted to sleep in, I would be ready to get up and now be too cold.

Just as the house doesn't heat up immediately, it doesn't cool down immediately either.

As you have said, they just need to experiment until they find what suits them.