Basic ASHP Efficiency Questions

[soapstar]

Hi Folks,

 

This is probably a controversial subject and there is (before anyone points it out! ? ) lots of information out there regarding how to calculate the most efficient methods of running ASHP's. However for a simple minded person like myself I would like to understand the system better to allow me to make a judgement on how I should be running our ASHP.

 

We have an ASHP which feeds UFH downstairs and radiators upstairs. Heatmeisser stats are in every room downstairs to control the temperature of each room or UFH heating loop for that room as I understand. And one stat upstairs on the landing to control the radiators on the top floor for every room - each radiator has its own thermostat also.

 

Obviously the main goal here is to reduce running costs. We have several rooms downstairs which I feel do not need any heat as they will never be used (spare bedrooms). From a running cost perspective what is the consequences of setting the temperature very low in these rooms? Lets say 10 degrees (with the rest of the house at 19) - would this be detrimental to the overall running costs of the house? Is there a point where it can be too low? I understand the cost involved to bring these rooms up to temperature when required would be quite pricey however if your not using the room what's the point in heating it?!

 

Going even further is it beneficial at all to turn the heating off completely for these rooms? This part confuses me as I have been told its good practise to keep the flow running around each UFH loop - now if it was turned off this is going to stop correct?

 

In regards to 'turning off' the heating this begs the question how is this possible with thermostats in every room. These are set to keep a constant temperature, so if I wanted to turn off the heating for say between 10pm - 6am do I simply set the thermostat to 0 degrees between these hours, then at 6am set the temperature back to the normal temperature of 19 degrees? And hope the temperature does not drop considerably over night...

 

Obviously there are other variables like weather, drafts, insulation etc however lets assume these are all constant - I just want to get an understanding of how to reduce the ASHP output before it gets to the stage where its doing more harm than good.

 

Thanks

[ProDave]

The first "efficiency" hurdle you have, is having radiators in the system.  Even high output radiators will demand a higher operating temperature than the UFH.  If you had UFH throughout your best efficiency choice would be to run the flow temperature from the heat pump as low as possible.

 

If the only controls you have is simple thermostats, then replacing those with programable thermostats would be a good move then you can set indivisual room temperatures at different times of day.

[soapstar]

Just now, ProDave said:

The first "efficiency" hurdle you have, is having radiators in the system.  Even high output radiators will demand a higher operating temperature than the UFH.  If you had UFH throughout your best efficiency choice would be to run the flow temperature from the heat pump as low as possible.

 

If the only controls you have is simple thermostats, then replacing those with programable thermostats would be a good move then you can set indivisual room temperatures at different times of day.

 

hi @ProDave thanks. We looked into having UFh heating throughout however the costs involved were quite pricey!

 

Sorry I should have mentioned we do have programmable thermostats, these can be set at different times. My main query was how low can you go for an 'unused' room and is it wise to set the thermostats really low overnight (or off if possible?).

[SteamyTea]

I think the only risks you run about only heating some part of the house is that you run the risk of short cycling and you may have to run the flow temperature higher.  Both of those will hurt your CoP.

Have you done a room by room heat loss analysis?  This may show you what flow temperatures are optimal.

 

On @ProDave's point about having radiators on part of the system.  The power they can deliver is a function of the surface area and the temperature differences.

If they perform well at say a flow temp of 40°C, then this should not be a problem and the UFH should have a temperature blender to reduce from this.

It really does depend on the system design.

Do you have a buffer tank fitted?

[soapstar]

53 minutes ago, SteamyTea said:

I think the only risks you run about only heating some part of the house is that you run the risk of short cycling and you may have to run the flow temperature higher.  Both of those will hurt your CoP.

Have you done a room by room heat loss analysis?  This may show you what flow temperatures are optimal.

 

On @ProDave's point about having radiators on part of the system.  The power they can deliver is a function of the surface area and the temperature differences.

If they perform well at say a flow temp of 40°C, then this should not be a problem and the UFH should have a temperature blender to reduce from this.

It really does depend on the system design.

Do you have a buffer tank fitted?

 Hello @SteamyTea Yes we have a 50 litre buffer tank fitted.

 

Sorry can you explain in simple terms what a room/room heat analysis consists of? My thinking was to try out different scenarios and keep an eye on the meter which reads the ASHP only, although this is not very accurate as there are a lot of other factors at play

[SteamyTea]

9 minutes ago, soapstar said:

Sorry can you explain in simple terms what a room/room heat analysis consists of

Calculating the heat losses between each room of the house, as well as any losses to the outside.

From this the heat emitter sizes, and at what temperatures, they will operate best at, can be calculated.  From this the heat source can be sized correctly.

 

It is the same as going an external wall, floor, window, door and roof heat loss, except some rooms will have different temperatures i.e. living rooms and bedrooms.

It may seem a bit of a faff, but it should help with setting a system up.

Edited February 17, 2021 by SteamyTea

[soapstar]

5 minutes ago, SteamyTea said:

Calculating the heat losses between each room of the house, as well as any losses to the outside.

From this the heat emitter sizes, and at what temperatures, they will operate best at, can be calculated.  From this the heat source can be sized correctly.

 

It is the same as going an external wall, floor, window, door and roof heat loss, except some rooms will have different temperatures i.e. living rooms and bedrooms.

It may seem a bit of a faff, but it should help with setting a system up.

 Thanks @SteamyTea Is there anything which can assist me in calculating the heat loss?

 

On another note I understand there is a thermostat connected to the UFH manifold which is set at 45 degrees - how can I determine if this is an adequate temperature? I assume if this is set too low the ASHP will struggle to heat the rooms to the set temperatures and never be off. On the other hand what are the consequences if this is set too high? 

[SteamyTea]

1 minute ago, soapstar said:

Is there anything which can assist me in calculating the heat loss

Excel and Google.

 

1 minute ago, soapstar said:

On another note I understand there is a thermostat connected to the UFH manifold which is set at 45 degrees - how can I determine if this is an adequate temperature

Someone like @PeterW will know more about the intricates of the actual set up..

But 45°C seems high if it the flow temperature into the UFH, probably fine for radiators.

[soapstar]

I believe the output temperature on our ASHP for space heating is set at 50 degrees with the hot water set at 55 degrees if this helps

[Ultima357]

If you have Heatmiser Neostats, simply set the overnight to 5 deg lower than daytime. Even in a poorly insulated place you'd be unlikely to drop this much. I'm guessing that as you have ASHP then you are decently insulated and draught proof. Not heating some of the house as high as the rest will save some pennies but if your internal walls are not insulated, a cold one could attract moisture if the temperature in these is too low. The cost difference between 10 and 15 deg is not huge and bear in mind that insurance policies usually have a minimum temperature required around the 13 to 15 mark if you're claiming for frozen pipes. 

[soapstar]

I have noticed by reducing our heat setting lower overnight does indeed save us money, all be it small but it is a saving!

 

Can anyone shed any light on the query regarding the UFH temperature? I would like to know what determines the correct thermostat setting at the UFH manifold if our ASHP output is 50 degrees for heating - therefore should the UFH heating be set at 50 degrees also?

 

[Ultima357]

Normally there is some mixing of the supply temp to the UFH by buffer tank giving a lower input to the UFH. You can run the UFH at 50 but it uses more energy. With a mix of radiators and UFH and without separate heating control/circuits you are juggling. The system designer should help you out if it's a new system. Basically I'd imagine that your ASHP has a set water out temperature which feeds both rads and UFH so to get the best performance the system should either have oversized rads that can run at a lower temperature or some mixing to lower the UFH feed. UFH is normally set between 35 to 45 deg. If no system designer help is available then try setting the water out temperature back to 45 deg and see how it all performs. If all is well, then try lower still. It is normal for the ASHP to have weather compensation or water law as some call it whereby it increases the water outlet temperature in colder (Eg near freezing) weather. 

Edited February 22, 2021 by Ultima357
More info

[soapstar]

Thanks @Ultima357. I can confirm we are running oversized radiators with a 50 litre buffer tank. I have attached a picture of our UFH manifold setup.

 

The thing that confuses me is if the ASHP is set at 50 degrees for heating will lowering this thermostat contradict the ASHP and cause issues? Where will the 'extra' heat go if we were to set the thermostat to say 45 degrees?

 

[dpmiller]

The ASHP will "slow down" as the energy is not being removed that it is adding. Just like a modulating boiler....

 

Remember that the underfloor loops share water with the heatpump but aren't directly connected due to the magic of the mixer valve...

[CallMeMeryl]

I am hoping that all you knowledgeable people will be kind enough to help me out with advice. I do not have a technical bone in my body and am struggling with the ASHP I have just had installed. It is costing me a fortune!  I have been scouring the net for help and came upon this site.

 

I have of course approached the installer, and also Panasonic directly. I have also looked at the 'paperwork' from Panasonic but no further forward. I just need to know the best way to use it.

 

The unit is a Panasonic WH-MXC12H6E5. Admittedly it was installed right before we had the snow this year.  The installer kept referring to the curve (don't entirely understand but get the gist that these settings have something to do with performance/efficiency).  When I displayed shock at how much it was costing me to run, the installer amended the curve from 55 to 45 @ -15 degrees, and 35 @+15 degrees.  Does this make sense?

 

The installer asked us what time settings we wanted so we started with warmer during the day and cooler at night (mimicing the old gas system ). But the trouble with this is the length of time it takes to effect a change of 1 degree - typically 5 hours! This is useless so I am currently keeping it at a steady temperature.

 

We live in an Edwardian house but with double glazing, cavity fill and loft insulation in the south of England. We are running radiators, no UFH. We have TRV on the bedroom rads and most of these are turned down low as no children home at the moment.

 

So specific questions:

1  What should the curve settings be ?

2  Should I alter temp throughout day/night or keep it on a constant heat? (I want to be warm at night but only to a point!)

3  Am I right to turn TRVs right down in unused rooms?

 

I wasn't sure if to post a new thread or come on in this one , I am just hoping that members get new messages flagged up and someone will take pity on me.  Thank you so much. The more I find out about ASHPs and my circumstances the whole situation is getting like the old joke about asking for directions someplace and being told "well I wouldn't start from here"!

[Temp]

On 17/02/2021 at 11:42, soapstar said:

My main query was how low can you go for an 'unused' room and is it wise to set the thermostats really low overnight (or off if possible?).

 

In general the cost of running a heating system depends on the heat loss. So reducing the room temperature reduces heat loss and running cost.

 

Any other effect on overall efficiency is a secondary factor. So I say go ahead and turn off/down rooms that don't need to be heated.

 

[Temp]

It would be interesting to see what other members think of adding something like these fans to radiators to improve their ability to emit heat at lower flow temperatures. Obviously it might be better to fit larger or double rads but if that's already been done what about these? Would the electricity they consume out weigh the improvement? They claim a running cost of £2 a year.

 

https://www.amazon.co.uk/Radfan-Classic-Small-Power-Radiator/dp/B00GR0STTG

 

[Temp]

30 minutes ago, CallMeMeryl said:

I am hoping that all you knowledgeable people will be kind enough to help me out with advice. I do not have a technical bone in my body and am struggling with the ASHP I have just had installed. It is costing me a fortune!  I have been scouring the net for help and came upon this site.

 

I have of course approached the installer, and also Panasonic directly. I have also looked at the 'paperwork' from Panasonic but no further forward. I just need to know the best way to use it.

 

The unit is a Panasonic WH-MXC12H6E5. Admittedly it was installed right before we had the snow this year.  The installer kept referring to the curve (don't entirely understand but get the gist that these settings have something to do with performance/efficiency).  When I displayed shock at how much it was costing me to run, the installer amended the curve from 55 to 45 @ -15 degrees, and 35 @+15 degrees.  Does this make sense?

 

The installer asked us what time settings we wanted so we started with warmer during the day and cooler at night (mimicing the old gas system ). But the trouble with this is the length of time it takes to effect a change of 1 degree - typically 5 hours! This is useless so I am currently keeping it at a steady temperature.

 

We live in an Edwardian house but with double glazing, cavity fill and loft insulation in the south of England. We are running radiators, no UFH. We have TRV on the bedroom rads and most of these are turned down low as no children home at the moment.

 

So specific questions:

1  What should the curve settings be ?

2  Should I alter temp throughout day/night or keep it on a constant heat? (I want to be warm at night but only to a point!)

3  Am I right to turn TRVs right down in unused rooms?

 

I wasn't sure if to post a new thread or come on in this one , I am just hoping that members get new messages flagged up and someone will take pity on me.  Thank you so much. The more I find out about ASHPs and my circumstances the whole situation is getting like the old joke about asking for directions someplace and being told "well I wouldn't start from here"!

 

Can we start by doing a before and after comparison. What were you paying for gas and what are you paying now for electricity? For example do you have old bills that tell you how many units of gas you used? 

 

By my reckoning electricity is so much more expensive than mains gas (per kWH) that ASHP will definitely cost more to run in very cold weather.  

 

 

 

 

 

 

Edited March 1, 2021 by Temp

[SteamyTea]

At the most basic level, any heatpump will loose efficiency as the temperature difference increases.

It is also not linear.

The idea is that the system has to be designed to work in the most effective temperature range.  This is achieved with a combination of system size and refrigerant gasses used.

 

(the CoP is the extra energy from the air or water, not the total energy)

Edited March 1, 2021 by SteamyTea

[CallMeMeryl]

My gas last Jan/Feb cost me approx £5 per day using 382 units (4210 kWh).  

 

My average daily electricity usage for this January using heat pump (but only fitted 22nd Jan) was 35kWh per day, and for February that came down to an average of 27 kWh per day.  I have been keeping a simple spreadsheet of external temperature and heat pump settings in order to try and suss it out.

 

So on the face of it it is averaging out but the difference is we are not as warm as we were even with settings on average 2 degrees higher than previously.

 

The way the heat pump was 'sold' to use was that although electricity is dearer than gas, it would be much more efficient. Am I just misinformed do you think?  My rationale was tat the gas boiler didn't have much life left in it and I was making a 'greener' choice by installing the heat pump.

[CallMeMeryl]

37 minutes ago, Temp said:

 

In general the cost of running a heating system depends on the heat loss. So reducing the room temperature reduces heat loss and running cost.

 

Any other effect on overall efficiency is a secondary factor. So I say go ahead and turn off/down rooms that don't need to be heated.

 

Thank you, that was my logic. So, I will keep doing that.

[ProDave]

14 minutes ago, SteamyTea said:

At the most basic level, any heatpump will loose efficiency as the temperature difference increases.

It is also not linear.

The idea is that the system has to be designed to work in the most effective temperature range.  This is achieved with a combination of system size and refrigerant gasses used.

 

(the CoP is the extra energy from the air or water, not the total energy)

That graph looks wrong.

 

It says when I am heating my DHW at 50 degrees even when it is a balmy 10 degrees outside, so delta T 40 degrees, it is running at a COP of 0.

[SteamyTea]

32 minutes ago, ProDave said:

That graph looks wrong

Look at it again.

As the ∆T increases, the CoP reduces.

It has nothing to say about absolute numbers (in fact it is just made up numbers, I could change the chart to fit what you want though).

The important thing is that it is sizing and system design that is important, not the underlying technology.

A thermal boiler is just a heat engine, just that very little excess energy comes from the air that is drawn in to aid combustion. Though with a condensing boiler, energy, via phase change, is sometimes recovered.

[dpmiller]

so your chart is fake then.

[SteamyTea]

2 minutes ago, dpmiller said:

so your chart is fake then.

No. It is illustrative.

There will be a system that fits that curve, that is the beauty of physics. The concepts are simple.