Heat Pump Pricing versus current costs

Reed_Richards

@matt_drummer,  I'll have a look at flow rate but:

• There are two flow rates I can change, because I have two pumps.  One sets the flow rate round my heating system, the other the flow rate from my heat pump.  Increasing the flow rate round my heating system would reduce the temperature differential across the radiators and so cause them to give out a bit more heat for the same input water temperature.  But it would also increase the noise level from the water flowing through the radiators, which is borderline acceptable at present (at setting 2 of 3).
• I'll have to check but I think the flow rate from the heat pump is near the maximum setting at present, in which case I could only reduce it.

Your point that I don't have monitoring is well-made.  With hindsight I wish I did but here I am without it. 

 

michaels

Not sure I understand how having the same water temp and delta T can result in different levels of radiator heat output to match the different levels of heat demand resulting from changes in outside temps?

I assume the level of insulation means that the heat demand does not differ much as the outside temp changes (almost flat weather comp slope) but in that case the 4.5kw at every outside temp sounds strangely high?

Surely the only way of having lower heat output is that the radiators are colder, whether that is by using weather comp so they are constantly warmer or colder depending on the heat demand/loss or by having them vary in temp via cycling of the input heat on and off so that the average temp is lower.

Edit:  Thinking out loud, perhaps it can be achieved by a lower flow rate so the difference between the water entering and leaving the rads is greater. 
EG high flow, water entering and leaving rad is 30C and 28C so average rad temp is 29C
Low flow, water entering is still 30C but water leaving is 24C so average rad temp is 27C

So effectively a very shallow weather comp rad temp effect but achieved via adjusting the flow to adjust the average rad temp.  Seems like it might be hard to configure and depend on the rad circuit being very high volume to make sure that even with low flow the whole circuit still gets the hot flow.

FreeBear

matt_drummer said: Why increase flow temperature to get more heat when you can increase flow rate and leave the temperature low to get the same result?

There is a limit on how high the flow rate can go. Either a limitation of the pump, or the sizing of the pipes. With 28/22/15mm pipes, you are unlikely to hit either limit.

matt_drummer

FreeBear said:

matt_drummer said: Why increase flow temperature to get more heat when you can increase flow rate and leave the temperature low to get the same result?

There is a limit on how high the flow rate can go. Either a limitation of the pump, or the sizing of the pipes. With 28/22/15mm pipes, you are unlikely to hit either limit.

My pump runs at 6.5 litres per minute and can go up to 25 litres per minute.

Plenty of scope to vary heat output.

My dT between flow and return can be set between 1c and 10c, again plenty to play with.

With all of that, I never need to change my flow temperature as my radiators can handle it.

matt_drummer

michaels said:

Not sure I understand how having the same water temp and delta T can result in different levels of radiator heat output to match the different levels of heat demand resulting from changes in outside temps?

I assume the level of insulation means that the heat demand does not differ much as the outside temp changes (almost flat weather comp slope) but in that case the 4.5kw at every outside temp sounds strangely high?

Surely the only way of having lower heat output is that the radiators are colder, whether that is by using weather comp so they are constantly warmer or colder depending on the heat demand/loss or by having them vary in temp via cycling of the input heat on and off so that the average temp is lower.

Edit:  Thinking out loud, perhaps it can be achieved by a lower flow rate so the difference between the water entering and leaving the rads is greater. 
EG high flow, water entering and leaving rad is 30C and 28C so average rad temp is 29C
Low flow, water entering is still 30C but water leaving is 24C so average rad temp is 27C

So effectively a very shallow weather comp rad temp effect but achieved via adjusting the flow to adjust the average rad temp.  Seems like it might be hard to configure and depend on the rad circuit being very high volume to make sure that even with low flow the whole circuit still gets the hot flow.

Almost right.

Two times more water at the same flow temperature and dT is double the heat.

You only need to vary one of the three to change the heat produced. What you can vary and by how much depends on the heat pump controls, the circulation pump and the emitter's ability to deliver heat. 

But your example is almost exactly what I do, my flow temperature remains the same as does my flow rate. All that happens is that as it gets colder outside the dT between flow and return gets bigger. The radiators cool the water more as the difference between inside and outside increases.

My pipework is not ideal so I favour low flow rates.

I balanced all my radiators using a thermal imaging camera. I fitted high quality Danfoss lock shield valves that allow good flow and fine adjustment. No trvs.

The key was getting the radiators in the right proportion to the desired room temperature and heat loss. With monitoring I was able to play and play until I found something that worked.

In all honesty I wouldn't have done any of this but for having to find a way to make my original heat pump work in this house. I spent a lot of time on it.

I am not suggesting that anybody should copy me, although you can if you want as it works really well

I am trying to demonstrate the relationship between flow temperature, flow rate and dT. Adjusting any one of those affects the heat produced. Increasing flow temperature is not the only way to get more heat.

In the context of the topic, high flow temperatures are not the only way to get more heat and if you can get the heat you need by a bigger change in water temperature (increased dT by using bigger emitters) and/or by flowing more water (again probably needs bigger emitters) you end up with a lower flow temperature which is more efficient and will use less electricity.

With any systems, mine included, you will reach a limit where the radiators cannot deliver any more heat at your chosen flow temperature. Then the only way to get more heat is to increase the flow temperature which then allows the radiators to give out more heat.

And that is where these high flow temperature heat pumps come in, they allow you to get the heat you need with the same radiators that you used with a gas or oil boiler.

The reality is that most houses have radiators that are already bigger than needed. The radiators that I had with my gas boiler allowed me to run a heat pump at 43c to get the heat I needed.

All homes are different though so the flow temperature required could be higher or lower than I experienced.

But I think most of us agree, running at anywhere near 65c will be expensive and inefficient and should be avoided if possible.

wrf12345

With individual air to air heat pumps (aka air-conditioners running in reverse) do the same rules of running for a long time at a low heat output apply? Apart from the frightening money that installers tend to charge, I am quite tempted by these as I tend to close down a couple of rooms in the winter, don't use hot water, and feel they might be easier to understand and control, and can be installed in stages rather than all at once. No grant, though.

matt_drummer

wrf12345 said:

With individual air to air heat pumps (aka air-conditioners running in reverse) do the same rules of running for a long time at a low heat output apply? Apart from the frightening money that installers tend to charge, I am quite tempted by these as I tend to close down a couple of rooms in the winter, don't use hot water, and feel they might be easier to understand and control, and can be installed in stages rather than all at once. No grant, though.

Not really.

I have experience of air to air heat pumps and they work really well. Easier to control and set up and much easier to understand. Probably more efficient but it is difficult to measure.

They still use more electricity when first turned on but quickly settle down to quite low levels of electricity consumption.

They have many advantages, the heat is instant and they control the temperature pretty well. Having heating and cooling is a bonus.

They are not necessarily cheap to install and with the grant that is currently available probably more expensive to the home owner if you want to heat the whole home.

Strictly speaking, you need planning permission too.

I still think you underestimate what is required swapping a gas boiler for a heat pump following the MCS rules to enable you to get a grant.

An Octopus heat pump installation is a total bargain when you see what is actually involved and the materials required.

FreeBear

matt_drummer said:

wrf12345 said:

With individual air to air heat pumps (aka air-conditioners running in reverse) do the same rules of running for a long time at a low heat output apply? Apart from the frightening money that installers tend to charge, I am quite tempted by these as I tend to close down a couple of rooms in the winter, don't use hot water, and feel they might be easier to understand and control, and can be installed in stages rather than all at once. No grant, though.

Not really.

I have experience of air to air heat pumps and they work really well. Easier to control and set up and much easier to understand. Probably more efficient but it is difficult to measure.

...
Strictly speaking, you need planning permission too.

Permitted development* only applies to a single outdoor unit being used for heating. If you use the unit for cooling, or install more than one, planning permission is required. With changes to legislation introduced last year, the council now have up to ten years to serve enforcement notices for planning & building regs breaches.

Another advantage of using Octopus or any other MSC installer, is they (should) take care of getting the relevant permissions and provide compliance certificates once the work is complete.

Reed_Richards

wrf12345 said:

With individual air to air heat pumps (aka air-conditioners running in reverse) do the same rules of running for a long time at a low heat output apply? Apart from the frightening money that installers tend to charge, I am quite tempted by these as I tend to close down a couple of rooms in the winter, don't use hot water, and feel they might be easier to understand and control, and can be installed in stages rather than all at once. No grant, though.

The big advantage of air-to-air is that you only heat the air to slightly hotter than you want your room temperature to be.  Whereas with radiators (or UFH) you need to make the water quite a bit hotter than you want your room to be.  That is always going to be the less-efficient option so air-to-air ought to give you better efficiency and lower running costs.

The big disadvantage is that unless you have a very small property you will need more than one air-to-air unit.  Installation could work out quite expensive, particularly without the grant that you get for air-to-water.  And you would need planning permission for more than one unit.  

matt_drummer

Reed_Richards said:

wrf12345 said:

With individual air to air heat pumps (aka air-conditioners running in reverse) do the same rules of running for a long time at a low heat output apply? Apart from the frightening money that installers tend to charge, I am quite tempted by these as I tend to close down a couple of rooms in the winter, don't use hot water, and feel they might be easier to understand and control, and can be installed in stages rather than all at once. No grant, though.

The big advantage of air-to-air is that you only heat the air to slightly hotter than you want your room temperature to be.  Whereas with radiators (or UFH) you need to make the water quite a bit hotter than you want your room to be.  That is always going to be the less-efficient option so air-to-air ought to give you better efficiency and lower running costs.

The big disadvantage is that unless you have a very small property you will need more than one air-to-air unit.  Installation could work out quite expensive, particularly without the grant that you get for air-to-water.  And you would need planning permission for more than one unit.  

You actually need planning permission for one air to air heat pump that is capable of cooling.