Heat Pump Sizing?

michaels

Reed_Richards said:

Gas and electricity prices vary but if you divide the price you pay per kWh for electricity by the price you pay per kWh for gas then that is the SCOP you will need to make the heat pump slightly cheaper to run (since your gas boiler is not 100% efficient).  If you cannot operate a heat pump at conditions that would achieve that SCOP then a heat pump is likely to be uneconomic, at current energy prices.

But the chances are you will be able to achieve that SCOP or better quite easily.  After that it comes down to a trade-off between running costs and the inconvenience/expense of squeezing in suitably large radiators.      

In the short term at least I can use my V2H to store overnight electricity at 7.5p per unit hopefully in enough volume to run the whole house on all but the highest use days - factor in some round trip losses and perhaps the electricity cost would be about 8.5 to 9p per unit thus I would need to get very inefficient on the SCOP not to be cheaper than gas at 5.5p per unit.

However who knows if this type of tariff will be available long term or whether my V2H unit will remain operational and supported long term although hopefully it will be fine until at least alternative V2H solutions become available (a new BYD UK trial was announced today).

Are there any 'rule of thumb' rules out there re flow temps - eg a 50-35 heat curve will use 25% more electricity than a 40-25 curve and 15% more than a 45-30 curve?!

(Aside: as a household we use considerably more hot water than for example the standard MCS assumption so will need a bigger proportion of 'higher temp' output than the standard models which might impact on the decision re any oversizing)

matt_drummer

It is very difficult to give detailed advice with very little knowledge of your house.

I remember in one post that you said that cost was not an issue, just getting the correct heat pump, is that not the case?

I am not that familiar with Samsung controls but I have read many topics and posts and the feeling I have is that there is very little in the way of adjustment compared to what I am used to with Daikin.

With Samsung it seems you need high flow rates which can be an issue with retrofits because of the issues you have described.

You can't just put two circulation pumps in series on a heating system, it takes more work than that.

I would forget weather curves at this stage, it is a detail you cannot address right now.

I don't run a weather curve at all.

I just run a fixed flow temperature which I increase slightly as it gets warmer.

I have a requested dT between flow and return that I cannot achieve.

What happens is that my heat pump runs at its lowest flow rate all of the time.

As the weather gets colder and the heat loss increases the dT between flow and return increases and the heat output goes up.

This is because I very carefully matched the radiator sizes to the heat loss of each room and the overall capacity to the total heat loss of our house. It is self regulating and I just choose a flow temperature and pretty much leave it alone. By either luck or judgement my heat pump works at around 30c in any weather.

You don't need manufacturer data.

heatpumpmonitor.org gives you all you need, virtually every heat pump going and data in great detail.

You can see flow and return temperatures, flow rates and energy consumption, what more do you need to decide what is the right heat pump for your house?

The lower the flow temperature in general is what will work best.

There's always a way to get enough radiators, by increasing sizes, increasing panels/convectors and by adding more radiators.

How far you go is up to you.

michaels

Thanks.  Difficult to add rads in our house downstairs as solid floors and the rooms being designed for the rads in certain positions.  The info I found on the relationship between delta T and rad output means I can't imagine how people could get to a 30C flow rate so a delta T of 10, our biggest room needs 2.2kw of heat for example and another needs 1kw and currently has a 450x1800 dual penl vertical and there doe snot seem to be a vertical replacement that would produce enough output even at dT20.

My mindset is very much attached to weather comp as this is what we have now and it works beautifully with a gas boiler that will output between 2kw and 19kw (although the max we need is about 9kw) and just runs continually with the rads being warmer or cooler and so always emitting enough heat to balance the heat loss.

This means I find things like the viessman documentation showing output ranges at different air temps as being very helpful. (Technical Guide.pdf (viessmanndirect.co.uk) )

Cost is always an issue, I'm willing to pay more to not have the heat pump on the back of the house (although a shorter pipe run to the side should help offset this) but if paying 3.5k more for a Viessman would take 20 years to recoup then that may not make sense, there is also the option of replacing the existing gas boiler for say 3k to compare against.

matt_drummer

A heat pump doesn't work like a gas boiler.

The amount of heat produced by a heat pump is not governed by the flow temperature.

It is a product of the dT between flow and return and the amount of water flowing.

So you can either maintain a dT and vary the flow temperature and flow rate to change the amount of heat produced.

Or, what I do is leave my flow rate and flow temperature fixed and let my radiators control the dT based on how much heat the house requires.

The same end result in terms of heat and comfort, but the big difference is that I am never heating water up more than I need to.

It is a huge change in mindset. Gas and oil boiler have a fixed energy output per unit of energy consumed and that doesn't really change much. With a heat pump, the amount of heat produced for each unit of energy consumed varies wildly depending on many factors. The biggest factor though is how much you need to heat the water by, less is better.

I understand the problem with radiators.

You can't solve the problem without expense and disruption. Adding radiators or moving them will either be expensive or spoil the decor.

You would need two large k3 and four large k3 radiators to heat those rooms at 30c flow.

I have k3 radiators downstairs and I think my two in the lounge put at about 1.1kW without the fans but they are 600mm x 1800mm, not cheap and not necessarily that easy to accommodate.

matt_drummer

Out of interest, what did various installers offer and recommend?

michaels

matt_drummer said:

A heat pump doesn't work like a gas boiler.

The amount of heat produced by a heat pump is not governed by the flow temperature.

It is a product of the dT between flow and return and the amount of water flowing.

So you can either maintain a dT and vary the flow temperature and flow rate to change the amount of heat produced.

Or, what I do is leave my flow rate and flow temperature fixed and let my radiators control the dT based on how much heat the house requires.

The same end result in terms of heat and comfort, but the big difference is that I am never heating water up more than I need to.

It is a huge change in mindset. Gas and oil boiler have a fixed energy output per unit of energy consumed and that doesn't really change much. With a heat pump, the amount of heat produced for each unit of energy consumed varies wildly depending on many factors. The biggest factor though is how much you need to heat the water by, less is better.

I understand the problem with radiators.

You can't solve the problem without expense and disruption. Adding radiators or moving them will either be expensive or spoil the decor.

You would need two large k3 and four large k3 radiators to heat those rooms at 30c flow.

I have k3 radiators downstairs and I think my two in the lounge put at about 1.1kW without the fans but they are 600mm x 1800mm, not cheap and not necessarily that easy to accommodate.

I'm guessing you are saying a combined output of 1.1kw from both K3s rather than individually.

I found this correction factor website suggesting that you need basically double the nominal rad output if you take the flow down from 70 to 50 (dt from 50 to 30) and reducing it from 30 to 10 means another 4x as much rad output!!!

Delta T Conversion l Explanation, Calculations & More (radiatorsonline.com)

Octopus wanted a 10kw Daikin with a 50 plus flow temp

Others proposed a split unit Grant, a Vaillant, an Ecodan and the Viessman people a Viessman, sizes from 8kw up to 16kw....Octopus were 4.4k including about 8 rads, others were between 6 and 8k plus any rads (tbc) on top.  Some would install on the flat roof, others preferred not too, only one said get planning permission, others said would happily install at my risk or that it probably wouldn't be a problem if/when the rules change....

Seem to be some very large sums quoted for plumbing and MCS considering I reckon I could do it myself and it wouldn't take too long.

DougMLancs

You can’t do MCS yourself though and without MCS you can’t get the BUS grant. MCS are supposedly changing to a more hands-on, less paperwork approach but as it stands, there is a massive load of paperwork for MCS and even more for the BUS grant. It’s not just the time on the tools you’re paying for, it’s the design, admin, MCS, BUS, DNO and Building Control, etc. 

matt_drummer

Yes 1.1kW is for both radiators, they are 5.6kW each at dT 50c

I think you may be surprised by the amount of work required in changing from a gas boiler to a heat pump and the amount of stuff required.

It took Octopus a full week to fit mine.

Once done though, swapping heat pumps took 3 hours or so.

Reed_Richards

I can't help thinking that the reason @matt_drummer goes not use Weather Compensation is that his starting point (30 C) is so low that there isn't room to go any lower.  I use WC very successfully with my heat pump but my starting point is about 44 C flow at -3 C outside.  Matt will be achieving better efficiencies than I get; I didn't have the space for radiators that would work at 30 C at outside temperatures less than 12 C. 

NedS

michaels said:

Thanks

Funnily enough I was just pouring over the specs of the Samsung 12kw unit (is yours the Gen 6 or Gen 7?) - I think the 8kw would be too small - lots of info on the max output at different temps but nothing on the minimum.  Seems to be 59DB sound power.

Mine is a Gen 6 (R32)

The Samsung controller is not too bad, IMHO, as someone without any experience of anything else.

It's a wired controller - you can either install it at the cylinder/plant room with a 3rd party wireless room stat to control heating, or you can install it in a suitable location in room (wired) to also act as a room stat (I think the former is very preferable). You can programme on/off schedules for heating and DHW - we use the Samsung controller to schedule DHW runs, but use our 3rd party wireless room stat controller to programme heating.

The weather comp settings are easy to programme / understand - simply high/low points and the flow temps at those ambient outside temps. I have mine set for 40C at -3C and 30C at 15C as a starting point.

You can control the cycling of the unit either by room stat (call for heat), and/or by LWT. My unit is oversized for my property, so the LWT can never achieve the 30C I've set it to when mild - it sits at around 33C, so I've switched off that facility in the Samsung controller and simply allow the room stat to control the heat pump, turning it on/off depending on room temp. When it's mild, the heat pump can't achieve it's target LWT, but will run as low as it can, and the room stat will turn off once the room reaches the set point. It is not possible to run 24/7 in milder weather, so the heating is on for a bit, then off for a bit. Once we hit the cold weather, we should be able to tick over constantly at LWT of 33C or so.

Happy to sell you my barely used 12kW unit and replace with an 8kW unit