ASHP Sizing

[Triassic]

On 23/02/2021 at 18:21, LA3222 said:

Wait time - 84 days!

Just checking elsewhere and  can’t find anyone with them in stock. 
 

Im looking at the Daikin Altherma and City Plumbing have 9 in stock.

[Stones]

I bought my ASHP from here:

 

https://www.seconrenewables.com/

 

Whilst the website does say trade only, they were quite happy to sell to me. That was maybe just luck of course as when I phoned to enquire, the person who answered happened to be the managing director.

 

Just to add if you look at their trade account application, 'self build' is listed as one of the business they will deal with

[Triassic]

12 hours ago, Stones said:

I bought my ASHP from here:

 

https://www.seconrenewables.com/

I’ll be calling them on Monday.

[J1mbo]

On 14/02/2021 at 21:14, LA3222 said:

I am trying to decide on ASHP sizing. Using the heat loss spreadsheet I believe I can get by with an 8.5kW ASHP.

 

The problem is that I want a 300l DHW tank. All the sellers i find with the Ecodan seem to pair the 8.5kW version with maximum 210l tank. I am considering to go up to the 11kW ASHP so I can get the larger tank - is there any reason 'not' to go ahead and oversize the ASHP?

 

The UFH will run off a buffer so there shouldn't be cycling issues with that part of the system?

 

I don't know the Ecodan system, but I do have a Vaillant Arotherm plus installed. Its DHW has three modes - eco, balance or normal. Essentially normal runs flat-out until cylinder is satisfied, eco heats the tank at 45% compressor modulation, and balance (I think) aims for a target flow temperature whilst heating the tank, so reduces compressor load as flow temperature increases.

 

The DHW side is worth getting right. My household is 5 and we have pumped showers. In Eco mode, the system can be plodding along doing DHW for hours during which time there is no heating. I've more recently lowered the tank temp (to 48°C - which is counterintuitive, but bear with) and changed the mode from Eco to normal. Because the tank is cooler, running at max the HP can deliver more KW to the cylinder (12-18kW in my case, as it recharges). Although this mode sacrifices efficiency - DHW is averaging about 2.4 - it greatly reduces the recharge time to the point that the DHW isn't really noticed in terms of impact on heating. DHW load in my case is small compared to heating load. Tank is 250 litres.

 

Re oversizing the HP. System volume is a key factor. The challenge is mid season. So the HP modulates down to 30% or so, however when it's warmer the efficiency is also higher as outside is warmer and the flow temp target is lower. An 11kW system running with 30% modulation at 30°C flow target might still produce 6kW or more. This means it will not be able to match the fabric loss, so will cycle, which needs to be minimised. I found this study an interesting read, it basically says 11kW HP needs 250 litres system volume, so a small buffer like 50 litre specified basically for defrost may not be sufficient.

 

Hope that helps.

Edited March 7, 2021 by J1mbo

[dpmiller]

With an inverter unit I'm not convinced short-cycling is such an issue ,never mind that the controller should have a delay/cycles per hour setting to assist.

 

But there are many options there in terms of modulation control for sure. Mine has three modes for fan and compressor control somewhat like the Vaillant- Day, Night, and weather compensated. All change the response in subtley-different ways, and then you've got the pump and your chosen deltaT for the system to consider.