ASHP installed together with existing heating system

[Bruno]

Hi all.

 

I currently have the setup as attached. Because I'm really fed up with wood (as it produces a lot of dirt, it's not possible to preheat and I need to burn wood every day for DHW) I'd like to install an ASHP, so I asked for some quotations and all the calculations point to 9kW. The goal is to install both systems together, so that I'm able to use the wood burner sporadically if I wish.

I'd like to consider buying the equipment myself, then get a certified installer to... install ?

- should I add an additional buffer tank? I've seen several schematics where a buffer tank is not represented, but the quotations I got do mention it

- is it a good idea to buy online or is it too risky? The risk seems to pay off, as I've seen deals online much, much cheaper than the quotations I got.

- I'm getting quotations around 7000€ (heat pump, buffer tank, accessories and installation). What do you think about this value?

 

Thanks.

Edited March 14, 2021 by Bruno

[SteamyTea]

My feeling is that you should always fit a buffer tank.

Are your current radiators large enough?

[Gone West]

I'm no expert but I thought if you're using two different heat sources with very different temperatures you should use a thermal store. Don't know if that's what you mean by a buffer tank but buffer tanks are usually much smaller. I'm not sure how well thermal stores work with ASHPs.

Edited March 15, 2021 by PeterStarck

[Bruno]

1 hour ago, SteamyTea said:

My feeling is that you should always fit a buffer tank.

Are your current radiators large enough?

I have 63 "elements" in 8 radiators (from 12 to 5 elements each, depending on the room) and 2 towel racks. I'm considering replacing one of the radiators, the one on the end of the circuit as this usually gets less hot than the rest. 

 

13 minutes ago, PeterStarck said:

I'm no expert but I thought if you're using two different heat sources with very different temperatures you should use a thermal store. Don't know if that's what you mean by a buffer tank but buffer tanks are usually much smaller. I'm not sure how well thermal stores work with ASHPs.

As far as I know, a buffer tank is just a mass of water (the recommendation I see more often is 10l per kWh) between the heat pump and the heating circuit. 

The difference in the temperature of both sources is something I've been thinking about. All installers said that that's not a problem, even if the temperature is lower with the heat pump, it will be constant so the house will get comfortable. 

[SteamyTea]

1 hour ago, Bruno said:

elements

Not sure what you mean by that.  Are the radiators able to deliver the necessary power at say 40°C rather than 65°C.

 

A buffer tank is really there to limit short cycling of the heat source.

ASHPs perform best when they have been on for a few minutes and then deliver a consistent temperature at a fixed flow rate.

Radiators on the other hand may be switching on and off as the room temperatures vary, this changes the volume of water in the system, that needs heating, to change.

[J1mbo]

Probably Tropical95 rad elements based on the pic.

[Bruno]

Sorry for missing the correct English words. 

Elements is what is called here to the parts which make a radiator. In the attached image, the radiator is made of 8 elements. 

 

4 hours ago, SteamyTea said:

Are the radiators able to deliver the necessary power at say 40°C rather than 65°C.

I'm not sure. The installers said that, although it would be a good idea to replace the radiators, as they're aluminium and with good surface, they should work. 

To be honest, that scares me a bit. 

Edited March 15, 2021 by Bruno

[SteamyTea]

5 minutes ago, Bruno said:

I'm not sure

You can look them up on the manufactures website, there should be a temperature/power chart.

The material does not make much difference as they are thin and of low SHC.  The water in the system will have a much greater effect than the emitters.

[Bruno]

I did look into it before, but although I understand the relation between the temperature and power, I have no idea how the difference will feel. 

The thing is, with wood I can run the heating during 3-4h at 70° and then leave it off for 20h until I turn it on again (and rest assured, if it's really cold outside, then after 20h it'll be really cold inside...) 

But what it if I have it on at 40° 24/7? Worse? Better? Can't tell. 

[J1mbo]

Take your target flow temperature, subtract the room temperature, that gives dT. Then use the formula.

 

e.g. 45°C flow and 21°C room temp, dT = 24.

Power output per section = 0.80314 x (24^1.32266) = 53.75 Watts

System comprised of 63 sections therefore total output = 63 x 53.75 = 3,385 Watts. Plus the two towel radiators, probably 100 W each.

 

If the flow temp were increased to 50°C then the power output would increase to 4.3kW. At 70°C flow, it's about 9kW.

 

So currently you are heating 9kW x 4 hours = 36 kWhr per day. This is an average load of only 1.5kW. With 3.5kW running 24x7, the house will have a higher average temperature but probably a lower peak temperature everything else being equal, because the total energy added per day would increase from 36 to 84kWhr.

 

Re buffer - the system needs enough capacity to avoid short cycling at part load and enough available circulating water to provide for the defrost cycle. These radiators probably have about 0.6 litres per section so system volume will be only 40 litres plus the volume in the pipes plus the towel radiators. A 9kW heater will heat 200 litres from 40 to 45°C in about 12 minutes (assuming 3kW losses from the radiators in the process) so a buffer of something like 150 litres is probably about right.

 

You could try running electric heaters totalling 3.5kW for a couple of days 24x7 instead of the wood and see how warm the house gets. But the heat load increases as the temperature outside decreases so that needs to be factored in too.

Edited March 16, 2021 by J1mbo

[Bruno]

@J1mbo

Thanks. That, sir, was awesome ✌️

[SteamyTea]

9 hours ago, J1mbo said:

 

You could try running electric heaters

I am always amazed that people are reluctant to do this.

It is probably the cheapest way to test your heat load.

Could even out energy meters on them, and a cheap data logger.

Tempted to design a box of tricks to do just this.

Edited March 16, 2021 by SteamyTea

[Bruno]

I suppose that the biggest problem would be to have the necessary number of heaters as well as balanced power ratings. I can get a 2kW heater in the living room and another in a bedroom, but in reality I suppose that the proper way to do it would be to have several smaller heaters throughout the house. 

 

Nevertheless it's a great idea and I honestly never thought of that. And the math behind it is quite simple and was brilliantly explained ?

[Bruno]

 

I asked some local companies for quotations, and I have to say I'm confused. 

 

I got:

- 9kW LG Therma V

- 8,5kW Vaillant aroTHERM Plus  (2 quotations) 

- 12,5kW Vaillant aroTHERM Plus 

- 15kW "White brand" 

 

According to what we calculated here, the smaller ones should be more than enough, but the suppliers who quoted the bigger ones say that these are needed as the smaller ones would struggle. 

 

I also got quotations with or without buffer tank, the ones without include an "hydraulic separator" which I never heard of before. 

 

Oppinions? 

Thanks

[J1mbo]

Hydraulic separator is a plate heat exchanger. It has the advantage that the radiator/UFL circuit is independent of the ASHP, so any maintenance on it doesn't mean refilling everything with expensive glycol. The disadvantage is that it will create a flow temp step across it, i.e. the ASHP will need to product 3-5°C warmer flow than without it.

 

Buffer tank requirements are set by the ASHP vendor and are there for defrost. Again, they will create a flow temp step across them when plumbed in a 4-pipe configuration.

 

DHW demand will likely impact your choice of output. The heat pump provides no central heating whilst it's warming the cylinder.

[Bruno]

35 minutes ago, J1mbo said:

Buffer tank requirements are set by the ASHP vendor and are there for defrost.

Uh, I thought it was mainly to prevent short cycling of the ASHP. I have a situation with my neighbour (which I'll probably discuss in another topic) where he has a 14kW LG, connected directly to radiators (no buffer) which is constantly working in peaks from 0 to 6kW (electrical power).

Having the DHW in my case will also probably help having the buffer tank, right? When the cylinder is being heated, the buffer tank can continue to supply the radiators (although I would expect a quick drop in temperature in this case).

 

What about the very different power ratings which I received, what do you think? And about the different models, any recommendation? (I believe the Vaillant would be better as it's more efficient and has a higher output temperature).

[J1mbo]

Probably your Vaillant designs have a small c.45 litre buffer in them. Drawing 5K from it is only 0.25kWhr. In any case, the system will not run the CH pump whilst serving DHW.

[George]

If it's mainly for the DHW I'd be tempted to undersize it - when it's really cold you've still got the wood burner to fall back on.

Seems like a good idea, though. Similar to me except I've got wood burners purely for space heating (and to save electricity - the wood is free!) as I found the connection to a HWC too difficult without using a thermal store, which I didn't really have space for (just have a 300l HWC).

 

So the heat pump heating/hot water just chug along merrily and if I can be bothered I light the fire. It's night and day compared to what I used to do... get home and shiver while lighting the fire.

[SteamyTea]

11 minutes ago, George said:

get home and shiver while lighting the fire.

Sort the building out, then you only need a smaller thermal load.

[George]

1 minute ago, SteamyTea said:

Sort the building out, then you only need a smaller thermal load.

 

Yeah don't worry. Spent 4 months insulating... but as its locally listed all internal wall using wood fibre so never going to meet passiv haus type performance. However electric heat pump heating is a fraction of what oil used to be and that only effectively heated one room.

[Bruno]

6 hours ago, J1mbo said:

The disadvantage is that it will create a flow temp step across it, i.e. the ASHP will need to product 3-5°C warmer flow than without it.

 

Buffer tank requirements are set by the ASHP vendor and are there for defrost. Again, they will create a flow temp step across them when plumbed in a 4-pipe configuration.

Sorry for getting back to this. One of my quotations (the one I like the most) has both hydraulic separator and buffer tank. In this case I don't need a 3 way valve, but I don't understand how does it choose where to send the water to. 

In this case, should I expect a big temperature drop because of losses in both systems? 

[Bruno]

6 hours ago, Bruno said:

the one I like the most

So I was on the phone with the seller who provided this quotation. Turns out the 3-way valve isn't required because the DHW cylinder is to be connected as if it was a radiator, always connected and can't be disconnected, neither the radiators. 

According to him:

- as the DHW cylinder is small and has a small "serpentine" (? It's the heating water circuit on the tank) is too small, it won't dissipate the heat produced by the ASHP, thus resulting in errors in the unit as the returning water will be the same temperature as the output. Q: is this really an issue? Can't the heat pump keep a big outside temperature (no matter the returning water until the temperature of the DWH cylinder reaches its setpoint and then shut off or divert to the radiators? 

- it makes no sense to use the heat pump to heat DHW without heating the radiators (eg summer) as the unit's COP will be 1, so it would be better to use an electric resistor. Q: does this make any sense? If there are 30 degrees outside and I want to heat the DHW to 55deg, is the COP worse? It goes against everything I read... 

 

I'm now a bit lost. Help? 

[J1mbo]

It sounds like an odd solution. Usually the DHW and CH will be controlled with a diverter valve because the required flow temperatures are very different. There will be quite a step across a heat exchanger and buffer but this will vary with load. When continuously heated to a steady state and with the pump speeds all balanced it might 5°C or less.

[Bruno]

That's what I thought too. 

But about the DHW cylinder, do you think he's right? As the circuit from the HP to the the cylinder and back would have only some 3-4m, can't the HP handle this? (having the "same" temperature in and out while doing DHW) 

[ProDave]

If the flow and return to the HW cylinder is "the same" then something is wrong, like the HW is already up to temperature and the system should have turned off.

 

You have got a proper "heat pump" HW cylinder with a large area input coil?